Quantifying nutrient fluxes with a new hyporheic passive flux meter (HPFM)

Abstract

The hyporheic zone is a hotspot of biogeochemical turnover and nutrient removal in running waters. However, nutrient fluxes through the hyporheic zone are highly variable in time and locally heterogeneous. Resulting from the lack of adequate methodologies to obtain representative long-term measurements, our quantitative knowledge on transport and turnover in this important transition zone is still limited.In groundwater systems passive flux meters, devices which simultaneously detect horizontal water and solute flow through a screen well in the subsurface, are valuable tools for measuring fluxes of target solutes and water through those ecosystems. Their functioning is based on accumulation of target substances on a sorbent and concurrent displacement of a resident tracer which is previously loaded on the sorbent.Here we evaluate the applicability of this methodology for investigating water and nutrient fluxes in hyporheic zones. Based on laboratory experiments we developed hyporheic passive flux meters (HPFMs) with a length of 50cm which were separated in 5-7 segments allowing for vertical resolution of horizontal nutrient and water transport. The HPFMs were tested in a 7 day field campaign including simultaneous measurements of oxygen and temperature profiles and manual sampling of pore water. The results highlighted the advantages of the novel method: with HPFMs, cumulative values for the average N and P flux during the complete deployment time could be captured. Thereby the two major deficits of existing methods are overcome: first, flux rates are measured within one device instead of being calculated from separate measurements of water flow and pore-water concentrations; second, time-integrated measurements are insensitive to short-term fluctuations and therefore deliver more representable values for overall hyporheic nutrient fluxes at the sampling site than snapshots from grab sampling. A remaining limitation to the HPFM is the potential susceptibility to biofilm growth on the resin, an issue which was not considered in previous passive flux meter applications. Potential techniques to inhibit biofouling are discussed based on the results of the presented work. Finally, we exemplarily demonstrate how HPFM measurements can be used to explore hyporheic nutrient dynamics, specifically nitrate uptake rates, based on the measurements from our field test. Being low in costs and labour effective, many flux meters can be installed in order to capture larger areas of river beds. This novel technique has therefore the potential to deliver quantitative data which are required to answer unsolved questions about transport and turnover of nutrients in hyporheic zones.

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/ doi:10.5194/bg-14-631-2017 Author(s) 2017. CC Attribution 3.0 License.

Julia Vanessa Kunz1, Michael D. Annable2, Jaehyun Cho2, Wolf von Tmpling1, Kirk Hateld2, Suresh Rao3, Dietrich Borchardt1, and Michael Rode1

1Helmholtz Centre for Environmental Research UFZ, Magdeburg, Germany

2University of Florida, Gainesville, Florida, USA

3Purdue University, Lafayette, Indiana, USA

Correspondence to: Julia Vanessa Kunz ([email protected])

Received: 11 August 2016 Discussion started: 19 August 2016

Revised: 20 January 2017 Accepted: 23 January 2017 Published: 9 February 2017

Abstract. The hyporheic zone is a hotspot of biogeochemical turnover and nutrient removal in running waters. However, nutrient uxes through the hyporheic zone are highly variable in time and locally heterogeneous. Resulting from the lack of adequate methodologies to obtain representative long-term measurements, our quantitative knowledge on transport and turnover in this important transition zone is still limited.

In groundwater systems passive ux meters, devices which simultaneously detect horizontal water and solute ow through a screen well in the subsurface, are valuable tools for measuring uxes of target solutes and water through those ecosystems. Their functioning is based on accumulation of target substances on a sorbent and concurrent displacement of a resident tracer which is previously loaded on the sorbent.

Here we evaluate the applicability of this methodology for investigating water and nutrient uxes in hyporheic zones. Based on laboratory experiments we developed hyporheic passive ux meters (HPFMs) with a length of 50 cm which were separated in 57 segments allowing for vertical resolution of horizontal nutrient and water transport. The HPFMs were tested in a 7 day eld campaign including simultaneous measurements of oxygen and temperature proles and manual sampling of pore water. The results highlighted the advantages of the novel method: with HPFMs, cumulative values for the average N and P ux during the complete deployment time could be captured. Thereby the two major decits of existing methods are overcome: rst, ux rates are measured within one device instead of being calculated from separate measurements of water ow and pore-water concentra-

tions; second, time-integrated measurements are insensitive to short-term uctuations and therefore deliver more representable values for overall hyporheic nutrient uxes at the sampling site than snapshots from grab sampling. A remaining limitation to the HPFM is the potential susceptibility to biolm growth on the resin, an issue which was not considered in previous passive ux meter applications. Potential techniques to inhibit biofouling are discussed based on the results of the presented work. Finally, we exemplarily demonstrate how HPFM measurements can be used to explore hyporheic nutrient dynamics, specically nitrate uptake rates, based on the measurements from our eld test. Being low in costs and labour effective, many ux meters can be installed in order to capture larger areas of river beds. This novel technique has therefore the potential to deliver quantitative data which are required to answer unsolved questions about transport and turnover of nutrients in hyporheic zones.

1 Introduction

Rivers export high loads of nitrogen from inland catchments to the marine environment (Smith et al., 2006). The ecological and economic problems caused by eutrophication of coastal and riverine ecosystems have been recognised years ago (Patsch and Radach, 1997; Artioli et al., 2008; Skogen et al., 2014). Decades of nutrient studies have revealed that rivers cycle rather than only transport nutrients (Garcia-Ruiz et al., 1998a; Seitzinger et al., 2002; Galloway et al., 2003). In agriculturally dominated areas, in-stream processes may

Published by Copernicus Publications on behalf of the European Geosciences Union.

Quantifying nutrient uxes with a new hyporheic passive ux meter (HPFM)

632 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

for example retain up to 38 % of nitrate (NO3) and 48 % of soluble reactive phosphate (SRP) inputs (Mortensen et al., 2016). The hyporheic zone, the subsurface region of streams and rivers that exchanges water, solutes and particles with the surface (Valett et al., 1993) and may mix streamwater during the transport through the sediments with underlying ground-water (Triska et al., 1989; Fleckenstein et al., 2010; Trauth et al., 2014), is one key compartment for in-stream nutrient cycling (Fischer et al., 2005; Zarnetske et al., 2011b; Basu et al., 2011; Stewart et al., 2011). For instance, denitrication, the anaerobic reduction of NO3 to gaseous N2 and in most river systems the dominant dissimilatory process which removes N out of the system (Laursen and Seitzinger, 2002;Bernot and Dodds, 2005; Lansdown et al., 2012; Kunz et al., 2017), often exclusively happens at reactive sites in the hyporheic zone (Duff and Triska, 1990; Rode et al., 2015). In addition to biological nutrient uptake, intermediate physical storage in the hyporheic zone disperses the propagation of pollutant and nutrient spikes which could be harmful for receiving water bodies (Runkel, 2007; Brookshire et al., 2009;Covino et al., 2010; Findlay et al., 2011). For those reasons, it is of interest to quantify the amount of nutrients actually reaching the reactive sites in the subsurface collateral to the processes they undergo there (Seitzinger et al., 2006; Zarnetske et al., 2012). Transport rates of water and nutrients from the surface to the subsurface could be attributed to water level, sediment properties and various other hydrological, biological, chemical and physical factors (Bhlke et al., 2009; Boano et al., 2014; Trauth et al., 2015). The complex interactions between these inuencing factors and the temporal variability and local heterogeneity of hyporheic processes often cause high uncertainties in quantitative models. However, due to methodological restrictions, experimental investigations of nutrient dynamics in the hyporheic zone are rare and commonly exclusively of qualitative nature (Mulholland et al., 1997; Grant et al., 2014).

Attempts to quantify hyporheic nutrient processing rates have primarily been based on benthic chamber and incubation experiments (Findlay et al., 2011; Kessler et al., 2012).Those laboratory (mesocosm and ume) experiments can estimate the denitrication potential of the substrates, usually via denitrication enzyme assays. However, the realised denitrication rates depend equally on environmental and hydro-logical conditions rather than on substrate type or denitrication potential alone (Findlay et al., 2011). Thus, owing to the high variability and complexity of natural systems, hyporheic transport of nutrients cannot satisfactorily be mimicked in articial set-ups (Cook et al., 2006; Grant et al., 2014).

Direct in-stream measurements of nutrient dynamics based on whole-stream tracer injections, mass balances (McKnight et al., 2004; Bhlke et al., 2009) and more recently high-resolution time series from automated sensors (Pellerin et al., 2009; Hensley et al., 2014; Rode et al., 2016a,b) can be used for determining general uptake rates on the reach scale, but do not allow to identify the reaction sites

(hyporheic versus in channel or algal canopies) or specic local uptake processes (Ensign and Doyle, 2006; Ruehl et al., 2007). Further, in-stream measurements exclusively account for water which is re-inltrating into the main stem after passage through the hyporheic zone. Under loosing conditions, where most of the nutrient-inux is owing towards the groundwater, processing rates in the subsurface cannot be observed in the surface water. Likewise, if groundwater is contributing signicantly to surface water chemistry, surface water mass balances do not characterise nutrient cycling in the hyporheic zone realistically (Trauth et al., 2014).

Conclusively, in situ assessments of hyporheic nutrient uxes are indispensable. Hyporheic nutrient uxes are commonly calculated from separate measurements of inltration rates and pore-water concentrations (Kalbus et al., 2006;Ingendahl et al., 2009). The exchange of water between the surface and subsurface is traditionally derived from hydraulic head differences or tracer injections (Fleckenstein et al., 2010; USEP, 2013). Time series of high-resolution vertical temperature proles have efciently been used to derive vertical Darcy velocity (qy) (m d1) in the streambed.

While measurements of vertical Darcy velocities are a valuable asset, primarily horizontal uxes are needed to assess hyporheic transport and residence time (Binley et al., 2013;Munz et al., 2016). Active heat-pulse tracing enables highly resolved in situ measurements of direction and velocity of hyporheic ow (Lewandowski et al., 2011; Angermann et al., 2012). These methods are protable in shallow sediments (max. 1520 cm) and rivers with ne sediments, but may not be implementable in streams with coarser sediments.

Pore-water solute concentrations are typically determined from analysis of grab samples extracted with drive points (Saenger and Zanke, 2009; USEP, 2013). Alternatively, dialysis cells so-called peepers (Hesslein, 1976; Teasdale et al., 1995) or gels (Krom et al., 1994), based on the diffusive equilibrium between the solute concentration in the pore water and the receiver solution in the peeper or the gel, have been used to measure small-scale solute distribution over highly resolved proles on the millimetre to centimetre scale.These techniques provide valuable insights into the time-specic conditions at the target site. However, they delivers effectively a snap shot of the highly temporally variable hyporheic zone processes (Cooke and White, 1987) that may not be representative for the overall conditions in the system.Only repeated sampling at high frequencies and over longer time spans as conducted for example by Duff et al. (1998) can account for the short-term variability. Attempting to characterise larger areas with these methods is laborious and costly.

In sum, direct quantitative methods for hyporheic ux measurements have two major deciencies. First, separate measurements of water ow velocities and pore-water concentrations are necessary to calculate mass uxes. This is not only labour intensive but can, due to the high temporal and spatial variability of hyporheic ow, also lead to incorrect estimates of real ux rates (Kalbus et al., 2006). Second, most

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

rections for convergence and divergence of owlines into or around the ux meter have been established in earlier studies (Klammler et al., 2007). However, accounting for an impermeable outer casing of a ux meter is much more complicated and requires additional factors which have to be determined experimentally for each specic application (Hateld et al., 2004; Klammler et al., 2007; Annable et al., 2005). We therefore intended to deploy the PFM in a way that allows direct contact with the surrounding sediments and minimal manipulation of the natural ow pattern.

Considering these requirements, we developed a modication of the PFM for the application in the hyporheic zone (hyporheic passive ux meter, HPFM). Based on the results from laboratory analysis and a rst eld test in a nutrient-rich 3rd-order stream (Holtemme, Germany), we demonstrate prospects and remaining limitations for hyporheic nutrient studies with HPFMs.

2 Methods

2.1 Construction and materials

The HPFMs consisted of a nylon mesh which was lled with a mixture of a macroporous anion exchange resin as a nutrient absorber and alcohol tracer loaded activated carbon (AC) for the water ow quantication. In the present study HPFMs were constructed 50 cm long and 5 cm in diameter.A stainless steel rod in the middle assured the stability of the device (Fig. 1). To measure vertical proles of horizontal uxes of both nutrient and water in the hyporheic zone, the HPFM was divided into several segments using rubber washers. Steel tube clamps were used to attach the nylon mesh to the steel rod placed in the centre of the HPFM. The nylon mesh was purchased from Hydro-Bios (Hydro-Bios Apparatebau GmbH, Kiel-Holtenau, Germany) and is available in a wide range of mesh size and thicknesses. We used a mesh size of 0.3 mm. In general, meshes should be as wide as possible because very ne mesh may act as a barrier to water ow limiting inltration of water and solutes into the HPFM (Ward et al., 2011). However, the mesh should be smaller than the nest sediments, AC or resin granules. As nal step, a rope was connected to the tube clamp on the upper end of the HPFM in order to facilitate retrieval.

2.2 Selection and characterisation of resin

The nutrient sorbent had to meet the following criteria:

1. have a high loading capacity for NO3 , PO4 and competing anions;

2. be free of compounds which could interfere with the alcohol tracer measurements (e.g. organic substances);

3. have a low background of NO3 and PO4 .

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 633

techniques for measuring pore-water concentrations exclusively capture the concentration at the sampling time, which may not reect the overall conditions at the sampling location. New, affordable and efcient methods for the long-term measurement of nutrient uxes through the hyporheic zone are therefore required to validate and improve models (Boyer et al., 2006; Wagenschein and Rode, 2008; Alexander et al., 2009) and to determine the site-specic extent of nutrient processing in the hyporheic zone (Fischer et al., 2009).

In groundwater studies, passive ux meters (PFMs) have successfully been used to quantify uxes of contaminants (Hateld et al., 2004; Annable et al., 2005; Verreydt et al., 2013) through screened groundwater monitoring wells, integrating time spans ranging from days to weeks. PFMs consist of a cylindrical, screened PVC casing lled with activated carbon (AC) as a porous sorbent. As long as the PFM is residing in the monitoring well, dissolved contaminants in the groundwater owing passively through the meter are retained on the AC. Furthermore, the AC is preloaded with water-soluble resident tracers. The horizontal water ux through the screened media can then be determined from the displacement of these resident tracers, while simultaneously the contaminant ux is quantied based on the solute mass captured on the AC. First bench-scale and column experiments for PO4 have been conducted in the laboratory (Cho et al., 2007). As AC did not prove efcient in capturing nutrients, an anion-absorbing resin, a granular matrix originally manufactured for purication processes, was used as sorbent for PO4 instead. In theory, the PFM principle should be extendable to other nutrients or other environments. For example, development of sediment bed passive ux meters (SBPFMs) for the measurement of vertical contaminant seepage has been initiated (Layton, 2015). However, specic tests for NO3 as well as assessments under the complexities associated with hyporheic zone processes have not been conducted yet.

In this study we evaluate the applicability of PFMs for the measurement of horizontal nutrient uxes in hyporheic zones, focusing on NO3. We hypothesised that, while the principal concept of PFM can be maintained, several adaptations will still be necessary: most importantly, a suitable sorbent for NO3 as target nutrient is required. The market of anion-absorbing resins is large and offers a wide range of products with varying characteristics (Annable et al., 2005;Clark et al., 2005). Various criteria, like possible interference of resin compounds with the resident tracer analysis or pre-existing background nutrient loads on the resin, have to be considered. As experience on resin behaviour under eld conditions is so far rare, we also expected unforeseen associated challenges, including for example biofouling of resin and/or nutrients. Additionally, a new deployment and retrieval procedure had to be developed. Existing ground-water PFMs have been installed into land-based wells. In hyporheic studies, underwater installation requires a technique which impedes contamination with surface water. Cor-

634 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

Figure 1. Photograph of an HPFM with alternating segments before deployment (left), schematic prole of a deployed HPFM (middle) and schematic steps of HPFM functioning (right): (1) directly after installation, tracer resides on activated carbon (AC), (2) inltrating water washes out the tracer, nutrients enter the HPFM and are absorbed on the resin, (3) after retrieval nutrients are xed on the resin, tracer concentration is diluted.

A pre-selection for anion-absorbing resins which were free of organic compounds was made based on information provided by the manufacturers (Purolite, Lewatit, Dowex).

2.2.1 Nutrient background

Nutrient background on the resins was determined by extracting and analysing NO3 and PO4 from each resin (n =

3). Therefore 30 mL of 2M KCl was added to 5 g of each pure resin and rotated for 24 h. The solution was then analysed on a Segmented Flow Analyser Photometer (DR 5000, Hach Lange) for NO3 at 540 nm (detection limit0.042 mg NO3-N L1) and for SRP at 880 nm (detection limit 0.003 mg P L1). In order to estimate the effect of background concentrations on nal results in the actual eld application of HPFM, the extractable background concentrations were then converted to nutrient uxes using a Darcy ux of qx = 4 m d1, an estimate based on hyporheic ow

velocities which were measured previously with salt tracer tests at the study site. Likewise, the expected hyporheic nutrient ux was computed from previously examined concentrations in pore-water samples and the Darcy ux. The only resin with nutrient background below 5 % of expected concentrations was Purolite A500 MB Plus (Purolite GmbH, Ratingen, Germany), which had extractable background NO3 of 8 g NO3-N g1 wetted resin ([notdef]1.6 g g1, n = 3)

and 0.08 g PO4-P g1 resin ([notdef]1.7 [notdef] 103 g g1, n = 3).

Purolite A500 MB Plus was then considered for testing the loading capacity. The limit of quantication LQ for the nutrient extraction resulting from this background was calculated according to the EPA Norm 1020B (Greenberg et al., 1992) as the sum of background concentration and 10 times the standard deviation and amounted to 24 g NO3-N g1 resin and 0.097 g PO4-P g1 resin.

2.2.2 Loading capacity and biofouling

Purolite A500 MB Plus is a macroporous anion exchanger on the basis of polyvinylbenzyl-trimethylammonium with a typical granular size of 0.88 mm diameter, an average density of 685 g L1 and an effective porosity of 63 %. The theoretical absorbing capacity is indicated in the product sheet as 1.15 eq L1 (molar weight equivalences per litre of resin), corresponding to 71.3 g NO3-N L1. Assuming hyporheic ow velocities of qx = 4 m d1 and a concentra

tion of 10 mg NO3-N L1, the volume of one HPFM could adsorb NO3 for 89 days. However, if multiple anions are present, real loading capacities for NO3 are expectedly lower.

For the determination of a realistic loading capacity, three 5 cm diameter columns were lled to a height of 5 cm with wetted Purolite A500 MB Plus resin, placed in a vertical position and inltrated with water collected from the study reach. The columns were covered with tin foil to keep them dark and ensure stable temperature. A constant supernatant of 1 cm was kept on all three columns to ensure uniform inltration at the surface of the column. Water was continuously pumped (peristaltic pump, ISMATEC BVP Standard, ISM444) through the columns from top to bottom for 22 days at a speed of 20 mL h1, which also equals the expected Darcy velocity of qx = 4 m d1. River water was sup

plied from a 22 L HDPE canister (RotilaboEPK0.1). SRP and NO3 concentrations in this reservoir were revised daily.

The draining water at the bottom outlet of the columns was sampled twice a day and analysed for SRP and NO3.

Biolm growth on the resin was assessed by repeating the same experiment in smaller columns (n = 3) and extend

ing it for several days after break-through occurred. That way, nutrient consumption by biolm after the exhaustion of

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 635

the loading capacity could be monitored. After the experiment we coloured samples (n = 3) of resin granules from the

columns with SybrGreen (C32H37N4S+) on nucleic acid and examined them under a confocal laser scanning microscope to depict the degree of bacterial fouling on the granular surface.

2.3 Preparation of activated carbon with alcohol tracers

As designed for the groundwater PFMs, silver impregnated activated carbon (AC) was used as sorbent for the resident alcohol tracers. The same AC as in previous PFM applications (Annable et al., 2005) was used for the HPFM in this study and was provided by the University of Florida, Gainesville. The AC had a bulk density of 550 g L1, a grain size ranging from 0.42 to 1.68 mm and a hydraulic conductivity k = 300 m d1.

Since the magnitude of water ow through the ux meter is unknown a priori, multiple resident tracers with a wide range of tracer elution rates were needed. The retardation factor of a substance Rd is a measure for the rate of elution of the substance from a particular carrier. Alcohols offer a wide range of retardation factors and can easily be mixed and sorbed to the AC (Hateld et al., 2004; Cho et al., 2007).By choosing the same manufacturer for the AC and the same alcohol mixture as used in the above-mentioned studies, we could rely on physical and chemical characterisations and calculated Rd for alcohol partitioning behaviour which have been established by Hateld et al. (2004), Annable et al. (2005) and Cho et al. (2007) (Table 1).

An alcohol tracer mixture for approximately 10 HPFMs was prepared by combining 100 mL of methanol, 100 mL of ethanol, 200 mL of isopropanol (IPA), 200 mL of tert-butanol (TBA) and 66 mL of 2,4-dimethyl-3-pentanol (2,4 DMP).

In order to prepare the resident alcohol tracers on the AC, the AC was soaked in an aqueous solution containing the resident alcohol tracers. A standard ratio of 13 mL tracer mixture was added to 1 L of water in a Teon sealed container and was then shaken by an automated shaker over a period of several hours. Subsequently, 1.5 L of dry activated carbon was added to the aqueous tracer solution and rotated for 12 h to homogenise the AC tracer mixture. After mixing, the supernatant water was discarded and the AC tracer mixture was stored in a sealed container and refrigerated, preventing the evaporation of the alcohol tracers.

Similarly to the resins, the AC was tested for background nutrients by extraction with 30 mL KCl per 5 g AC.

The activated carbon contained 0.01 mg PO4-P g1AC ([notdef]7.5 [notdef] 104 mg g1, n = 3) and 0.08 mg NO3-N g1 AC

([notdef]5 [notdef] 103 mg g1, n = 3), which amounts to 75 % of the

expected concentration for NO3 and 320 % for PO4. To investigate whether the AC could be cleaned by washing, we repeatedly treated AC samples with distilled water or KCl as depicted in the extraction description above. Nutri-

ents did not leach off under water treatment and neither did KCl treatment satisfactorily reduce extractable background concentration on the AC. After the third washing of AC with KCl, 0.02 mg PO4-P ([notdef]3.3 [notdef] 104 mg g1, n = 3) and

0.04 mg NO3-N ([notdef]2.3 [notdef] 103 mg g1, n = 3) could still be

extracted per g AC. Further, it was unclear to which degree replacing absorbed nutrients by KCl would alter the alcohol tracer retardation and extraction on the AC. For those reasons, we decided to keep the nutrient-absorbing resin separated from the AC. As AC did not release background nutrients under water treatment, water owing rst through AC and afterwards resin layers was not considered problematic.

2.4 Deployment and retrieval procedure

HPFMs were built, stored dry and transported in 70 cm long standard polyethylene (PET) tubes (58 [notdef] 5.3 SDR 11)

purchased from a local hardware store (Handelshof Bitter-feld GmbH, Bitterfeld, Germany). To avoid resident alcohol tracer loss, the transport tubes with the HPFMs were sealed with rubber caps and cooled during storage and transport. In the eld, prior to installation, the HPFMs were transferred to a stainless steel tube, 5.3 cm inner diameter with a loose steel drive point tip on the lower end. The diameter of the steel tube for installation tightly tted with the rubber washers at the top and bottom end of the HPFM, so that vertical water ow through tube and HPFM during installation was inhibited. The steel casing and HPFM were driven into the river bed using a 2 kg hammer until the upper end of the HPFM was at the same level as the surfacesubsurface interface. The metal casing was retrieved while the HPFM was held in place using a steel rod.

After 7 days of exposure, the HPFMs were retrieved by holding the transport tube in place and quickly drawing the HPFM into the tube using the rope xed to the upper end of the HPFM. The required length of the transport tube, steel drive casing and retrieval rope was determined by the depth of the water level in the stream.

After retrieval, the HPFMs were transported to the laboratory.

2.5 Analysis and data treatment

In the laboratory, the retrieved HPFMs were instantly (after maximal 12 h) sampled for analysis. Therefore one segment after the other was cut open and the sorbent was segment-wise recovered, homogenised and a subsample transferred to 40 mL glass vials. The subsamples from resin segments were then analysed for nutrient content, the subsamples from AC segments were analysed for the remaining alcohol tracers as described in the following paragraphs.

2.5.1 Water ux

The AC samples were shipped to the University of Florida for analysis. In the laboratory, the mass of the previously applied

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017

636 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

Table 1. Resident tracers per litre of aqueous solution and their partitioning characteristics. Retardation factors (Rd) for the specic set of tracers and AC used in this study had previously been determined by Cho et al. (2007).

Resident tracers Aqueous concentration (g L1) Rd

Methanol 1.2 4.9 Ethanol 1.2 20 Isopropyl alcohol (IPA) 2.3 109 tert-Butyl alcohol (TBA) 2.3 309 2,4-Dimethyl-3-pentanol (DMP) 1.2 > 1000

mixture of alcohol tracers in standard AC samples and the tracer mass remaining in the nal AC samples were extracted with iso-butyl alcohol (IBA). About 10 g of AC samples were transferred into pre-weighed 40 mL vials containing 20 mL IBA. Vials were rotated on a Glas-Col Rotator, set at 20 % rotation speed, for 24 h. Then, subsamples were collected in 2 mL GC vials for alcohol tracer analysis. The samples were analysed with a GC-FID (Perkin Elmer Autosystem) (Cho et al., 2007).

The relationship between time-averaged specic horizontal discharge qx (m s1) through the device and tracer elution is given by (Hateld et al., 2004):

qx =

outer casing or well wall), can be estimated after Strack and Haitjema (1981):

=

2 !, (3)

where KD = kDk10 is the dimensionless ratio of the uni

form hydraulic conductivity of the HPFM sorptive matrix kD (L T1) to the uniform local hydraulic conductivity of the surrounding sediment k0 (L T1). For more details on the correction factor and applications where a solid casing is required or the permeability of the surrounding sediments is higher than of the device see Klammler et al. (2007) and Hateld et al. (2004).

2.6 Field testing of HPFMs

2.6.1 Study site

A 30 m long stretch of the Holtemme River, a 3rd-order stream in the Bode catchment, TERENO Harz/Central German Lowland Observatory, served as study site (51 56[prime]30.1[prime][prime] N, 11 09[prime]31.8[prime][prime] E). The testing reach is located in the lowest part of the river, where the water chemistry is highly impacted by urban efuent and agriculture (Kamjunke et al., 2013). Long stretches have been subjected to changes in the natural river morphology by canalisation (Landesbetrieb fr Hochwasserschutz und Wasserwirtschaft Sachsen-Anhalt, 2009).

The sediments at the selected site are sandy with gravel and small cobbles. Sieving of sediment samples delivered the effective grain size d10 = 0.8 mm and a coefcient of uni

formity Cu = 3.13. The effective porosity nef is 13 %. Af

ter Fetter (2001) the intrinsic permeability was estimated to be Ki = 96 m2 and the hydraulic conductivity k = 81 m d1

Clay lenses are present in the deeper sediments below 35 cm.

Mean discharge in the stream is 1.35 m3 s1 with highest peaks around 56 m3 s1. Discharge is continuously recorded by the local authorities at the gauge Mahndorf, 15 km upstream of the testing site. In the course of the year, NO3 concentrations in the lower Holtemme vary between 2 and 8 mg NO3-N L1 (Hochwasservorhersagezentrale, 2015/2016).

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

1 KD

1.67r (1 MR)Rd

t , (1)

where r (m) is the radius of the HPFM, is the volumetric water content in the HPFM (m3 m3), MR () is the relative mass of tracer remaining in the HPFM sorbent, t (s) is the sampling duration and Rd () is the retardation factor of the resident tracer on the sorbent.

2.5.2 Nutrient ux JN

NO3 and PO4 were extracted and analysed in the laboratory at UFZ in Magdeburg, Germany, similarly to the analysis of background concentrations on the resin: subsamples of 5 g resin were treated with 30 mL of 2 M KCl each and rotated for 24 h for extraction. The solution was then analysed as described above.

The time-averaged advective horizontal nutrient ux JN (mg m2 d1) can be calculated using the following relationship (Hateld et al., 2004):

JN =

qxMN

2 rLt , (2)

where MN (kg) is the mass of nutrient adsorbed, L (m) is the length of the vertical thickness of the segment and () is a factor ranging from 0 to 2 that characterises the convergence ( > 1) or divergence ( < 1) of ow around the HPFM. If, like in the case presented here, the hydraulic conductivity of the HPFM sorbent (resin or AC) is much higher than that of the surrounding medium and the HPFM is in direct contact with the sediments (i.e. in absence of an impermeable

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 637

to the laboratory and stored until it was sampled and analysed along with the other HPFMs. The results from the control HPFM also include uncertainties arising from sample storage, analytical processing and the background concentration of nutrients on the resin. Measurements of the other HPFMs were corrected by subtracting the transport-, storage-and deployment-related tracer loss and nutrient accumulation detected in the control.

2.6.3 Additional measurements

Vertical Darcy velocity (qy)

The vertical vector of hyporheic Darcy velocities qy were measured supplementary to the horizontal uxes assessed with the HPFM in order to estimate the general direction of ow (upwards or downwards) and to calculate the angle of hyporheic ow.

The vertical Darcy velocity (qy) (m d1) in the streambed was calculated using temperature proles measured between January and October 2015. According to Keery et al. (2007) and Schmidt et al. (2014), vertical ow velocities can be computed from the temporal shift of the daily temperature signal in the subsurface water relative to the surface water. A multi-level temperature sensor (Umwelt- und Ingenieurtechnik GmbH, Dresden, Germany) was installed at the test site in January 2015. Temperature was recorded at the surface subsurface interface and at depths of 0.10, 0.125, 0.15, 0.2,0.3 and 0.5 m in the sediment at a 10 min interval (accuracy of 0.07 C over a range from 5 to 45 C and a resolution of0.04 C). A numerical solution of the heat ow equation was then used in conjunction with Dynamic Harmonic Regression signal processing techniques for the analysis of these temperature time series. The coded model was provided by Schmidt et al. (2014).

Oxygen proles

We monitored the subsurface oxygen concentration as a primary indication on the redox status of the hyporheic zone in order to evaluate the potential for NO3 reduction and PO4 mobilisation. Therefore two oxygen loggers (miniDO2T,

Precision measurement engineering Inc.) incorporated into steel tubes acuminated at the lower end were installed in the river bed. The tubes had lter-screens at the measuring depths of 25 and 45 cm below surfacesubsurface boundary.Installation was carried out 4 weeks prior to the experiments, allowing enough time for re-equilibration of the surrounding media. The measurement time step was 5 min.

Multi-level samplers (MLSs)

Pore-water nutrient concentrations were measured to substantiate the HPFM results. Multi-level samplers as described in detail by Saenger and Zanke (2009) are devices for the manual extraction of hyporheic pore water from several dis-

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017

Figure 2. Overview of the instrumental setup at the Holtemme for the testing phase in June 2015. R1, R2, resin-only HPFM; AC3, AC4, activated-carbon-only HPFM; L5, L6, alternating layered HPFMs; MLSA, MLSB, multi-level sampler; O2 25, O2 45, subsurface oxygen logger; C, vertical temperature prole.

The equipment was installed for a period of 7 days from 4 to 11 June 2015 as illustrated in Fig. 2.

2.6.2 HPFM testing

Based on the laboratory results for the nutrient backgrounds and the consequent necessity to keep resin and AC separated two approaches for constructing and deploying HPFM were tested in the eld.

Resin only and AC only HPFMs

Four HPFMs were constructed of which two contained only resin (R1 and R2) and the other two contained only AC (AC3 and AC4). The HPFMs were then installed in pairs: AC only and resin only next to each other with a separation distance of 30 cm. Those four HPFMs were sectioned in ve horizontal ow segments, each with a vertical length of 10 cm.

For the calculation of the nutrient ux through each segment of R1 and R2, we used the corresponding water ux through the respective segment of AC3 and AC4.

Alternating segments of AC and resin HPFMs

HPFMs L5 and L6 consisted of seven segments starting and ending with an AC segment and adjacent segments altering between resin and AC (also see Fig. 1). Each segment had a length of 7 cm.

For the calculation of the nutrient ux through the resin segments we used the interpolated water ow measured in the two adjacent AC segments.

One additional HPFM with alternating layers was used as a control HPFM, in order to assess potential tracer loss or nutrient contamination during storage, transport and deployment/retrieval. This control was stored and transported together with the other HPFMs. After deploying the control HPFM, it was immediately retrieved, transported back

638 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

tinct depths. The two samplers A and B used in these experiments were manufactured by the institutional workshop of the UFZ. Like the oxygen loggers both MLSs were installed 4 weeks prior to the experiment. They consisted of an outer stainless steel tube with a length of 50 cm and a diameter of 5 cm. Ceramic lters were inserted in this outer steel mantle marking the extraction depths at 5, 15, 25 and 45 cm. The inner sides of the lters were attached to steel pipes that ran to the top of the sampler so that Teon tubes could be attached.A protective hood was threaded on the upper end of the sampler to preclude particles and sediment entrance. Per sampler and depths 10 mL of pore water was manually extracted by connecting a syringe to the open end of the Teon tube and slowly sucking up water at a rate of 2 mL min1. The four extraction depths were sampled successively, always starting with the shallowest depths and continuing with ascendant depths. Manual pore-water samples were taken on the 4 and11 June 2015, both times between 01:00 p.m. and 04:00 p.m. local time.

The samples were ltered in the eld through a 0.45 m membrane lter and placed in boro-silica glass vials for transport to the laboratory. Analysis for NO3, SRP, sulfate (SO24) and boron (B) were conducted in the central analytical laboratory of the UFZ, Magdeburg, Germany. Analytical procedure for NO3 and SRP was as in the description above.

SO24 and B were used as natural tracers for groundwater and surface water respectively. SO24 was analysed on an ion chromatograph (ICS 3000, Thermo Fisher, formerly

DIONEX), B was analysed on an inductively coupled plasma mass spectrometer (ICP-MS 7500c, Agilent). As NO3 and

SRP concentrations in the pore-water samples taken on 4 and 11 June 2015 were unexpected and inconsistent with results from the HPFMs, the sampling was repeated on 8 October. The aim of this repeated sampling was to investigate whether diurnal variations in subsurface NO3 and SRP concentrations could explain the discrepancies between MLS and HPFM results. We assumed that the HPFM measurements integrated temporal oscillations, while MLS samples represented the specic concentrations around noon. In order to test this hypothesis, both MLS were sampled twice, the rst time in the early morning before sunrise and again in the early afternoon (around 02:00 p.m.) during the sampling in October. Those samples were analysed for NO3, SRP and

SO24. Due to technical issues, boron could not be measured in October.

Surface water chemistry

Surface water concentrations of SRP and NO3 were monitored in order to compare surface and subsurface water chemistry. Therefore we installed an automated UV absorption sensor for NO3 (ProPS WW, TriOS) at the beginning of the testing reach for the duration of the experiments. The pathway-length of the optical sensor was 10 mm,

measuring at wavelengths 190360 nm with a precision of0.03 mg NO3-N L1 and an accuracy of [notdef]2 %. The measure

ment time step was set to 15 min. SRP, SO24 and B concentrations in the surface water were assessed with grab samples taken simultaneously to the MLS measurements.

The UV sensor was supplemented with a multi-parameter probe YSI 6600 V2/4 (YSI Environmental, Yellow Springs, Ohio) recording the following parameters: pH (precision0.01 units, accuracy [notdef]0.2 units), specic conductivity (pre

cision 0.001 mS cm1, accuracy [notdef]0.5 %), dissolved oxygen

(precision 0.01 mg L1, accuracy [notdef]1 %), temperature (pre

cision 0.01 C, accuracy [notdef]0.15 C) and turbidity (precision

0.1 NTU, accuracy [notdef]2 %).

2.6.4 Estimates of nitrate turnover rates based on HPFM measurements

Estimates for hyporheic removal activity RN for the specic conditions at the study site during the HPFM testing phase were calculated using the morphological and hydrological parameters summarised in Table 2.

The absolute amount of water passing the screened area of the hyporheic zone QHZ (m s1) is the product of the average horizontal vector of the Darcy velocity qx (m s1) measured in the HPFM and the cross-sectional area of the upper 50 cm of the hyporheic zone AHZ (m2). The proportion of water inltrating the hyporheic zone %QHZ (%) was then calculated from the ratio QHZQSW , where QSW (m3 s1) is the average discharge at the study site during the days of measurements, derived from continuous records at the gauche Mahndorf, which were provided by the local authority Landesbetrieb fr Hochwasserschutz und Wasserwirtschaft Sachsen-Anhalt.

The NO3 removal activity of the hyporheic zone RN (%) was calculated from the difference in average surface water concentration CNO3-SW (mg NO3-N L1) and the average concentrations measured with the HPFM CNO3-HZ (mg NO3-

N L1), were CNO3-HZ is the quotient JNqx .

3 Results

3.1 Laboratory experiments

Loading capacity and biofouling

Break-through in the sorbent column experiments occurred after 300 pore volumes (PVs) or 21 days at selected drainage for both NO3 and SRP.

In the biofouling experiment, the NO3 concentration in the draining water gradually decreased again after breakthrough. SRP in the draining water was completely depleted 6 h after the break-through. The calculated amount of retained nutrient in comparison to manufacturer value loading capacities of Purolite A 500MB Plus indicate that the absorbing capacity of the resin in this small column experiment

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 639

Table 2. Selected morphological and hydrological parameters of the testing site for the duration of the testing phase from 411 June 2015. Ranges are indicated for directly measured parameters, the remaining parameters have been calculated from listed means. HZ: hyporheic zone.

Surface water Abbreviation Unit Mean Range

Cross-sectional area ASW m2 3.41

Depth h m 0.565 0.540.61

Width w m 6.03 5.576.29 Mean velocity v m s1 0.097

Discharge QSW m3 s1 0.32 0.300.34 NO

3 concentration CNO3 SW mg NO

3 -N L1 2.86 2.163.26

NO

PO

4 concentration CPO4 SW mg P L1 0.165 0.1110.231 PO

4 mass ux MPO4 SW mg P s1 51

Hyporheic zone upper 50 cm

Assessed depth of HZ hHZ m 0.5 Cross-sectional area of HZ AHZ m2 3.02

was exhausted after 25.5 h (Supplement S1). We attributed the decrease of nutrients in the draining solution after breakthrough to biotic consumption of SRP (limiting nutrient) and NO3. Under the laser scanning microscope growth of biolm could be observed on obviously brown stained Purolite

beads of the columns from the biofouling experiment and to a very low degree on beads from the same column which appeared still clean (Supplement S1). Browning of Purolite beads was not observed on Purolitebeads from the loading experiment (bigger columns, experiment not extended after break-through) but on the top 2 cm of the HPFM R2 after exposure at the study site.

3.2 Field testing

3.2.1 HPFMs and additional measurements

HPFMs

Deployment required approximately 15 min per HPFM and could be conducted by two persons. The water depth during the installation was 40 to 100 cm, depending on the specic location in the stream.

The average horizontal water ow qx and nutrient ux

JN measured in the HPFM during the 7 day eld testing are illustrated in Fig. 3. All ux meter except 5 L showed declining JN and qx with depth. Average horizontal qx was 76 cm d1, ranging from 115 cm d1 in the shallowest layer of 5 L to 20 cm d1 in the deepest layer of AC4. Over the 7 days duration of the experiment, accumulated horizontal ow velocities of qx = 8.4 cm 7 d1 ([notdef]0.02 cm 7 d1,

n = 3) and nutrient uxes of 29.4 mg NO3-N m2 7 d1

([notdef]0.7 mg m2 d1, n = 3) and 36.4 mg SRP m2 7 d1

([notdef]6.3 mg m2 d1, n = 3) were detected in the control

HPFM. Breaking these results down to dial values yields

3 mass ux MNO3 SW mg NO

3 -N s1 896

qx = 1.2 cm d1 ([notdef]0.003 cm d1, n = 3) and nutrient uxes

of 4.2 mg NO3-N m2 d1 ([notdef]0.1 mg m3 d1, n = 3) and

5.2 mg SRP m2 d1 ([notdef]0.9 mg m2 d1, n = 3). Comparing

these uxes to the JN values measured with the other

HPFM, an average 0.3 % of the uncorrected NO3 ux and 5 % of the uncorrected SRP ux were attributed to tracer loss or nutrient accumulation resulting from transport, deployment, retrieval, analytical processing of samples and the background concentrations on the resin.

Vertical Darcy velocity (qy)

Vertical water ow qy in the streambed was predominantly downward from January to October 2015. It was exclusively downward during the HPFM testing phase, ranging from 40 to 55 cm d1. With this, vertical ow qy was slightly lower than average horizontal ow qx. Resulting from the relationship between qy and qx the angle of hyporheic ow (tan = qyqx ) was 32 downwards.

Oxygen proles

We observed strong diel variations in oxygen concentration in the hyporheic zone. During several nights oxygen was nearly depleted (Fig. 4). The minima and maxima oxygen concentrations in the subsurface occurred contemporarily with the respective extremes in the surface water. Interestingly the amplitude in O2 oscillation was higher at 45 cm depths than at 25 cm depths.

Multi-level samplers

In order to facilitate direct comparison, nutrient uxes as measured in the HPFM were converted to ux-averaged concentrations which are the quotient of JN and the respective qx (Fig. 5). Overall, nutrient concentrations in the manually

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017

640 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

Figure 3. Time-integrative measurements for 411 June 2015. Left: horizontal NO

3 -N and SRP-P ux in mg m2 d1 through the resin HPFM R1 (a), R2 (b) and the layered HPFM L5 (c) and L6 (d). Right: corresponding Darcy velocities qx in cm d1 through the activated carbon HPFM AC3 (e) and AC4 (f) and the layered HPFMs 5L (g) and 6L (h).

sampled pore water taken in June 2015 were higher than the average concentration derived from the HPFM. While the expected increase of SRP and decrease of NO3 and water ow with depths was observed in the HPFM, pore water extracted with the MLS showed no change over depth neither for NO3 nor SRP. In the repeated manual pore-water samples taken in

October (Fig. 6) NO3 concentrations were uniformly lower in the early morning than in the afternoon, whereas SRP behaved the other way round. This trend was consistent in both samplers even though the average concentration and distribution over depths differed between the samplers A and B.

On both sampling dates in June (4 and 11 June 2015) neither SO24 nor boron showed a vertical gradient in concentrations in the pore-water samples. SO24 concentrations of 170 mg L1 on 4 June and 190 mg L1 on 11 June were in the same range as surface water concentrations. So too were boron concentrations with 50 to 60 g L1, consistent with the concentrations in the surface water, indicating no or only minor groundwater inuence. Also in October SO24 concentrations in the pore-water samples were in the range of surface water concentrations, slightly declining with depth.

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

Figure 4. Time series of dissolved oxygen concentrations in the surface water (green) and the subsurface water (depth 25 cm, purple; depth 45 cm, orange) at the study site from 411 June 2015.

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 641

Figure 5. Comparison between manually sampled pore water from MLS (red) and HPFM (blue) for NO

3 -N (top) and SRP (bottom).

Each MLS was sampled on 4 and 11 June 2015. Average surface water concentration during the deployment time and the concentrations at the time point of MLS sampling are marked in green.

Surface water chemistry

Temperature, O2 and pH showed the expected diurnal amplitudes whereas specic conductivity and NO3 did not display a distinct diurnal pattern (Table 4).

3.2.2 Estimates of nitrate turnover rates based on HPFM measurements

With an average water ow of QHZ = 2.65 [notdef] 105 m3 s1

through the assessed upper 50 cm of the hyporheic zone and across the 6 m width of the stream, 0.008 % of water transported in the river entered the hyporheic zone (Table 3).

While the average surface water concentration was2.86 mg NO3-N L1, the average concentration in the sub-surface measured with the HPFM was only 1.39 mg NO3-

N L1. Assuming that the difference between surface and subsurface concentration arose from hyporheic consumption of inltrating NO3, the average removal rate RN was 52 %.

Figure 6. Concentrations of NO

3 -N and SRP in time differentiating manually taken pore-water samples from MLS A (bottom) and

MLS B (top) on 8 October 2015. Corresponding surface water concentrations are marked as vertical lines.

For SRP the average surface water concentration from 4 to11 June 2015 was 0.165 mg P L1, the average concentration in the hyporheic zone was 0.11 mg P L1.

4 Discussion

The application of the HPFM for quantitative in situ measurement of horizontal NO3 and SRP uxes through the hyporheic zone is novel. An earlier study on passive ux meter (SBPFM) in river beds (Layton, 2015) only assessed vertical ow of contaminants and is therefore not comparable to the application presented here. In the current work, adaptations were developed, tested and improved. Those include the choice of an appropriate resin, assessment of biolm growth on the instruments and an approach that avoids challenges with contamination of the sorbent with nutrients. The results from the control HPFM showed that the uncertainty in measurement related to handling of the HPFM and processing of samples as conducted in this study is acceptable.Finally, the minimum and maximum deployment time will depend on the Darcy velocity and nutrient concentrations at a study site. Since the values derived from the control incorporate all the processing steps of HPFM and samples,

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017

642 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

Table 3. Summarised parameters of NO

3 transport and removal through the upper 50 cm of the hyporheic zone at the test site for the testing phase from 411 June 2015. Ranges are indicated for directly measured parameters, the remaining parameters have been calculated from listed means.

Abbreviation Unit Mean Range

Water ow through HZ QHZ L s1 0.0265% of river water entering HZ %QHZ % 0.008Horizontal Darcy velocity qx cm d1 76 20116

Average NO

3 concentration in the HZ CNO3 HZ mg NO

3 -N L1 1.39 0.312.86

% NO

3 entering the HZ which is removed RN % 52 Potential NO

3 load entering HZ MHZ theory mg NO

3 -N s1 0.08

NO

The high-nutrient background on the AC required the separation of resin and AC in the HPFMs. We tested two different HPFM designs in this study, of which each inherits designated characteristics being more or less benecial for different specications. The rst approach consists of pairs of two HPFMs where one is used to assess the water ux and the second to capture nutrients, and is preferable if a highly resolved depth prole is needed (a heterogeneous horizontal ux in the vertical direction). Since this approach assumes that local horizontal heterogeneity is negligible in the range of 2030 cm, we recommend this type only for the use in uniform systems such as channelised river reaches. Even in those systems however, small-scale variability in streambed and sediment characteristics can cause spatially heterogeneous ow distributions (Lewandowski et al., 2011; Mendoza-Lera and Mutz, 2013). The second approach with alternating nutrient sorbents and water ux measuring segments is therefore preferable in most other cases as long as a high resolution over the vertical prole is not required. In general, several HPFMs should be grouped together in order to obtain representative results.

Further improvements of the HPFM for nutrient studies in the subsurface of rivers could be achieved by identifying a nutrient-free carrier for the tracers. First, because this would allow measuring nutrient and water ux at the same location

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

3 load measured in HZ MHZ measured mg NO

3 -N s1 0.037

Table 4. Benchmark surface water parameters derived from the continuous sensor records from 411 June and 811 October 2015: Temp: temperature; SpC: specic conductivity; O2: dissolved oxygen.

Temp SpC pH O2 NO

3

C S cm1 mg L1 mg NO

3 -N L1

411 June 2015 mean 17.81 1063 8.42 9.37 2.86

SD 2.57 46 0.27 2.01 0.32 min 13.38 886 7.75 6.13 2.16 max 23.79 1224 8.84 13.12 3.26

811 October 2015 mean 11.22 951 8.21 10.48 2.75

SD 2.75 59 0.10 0.91 0.28 min 6.02 818 7.99 9.09 1.95 max 15.32 1056 8.44 12.44 3.40

they can be regarded as the method detection limit (MDL) (Greenberg et al., 1992). The MDL denes the lower limit for the use of HPFM in cases where nutrient uxes are very low and deployment time cannot be extended. Based on the accumulated values detected in the control, the minimum deployment time can be estimated. In systems with high nutrient concentrations, usually the ow velocity qx will be the limiting factor. In our application the MDL for qx derived from the control was 8.4 cm for the complete deployment time (7 days). If the method inherited uncertainty should not be more than 5 % of the total measurement, the product of duration (in days) and velocity (in cm) should be at least 168 (20 [notdef] 8.4). As an example: if measured qx is

around 200 cm d1, 1 day (24 h) of deployment is sufcient.

The lowest qx detected in our assessment was 21 cm d1 (in HPFM AC4, see Fig. 3f), so that actually a deployment duration of 8 days would have been optimal. The same estimation can additionally be derived for expected nutrient uxes. In systems with low nutrient concentrations, it would be preferable to start estimating the minimum deployment time based on the nutrient uxes. We recommend that a control HPFM is incorporated in each eld application of HPFM in order to determine the specic MDL. The upper limit is given by the loading capacity of the resin or complete displacement of all resident alcohol tracers.

coherently higher or coherently lower than the permeability of the HPFM matrix. Pre-measurements are therefore necessary for the selection of a suitable resin and tracer carrier.We believe that the presented approach and equations are applicable for a wide range of eld conditions. However, for very coarse sediments, a protection of the HPFM with a solid screen might still be preferred. If ne particles are observed to bypass the mesh and enter the HPFM, a ner mesh should be chosen. We did not observe clogging of the mesh or intrusion of particles at our study, though in highly permeable systems with ne particle transport this might have to be considered.

A major advantage of the HPFM method is highlighted by the ndings of the 7 day long eld testing. In June, we found discrepancies between the average concentrations measured in the HPFM and the concentration found using the MLS. From our measurements it is not possible to prove that the HPFM results are correct and the MLS results biased. Nevertheless, the HPFM showed the expected decline in JN with depths, whereas the MLS pore-water concentrations were similar at all depths. This can be related to two reasons. First, we might have sampled surface water which bypassed along the wall of the MLS. The question would then be why that happened in June but not in October; second, we might have sampled the MLS at a time point when the hyporheic zone was inactive in respect to nutrient processing. Considering the high diurnal amplitudes in hyporheic oxygen concentration, we assumed that the discrepancy between HPFM and MLS arose from oscillations in hyporheic nutrient concentrations similar to the oxygen pattern. Microbial consumption of O2 in the sediments can, depending on nutrient concentration in surface water and transfer of these nutrients to the sediments, result in O2 depletion in the subsurface. Especially in nutrient-rich streams the related diurnal oscillations in O2 concentration favour night time denitrication in the hyporheic zone (Christensen et al., 1990; Laursen and Seitzinger, 2004; Harrison et al., 2005;OConnor and Hondzo, 2008; Nimick et al., 2011). The redox conditions in the subsurface may also regulate the mobil-isation/demobilisation of phosphate (Smith et al., 2011). The repeated manual sampling of pore water from MLS in October showed diurnal variations of SRP and NO3 in the subsur-face of the testing reach, supporting the hypothesis that diurnal cycles in benthic metabolism caused temporal variations in hyporheic SRP and NO3 concentrations at our study site.

As the majority of sampling is commonly conducted during daylight hours, night time conditions are under-represented in studies relying on single manual sampling events. Flux average concentrations can deviate by more than 50 % from estimates based on single event sampling, as was illustrated by comparison between our manual samples and the average pore-water concentrations calculated from the HPFM data.

Repeated pore-water sampling at high frequencies can be used to determine diurnal dynamics. However, continuing this over a longer time span is laborious, whereas if only

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 643

within the device and thereby increase spatial resolution, and second, because in a mixed texture of nutrient absorber and tracer carrier the antibacterial nature of the activated carbon would suppress biofouling on the absorbent. We observed substantial biolm growth on the resin in the laboratory and on the top 2 cm of the eld-deployed HPFM R2. The results of the column experiments suggest that biolm growth on the resin porous media did not affect its loading capacity.Further, biolm growth was only visible on columns which were run beyond breakthrough, suggesting that considerable biofouling only started after the loading capacity of the tracer was exhausted. R2 detected higher NO3 uxes in the top layer than the other HPFM. This could be due to contamination of the top layer of this HPFM with surface water (if the HPFM was not introduced sufciently deep into the sediments). The further implication would be that this layer was exposed to much higher water and nutrient inltration, so that the loading capacity was exhausted before the end of the experiment allowing biolm accumulation. At the current state it is unclear to what extent the biolm bound nutrients can be extracted by the procedure used here. Further experiments would also be needed to clarify under which conditions biolm growth can occur and if bacterial uptake, transformation and release of nutrients inuence the concentrations of nutrients inside the HPFM. HPFM segments on which biolm is visible should be interpreted with caution. Finally, identifying a procedure or materials which completely inhibit biofouling will be an important step in the further development of HPFM.

In addition to instrumental adaptations we presented an installation procedure, which allows for smooth deployment with minimal disturbance of the system. Unlike typical well screen deployments where PFMs (Annable et al., 2005; Verreydt et al., 2013) or SBPFMs (Layton, 2015) have been inserted into a screened plastic or steel casing, our technique enabled the direct contact of the HPFM with the surrounding river sediments. Disturbing the natural structure of the sediment, potentially resulting in articial ow paths, is intrinsic to all intrusive techniques, including HPFMs. Still, dispensing of a solid wall improves the integration of HPFMs in the natural system and prevents the generation of preferential ow paths along the wall of the device. Additionally, the HPFMs include a measurement time that is long relative to the duration of the installation, suggesting that the presented method causes lower disturbance compared to other intrusive measurements. While the installation of mini-drive points or heat pulse sensors in sediments coarser than sand may be difcult or even impossible and also proved unfeasible at our eld site, installation of the HPFM with the presented technique was successful. The correction for convergence of owlines into the device or divergence around it is relatively simple and already incorporated in the equation for the ux calculation. Heterogeneous permeability of the hyporheic zone around the HPFM does not distort the correction term as long as the permeability of the surrounding is

644 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

few single time-specic snap shot samplings are conducted, the results may not realistically represent the overall conditions at the target site. Our comparison between MLS and HPFM reinforce the need for long-term recording of nutrient transport through the hyporheic zone. In general, most of our knowledge on hyporheic nutrient dynamics is based on measured surface water dynamics and models which project these dynamics on hyporheic processing. Theoretically, we could measure nutrient uxes in the hyporheic zone and estimate whole-stream uptake rates from these measurements.However, the substantially higher effort to obtain subsurface data is not justied in most cases. As long as the overall in-stream retention is the focus, surface water monitoring will remain the method of choice. Innovative tracer experiments may even allow quantifying hyporheic exchange in streams.Haggerty et al. (2009) proposed a smart tracer approach, where the injected substance resazurin converts irreversibly to resofurin under metabolic activity. While a promising tool for detecting metabolic activity at the sedimentwater interface in streams, rst, uncertainties about sorption and transformation characteristics of these tracers remain (Lemke et al., 2014) and second, those methods give no evidence about nutrient transport to those reactive sites.

Thus, whenever the nutrient processing function of the hyporheic zone and its quantitative contribution to stream nutrient retention is of interest, for example in the evaluation of restoration measures including a rehabilitation of the river bed, direct measurements of hyporheic uxes are indispensable. The HPFMs are a valuable approach that can be efciently used to characterise and quantify nutrient dynamics in a sediment system. We consider that a combination of HPFM, MLS and concurrent measurements of pore water oxygen concentrations, as presented in this study, provide a practical set-up to interpret hyporheic nutrient dynamics.

Like solute concentrations and water ow patterns, the vertical extension of the hyporheic zone varies in time and space and between different rivers and reaches. Our set-up assessed exclusively the upper 50 cm of the hyporheic zone. We found continuously decreasing NO3 concentrations with depths, suggesting that this entire area (and potentially deeper) of the subsurface contained active sites for nitrate removal. While it was stated that denitrication is limited to the upper few cm of the hyporheic zone close to the sedimentwater interface (Hill et al., 1998; Harvey et al., 2013), our results are in accordance to ndings by Zarnetske et al. (2011b) and Kessler et al. (2012) who also report extended active hyporheic zones. Conducting collateral tracer tests, as suggested for example by Abbott et al. (2016), could deliver further evidence and characterise distinct ow paths. Nevertheless, since vertical water movement was overall downward and the lowest concentrations of NO3 were observed in the deepest segments of the HPFM, it is very likely that the hyporheic zone at our study site extends deeper than the 50 cm evaluated. The length of an HPFM can easily be increased, depending on the individual site conditions.

Considering the high spatial heterogeneity of the hyporheic zone, a larger number of HPFM would be needed to derive reliable and statistically supportable rates of hyporheic nutrient dynamics. The following example aims to display further possibilities of interpreting HPFM measurements. At our study site, the hyporheic removal potential RN of more than 50 % of inltrating NO3 and 30 % of SRP suggests an active hyporheus. Evaluation of the effect of hyporheic removal activity on overall NO3 removal in the stream or the normalisation of hyporheic uptake to a benthic area requires a ow path length. In the presented example, this length refers to the horizontal vector of the distance the water travels in the subsurface before inltrating the HPFM. The horizontal vector can be derived from the residence time of water and solutes in the hyporheic zone HZ and the horizontal Darcy velocity qx. Assuming a downward ow direction, HZ could be inferred from the vertical Darcy velocity qy as assessed from the temperature proling and the hyporheic zone depths of 50 cm. Thereafter, HZ conceptually corresponds to the time the water travels through the hyporheic zone before exiting to groundwater and sHZ to the horizontal vector of the ow paths. The nitrate uptake rate UNO3-HZ (mg NO3-N m2 d1) is then the difference between the theoretically transported NO3 mass

MNO3-HZtheor, which is the product of QHZ and CNO3-SW and the measured mass ux MNO3-HZreal. During the testing phase UNO3-HZ was calculated as 693 mg NO3-N m2 d1.

The same procedure yields a removal (uptake or adsorption) rate for SRP of UPO4-HZ = 24 mg PO4 m2 d1. Calculating

UNO3-HZ in the same way for each single depth assessed with the HPFM can deliver additional information about vertical gradients on nutrient processing rates and help to identify the most active depth in hyporheic zone. UNO3-HZi of a particular layer in the hyporheic zone can be derived by the differences in uptake rate between the regarded layer and the overlying layer. For instance the removal rates attributed to the different layers of HPFM L6 would be UNO3-HZ15 =

567 mg NO3-N m2 d1 in the shallow layer (0 to 15 cm depths), UNO3-HZ30 = 174 mg NO3-N m2 d1 in the layer

from 15 to 30 cm depths and UNO3-HZ45 = 256 mg NO3-N m2 d1 in the deepest layer from 30 to 45 cm depths.

From this example one could conclude that the shallowest sediments are the most efcient ones in terms of nitrate removal. While removal activity is rst declining with depths it later increases again. This nding is consistent with the higher amplitudes of oxygen concentration in 45 cm depths compared to 25 cm depths, also suggesting higher biotic activity at the deepest layer. Potential reasons for this pattern could be decreasing NO3 penetration with depth (lower up-take at the middle layer than the shallowest one) which is in the deepest parts counter-balanced by increased residence time and stronger reducing conditions.

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 645

5 Conclusion and outlook

The role of the hyporheic zone as a hotspot for in-stream nutrient cycling is indisputable (Mulholland et al., 1997; Fellows et al., 2001; Fischer et al., 2005; Rode et al., 2015).Quantitative and qualitative knowledge about the inuence of mass transfer on hyporheic nutrient removal is crucial to manage streams and river, especially in the light of increasing worldwide morphological alterations (Borchardt and Pusch, 2009), eutrophication (Ingendahl et al., 2009) and sediment loading (Hartwig and Borchardt, 2015). Despite decades of research on hyporheic nutrient cycling, robust quantitative data on nutrient uxes through the hyporheic zone are limited, which is mainly due to methodological constraints in measuring nutrient concentrations and water ux in the sub-surface of streams (OConnor et al., 2010; Boano et al., 2014;Gonzalez-Pinzon et al., 2015). Passive ux meters have the potential to ll the gap in measured quantitative nutrient uxes to the reactive sites in the sediments of rivers. To date, HPFMs are virtually the only method which can simultaneously capture nutrient and water ux through hyporheic zone within the same device and at the same spatial location. The eld testing of several devices proved the general applicability of passive ux meters for quantifying NO3 and PO4 ux to reactive sites in the hyporheic zone. The hyporheic ux rates of nutrients and nitrate uptake rates measured in an agricultural 3rd-order stream were generally in agreement with rates reported in the literature. Our results clearly highlight the advantages of HPFM compared to commonly used methods (i.e. grab sampling of pore water and separate measurements of hyporheic exchange and Darcy velocities), rst of all the capacity to integrate over longer time periods.

Quantifying nutrient ux to the potentially reactive sites in the hyporheic zone is an essential step to further improve our process-based knowledge on hyporheic nutrient cycling.In the future, long-term measurements of nutrient uxes as obtained from HPFM can feed into and advance the transport part of nutrient cycling models.

We anticipate further improvement and increased use of passive ux meter approaches in order to advance conceptual models of nutrient cycling in the hyporheic zone. We demonstrated modications which extended PFM application from groundwater to hyporheic zones. Current limitations related to the potential bias of results due to biolm growth on sorbents require further analysis for the identication of more suitable sorbents. While we focused on nutrients, PFMs may also be used for a wide range of other substances like contaminants or trace elements.

Being labour efcient and attractive with respect to relatively low costs, numerous HPFMs can be efciently used to cover larger areas and assess the degree of local heterogeneity. Further, neither advanced technology, maintenance, or power supply are needed which can be extremely advantageous for the use in remote areas or study sites without infrastructure.

6 Data availability

All relevant data are either incorporated in gures/tables in the article or provided in the Supplement.

The Supplement related to this article is available online at http://dx.doi.org/10.5194/bg-14-631-2017-supplement

Web End =doi:10.5194/bg-14-631-2017-supplement .

Competing interests. The authors declare that they have no conict of interest.

Acknowledgements. We thank Uwe Kiwel for his technical support during the eld work and Andrea Hoff and Christina Hoffmeister from the analytical department of the UFZ for their assistance in the laboratory experiments. We are also grateful for fruitful discussions with James Jawitz, Andreas Musolff, Christian Schmidt and Nico Trauth.

The article processing charges for this open-access publication were covered by a ResearchCentre of the Helmholtz Association.

Edited by: T. J. BattinReviewed by: R. Gonzlez-Pinzn, J. Rozemeijer, J. Lewandowski, and one anonymous referee

References

Abbott, B. W., Baranov, V., Mendoza-Lera, C., Nikolakopoulou,M., Harjung, A., Kolbe, T., Balasubramanian, M. N., Vaessen,T. N., Ciocca, F., Campeau, A., Wallin, M. B., Romeijn, P., Antonelli, M., Gonalves, J., Datry, T., Laverman, A. M., De Dreuzy, J.-R., Hannah, D. M., Krause, S., Oldham, C., and Pinay, G.: Using multi-tracer inference to move beyond single-catchment ecohydrology, Earth-Sci. Rev., 160, 1942, 2016.Alexander, R. B., Bhlke, J. K., Boyer, E. W., David, M. B., Harvey, J. W., Mulholland, P. J., Seitzinger, S. P., Tobias, C. R., Tonitto, C., and Wollheim, W. M.: Dynamic modeling of nitrogen losses in river networks unravels the coupled effects of hydrological and biogeochemical processes, Biogeochemistry, 93, 91116, doi:http://dx.doi.org/10.1007/s10533-008-9274-8

Web End =10.1007/s10533-008-9274-8 http://dx.doi.org/10.1007/s10533-008-9274-8

Web End = , 2009.

Angermann, L., Krause, S., and Lewandowski, J.: Application of heat pulse injections for investigating shallow hyporheic ow in a lowland river, Water Resour. Res., 48, W00P02, doi:http://dx.doi.org/10.1029/2012WR012564

Web End =10.1029/2012WR012564 http://dx.doi.org/10.1029/2012WR012564

Web End = , 2012.

Annable, M. D., Hateld, K., Cho, J., Klammler, H., Parker, B. L., Cherry, J. A., and Rao, P. S. C.: Field-Scale Evaluation of the Passive Flux Meter for Simultaneous Measurement of Groundwater and Contaminant Fluxes, Environ. Sci. Technol., 39, 71947201, doi:http://dx.doi.org/10.1021/es050074g

Web End =10.1021/es050074g http://dx.doi.org/10.1021/es050074g

Web End = , 2005.

Artioli, Y., Friedrich, J., Gilbert, A. J., McQuatters-Gollop, A., Mee,L. D., Vermaat, J. E., Wulff, F., Humborg, C., Palmeri, L., and

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017

646 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

Pollehne, F.: Nutrient budgets for European seas: A measure of the effectiveness of nutrient reduction policies, Mar. Pollut. Bull., 56, 16091617, doi:http://dx.doi.org/10.1016/j.marpolbul.2008.05.027

Web End =10.1016/j.marpolbul.2008.05.027 http://dx.doi.org/10.1016/j.marpolbul.2008.05.027

Web End = , 2008.Basu, N. B., Rao, P. S. C., Thompson, S. E., Loukinova, N. V., Donner, S. D., Ye, S., and Sivapalan, M.: Spatiotemporal averaging of in-stream solute removal dynamics, Water Resour. Res., 47, W00J06, doi:http://dx.doi.org/10.1029/2010WR010196

Web End =10.1029/2010WR010196 http://dx.doi.org/10.1029/2010WR010196

Web End = , 2011.

Bernot, M. J. and Dodds, W. K.: Nitrogen retention, removal and saturation in lotic ecosystems, Ecosystems, 8, 442453, doi:http://dx.doi.org/10.1007/s10021-003-0143-y

Web End =10.1007/s10021-003-0143-y http://dx.doi.org/10.1007/s10021-003-0143-y

Web End = , 2005.

Binley, A., Ullah, S., Heathwaite, A.L., Heppell, C., Byrne, P., Landsdown, K., Timmer, M., and Zhang, H.: Revealing the spatial variability of water uxes at the groundwater-surface water interface, Water Resour. Res., 49, 39783992, 2013.

Boano, F., Harvey, J. W., Marion, A., Packman, A. I., Revelli, R.,

Ridol, L., and Worman, A.: Hyporheic ow and transport processes: Mechanisms, models and biogeochemical implications, Rev. Geophys., 52, 603679, doi:http://dx.doi.org/10.1002/2012rg000417

Web End =10.1002/2012rg000417 http://dx.doi.org/10.1002/2012rg000417

Web End = , 2014.Bhlke, J. K., Antweiler, R. C., Harvey, J. W., Laursen, A. E., Smith,L. K., Smith, R. L., and Voytek, M. A.: Multi-scale measurements and modeling of denitrication in streams with varying ow and nitrate concentration in the upper Mississippi River basin, USA, Biochemistry, 93, 117141, doi:http://dx.doi.org/10.1007/s10533-008-9282-8

Web End =10.1007/s10533- http://dx.doi.org/10.1007/s10533-008-9282-8

Web End =008-9282-8 , 2009.

Borchardt, D. and Pusch, M. T.: An integrative, interdisciplinary research approach for the identication of patterns, processes and bottleneck functions of the hyporheic zone of running waters, Adv. Limnol., 61, 2009.

Boyer, E. W., Alexander, R. B., Parton, W. J., Li, C., Butterbach-

Bahl, K., Donner, S. D., Skaggs, R. W., and Grosso, S. J. D.: Modeling denitrication in terrestrial and aquatic ecosystems at regional scales, Ecol. Appl., 16, 21232142, 2006.Brookshire, E. N. J., Valett, H. M., and Gerber, S.: Maintenance of terrestrial nutrient loss signatures during in-stream transport, Ecology, 90, 293299, doi:http://dx.doi.org/10.1890/08-0949.1

Web End =10.1890/08-0949.1 http://dx.doi.org/10.1890/08-0949.1

Web End = , 2009.

Cho, J., Annable, M. D., Jawitz, J. W., and Hateld, K.: Passive ux meter measurement of water and nutrient ux in saturated porous media: Bench-scale laboratory tests, J. Environ. Qual., 36, 1266 1272, doi:http://dx.doi.org/10.2134/jeq2006.0370

Web End =10.2134/jeq2006.0370 http://dx.doi.org/10.2134/jeq2006.0370

Web End = , 2007.

Christensen, P. B., Nielsen, L. P., Sorensen, J., and Revsbech, N. P.: Denitrication in nitrate-rich streams dirunal and seasonal variation related to benthic oxygen metabolism, Limnol. Oceanogr., 35, 640651, 1990.

Clark, C. J., Hateld, K., Annable, M. D., Gupta, P., and Chirenje,T.: Estimation of arsenic contamination in groundwater by the Passive ux meter, Environmental Forensics, 6, 7782, doi:http://dx.doi.org/10.1080/15275920590913930

Web End =10.1080/15275920590913930 http://dx.doi.org/10.1080/15275920590913930

Web End = , 2005.

Cook, P. L. M., Wenzhoefer, F., Rysgaard, S., Galaktionov, O. S.,

Meysman, F. J. R., Eyre, B. D., Cornwell, J., Huettel, M., and Glud, R. N.: Quantication of denitrication in permeable sediments: Insights from a two-dimensional simulation analysis and experimental data, Limnol. Oceanogr.-Meth., 4, 294307, 2006.Cooke, J. G. and White, R. E.: Spatial distribution of denitrifying activity in a stream draining an agricultural catchment, Freshwater Biol., 18, 509519, 1987.

Covino, T., McGlynn, B., and Baker, M.: Separating physical and biological nutrient retention and quantifying up-take kinetics from ambient to saturation in successive moun-

tain stream reaches, J. Geophys. Res.-Biogeo., 115, G04010, doi:http://dx.doi.org/10.1029/2009jg001263

Web End =10.1029/2009jg001263 http://dx.doi.org/10.1029/2009jg001263

Web End = , 2010.

Duff, J. H. and Triska, F. J.: Denitrication in sediments from the hyporheic zone adjacent to a small forested stream, Can. J. Fish.Aquat. Sci., 47, 11401147, 1990.

Duff, J. H., Murphy, F., Fuller, C. C., and Triska, F. J.: A mini drive-point sampler for measuring pore water solute concentrations in the hyporheic zone of sand-bottom streams, Limnol. Oceanogr., 43, 13781383, 1998Ensign, S. H. and Doyle, M. W.: Nutrient spiraling in streams and river networks, J. Geophys. Res.-Biogeo., 111, G04009, doi:http://dx.doi.org/10.1029/2005jg000114

Web End =10.1029/2005jg000114 http://dx.doi.org/10.1029/2005jg000114

Web End = , 2006.

Fellows, C. S., Valett, H. M., and Dahm, C. N.: Whole-stream metabolism in two mountain streams: Contribution of the hyporheic zone, Limnol. Oceanogr., 46, 523531, 2001.

Fetter, C. W.: Applied Hydrogeology, 4th Edn., New Jersey: Prentice Hall, 2001.

Fleckenstein, J. H., Krause, S., Hannah, D. M., and Boano, F.:

Groundwater-surface water interactions: New methods and models to improve understanding of processes and dynamics, Adv.Water Resour., 33, 12911295, 2010.

Findlay, S. E. G., Mulholland, P. J., Hamilton, S. K., Tank, J. L.,

Bernot, M. J., Burgin, A. J., Crenshaw, C. L., Dodds, W. K., Grimm, N. B., McDowell, W. H., Potter, J. D., and Sobota, D.J.: Cross-stream comparison of substrate-specic denitrication potential, Biogeochemistry, 104, 381392, doi:http://dx.doi.org/10.1007/s10533-010-9512-8

Web End =10.1007/s10533- http://dx.doi.org/10.1007/s10533-010-9512-8

Web End =010-9512-8 , 2011.

Fischer, H., Kloep, F., Wilzcek, S., and Pusch, M. T.: A rivers liver

microbial processes within the hyporheic zone of a large lowland river, Biogeochemistry, 76, 349371, doi:http://dx.doi.org/10.1007/s10533-005-6896-y

Web End =10.1007/s10533- http://dx.doi.org/10.1007/s10533-005-6896-y

Web End =005-6896-y , 2005.

Fischer, J., Borchardt, D., Ingendahl, D., Ibisch, R., Saenger, N.,

Wawra, B., and Lenk, M.: Vertical gradients of nutrients in the hyporheic zone of the River Lahn (Germany): Relevance of surface versus hyporheic conversion processes, Adv. Limnol., 01/2009, 105118, 2009.

Galloway, J. N., Aber, J. D., Erisman, J. W., Seitzinger, S. P.,

Howarth, R. W., Cowling, E. B., and Cosby, B. J.: The nitrogen cascade, Bioscience, 53, 341356, 2003.

Garcia-Ruiz, R., Pattinson, S. N., and Whitton, B. A.: Kinetic parameters of denitrication in a river continuum, Appl. Environ.Microbiol., 64, 25332538, 1998a.

Gonzalez-Pinzon, R., Ward, A. S., Hatch, C. E., Wlostowski, A. N.,

Singha, K., Gooseff, M. N., Haggerty, R., Harvey, J. W., Cirpka,O. A., and Brock, J. T.: A eld comparison of multiple techniques to quantify groundwater-surface-water interactions, Freshwater Sci., 34, 139160, doi:http://dx.doi.org/10.1086/679738

Web End =10.1086/679738 http://dx.doi.org/10.1086/679738

Web End = , 2015.

Grant, S. B., Stolzenbach, K., Azizian, M., Stewardson, M. J.,

Boano, F., and Bardini, L.: First-order contaminant removal in the hyporheic zone of streams: Physical insights from a simple analytical model, Environ. Sci. Technol., 48, 1136911378, doi:http://dx.doi.org/10.1021/es501694k

Web End =10.1021/es501694k http://dx.doi.org/10.1021/es501694k

Web End = , 2014.

Greenberg, A. E., Clesceri, L. S., and Eaton, A. D.: Standard Methods for the Examination of Water and Wastewater, American Public Health Association, 18th Edn., Washington, D.C., 1992.Haggerty, R., Marti, E., Argerich, A., Von Schiller, D., and Grimm,N. B.: Resazurin as a smart tracer for quantifying metabolically active transient storage in stream ecosystems, J. Geophys. Res., 114, G03014, doi:http://dx.doi.org/10.1029/2008JG000942

Web End =10.1029/2008JG000942 http://dx.doi.org/10.1029/2008JG000942

Web End = , 2009.

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 647

Harrison, J., Matson, P. A., and Fendorf, S. E.: Effects of a diel oxygen cycle on nitrogen transformations and greenhouse gas emissions in a eutrophied subtropical stream, Aquat. Sci., 67, 308 315, 2005.

Hartwig, M. and Borchardt, D.: Alteration of key hyporheic functions through biological and physical clogging along a nutrient and ne-sediment gradient, Ecohydrology, 8, 961975, doi:http://dx.doi.org/10.1002/eco.1571

Web End =10.1002/eco.1571 http://dx.doi.org/10.1002/eco.1571

Web End = , 2015.

Harvey, J. W., Bhlke, J. K., Voytek, M. A., Scott, D., and Tobias,C. R.: Hyporheic zone denitrication: Controls on effective reaction depth and contribution to whole-stream mass balance, Water Resour. Res., 49, 62986316, doi:http://dx.doi.org/10.1002/wrcr.20492

Web End =10.1002/wrcr.20492 http://dx.doi.org/10.1002/wrcr.20492

Web End = , 2013.Hateld, K., Annable, M., Cho, J. H., Rao, P. S. C., and Klammler,H.: A direct passive method for measuring water and contaminant uxes in porous media, J. Contam. Hydrol., 75, 155181, doi:http://dx.doi.org/10.1016/j.jconhyd.2004.06.005

Web End =10.1016/j.jconhyd.2004.06.005 http://dx.doi.org/10.1016/j.jconhyd.2004.06.005

Web End = , 2004.

Hensley, R. T., Cohen, M. J., and Korhnak, L. V.: Inferring nitrogen removal in large rivers from high-resolution longitudinal proling, Limnol. Oceanogr., 59, 11521170, doi:http://dx.doi.org/10.4319/lo.2014.59.4.1152

Web End =10.4319/lo.2014.59.4.1152 http://dx.doi.org/10.4319/lo.2014.59.4.1152

Web End = , 2014.

Hesslein, R. H.: An in situ sampler for close interval pore water studies, Limnol. Oceanogr., 21, 912914, 1976.

Hill, A. R., Labadia, C. F., and Sanmugadas, K.: Hyporheic zone hydrology and nitrogen dynamics in relation to the streambed topography of a N-rich stream, Biogeochemistry, 42, 285310, 1998.

Hochwasservorhersagezentrale: available at: http://www.hochwasservorhersage.sachsen-anhalt.de/wiskiwebpublic/stat_1024005777.htm

Web End =http://www. http://www.hochwasservorhersage.sachsen-anhalt.de/wiskiwebpublic/stat_1024005777.htm

Web End =hochwasservorhersage.sachsen-anhalt.de/wiskiwebpublic/ http://www.hochwasservorhersage.sachsen-anhalt.de/wiskiwebpublic/stat_1024005777.htm

Web End =stat_1024005777.htm (last access: 12 May 2016), 2015/2016.Ingendahl, D., Haseborg, E. T., Van der Most, O., and Werner, D.:

Inuence of interstitial ow velocity on hyporheic oxygen consumption and nitrate production, Adv. Limnol., 61, 119137, 2009.

Kalbus, E., Reinstorf, F., and Schirmer, M.: Measuring methods for groundwater surface water interactions: a review, Hydrol. Earth Syst. Sci., 10, 873887, doi:http://dx.doi.org/10.5194/hess-10-873-2006

Web End =10.5194/hess-10-873-2006 http://dx.doi.org/10.5194/hess-10-873-2006

Web End = , 2006.Kamjunke, N., Bttner, O., Jager, C. G., Marcus, H., von Tmpling,W., Halbedel, S., Norf, H., Brauns, M., Baborowski, M., Wild,R., Borchardt, D., and Weitere, M.: Biogeochemical patterns in a river network along a land use gradient, Environmental monitoring and assessment, 185, 92219236, doi:http://dx.doi.org/10.1007/s10661-013-3247-7

Web End =10.1007/s10661-013- http://dx.doi.org/10.1007/s10661-013-3247-7

Web End =3247-7 , 2013.

Keery, J., Binley, A., Crook, N., and Smith, J. W. N.: Temporal and spatial variability of groundwatersurface water uxes: Development and application of an analytical method using temperature time series, J. Hydrol., 336, 116, 2007.

Kessler, A. J., Glud, R. N., Cardenas, M. B., Larsen, M., Bourke, M. F., and Cook, P. L. M.: Quantifying denitrication in rippled permeable sands through combined ume experiments and modeling, Limnol. Oceanogr., 57, 12171232, doi:http://dx.doi.org/10.4319/lo.2012.57.4.1217

Web End =10.4319/lo.2012.57.4.1217 http://dx.doi.org/10.4319/lo.2012.57.4.1217

Web End = , 2012.

Klammler, H., Hateld, K., and Annable, M. D.: Concepts for measuring horizontal groundwater ow directions using the passive ux meter, Adv. Water Resour., 30, 984997, 2007.

Kunz, J. V., Hensley, R. T., Brase, L., Borchardt, D., and Rode, M.: High frequency measurements of reach scale nitrogen uptake in a 4th order river with contrasting hydromorphology and variable water chemistry (Weie Elster, Germany), Water Resour. Res., 53, doi:http://dx.doi.org/10.1002/2016WR019355

Web End =10.1002/2016WR019355 http://dx.doi.org/10.1002/2016WR019355

Web End = , online rst, 2017.

Krom, M. D., Davison, P., Zhang, H., and Davison, W.: High-resolution pore-water sampling with a gel sampler, Limnol.Oceanogr., 39, 19671972, 1994.

Landesbetrieb fr Hochwasserschutz und Wasserwirtschaft

Sachsen-Anhalt: Elbegebiet, Teil 1 Von der Grenze zur CR bis zur Havelmndung, Magdeburg, 2009.

Lansdown, K., Trimmer, M., Heppell, C. M., Sgouridis, F.,

Ullah, S., Heathwaite, A. L., Binley, A., and Zhang, H.: Characterization of the key pathways of dissimilatory nitrate reduction and their response to complex organic substrates in hyporheic sediments, Limnol. Oceanogr., 57, 387400, doi:http://dx.doi.org/10.4319/lo.2012.57.2.0387

Web End =10.4319/lo.2012.57.2.0387 http://dx.doi.org/10.4319/lo.2012.57.2.0387

Web End = , 2012.

Laursen, A. E. and Seitzinger, S. P.: Measurement of denitrication in rivers: An integrated, whole reach approach, Hydrobiologia, 485, 6781, doi:http://dx.doi.org/10.1023/a:1021398431995

Web End =10.1023/a:1021398431995 http://dx.doi.org/10.1023/a:1021398431995

Web End = , 2002.

Laursen, A. E. and Seitzinger, S. P.: Diurnal patterns of denitrication, oxygen consumption and nitrous oxide production in rivers measured at the whole-reach scale, Freshwater Biol., 49, 1448 1458, doi:http://dx.doi.org/10.1111/j.1365-2427.2004.01280.x

Web End =10.1111/j.1365-2427.2004.01280.x http://dx.doi.org/10.1111/j.1365-2427.2004.01280.x

Web End = , 2004.

Layton, L.: Development of a passive sensor for measuring water and contaminant mass ux in lake sediments and streambeds, Dissertation, University of Florida, 153 pp., 2015.

Lemke, D., Gonzlez-Pinzn, R., Liao, Z., Whling, T., Osenbrck, K., Haggerty, R., and Cirpka, O. A.: Sorption and transformation of the reactive tracers resazurin and resorun in natural river sediments, Hydrol. Earth Syst. Sci., 18, 31513163, doi:http://dx.doi.org/10.5194/hess-18-3151-2014

Web End =10.5194/hess-18-3151-2014 http://dx.doi.org/10.5194/hess-18-3151-2014

Web End = , 2014.

Lewandowski, J., Angermann, L., Ntzmann, G., and Fleckenstein, J. H.: A heat pulses technique for the determination of small-scale ow directions and ow velocities in the streambed pof sand-bed streams, Hydrol. Process., 25, 3244 3255, doi:http://dx.doi.org/10.1002/hyp.8062

Web End =10.1002/hyp.8062 http://dx.doi.org/10.1002/hyp.8062

Web End = , 2011.

McKnight, D. M., Runkel, R. L., Tate, C. M., Duff, J. H., and Moor-head, D. L.: Inorganic N and P dynamics of Antarctic glacial meltwater streams as controlled by hyporheic exchange and benthic autotrophic communities, J. N. Am. Benthol. Soc., 23, 171 188, 2004.

Mendoza-Lera, C. and Mutz, M.: Microbial activity and sediment disturbance modulate the vertical water ux in sandy sediments, Freshwater Sci., 32, 2638, 2013.

Mortensen, J. G., Gonzlez-Pinzn, R., Dahm, C. N., Wang, J.,

Zeglin, L. H., and Van Horn, D. J.: Advancing the Food-Energy

Water Nexus: Closing Nutrient Loops in Arid River Corridors, Environ. Sci. Technol., 50, 84858496, 2016.

Munz, M., Oswald, S. E., and Schmidt, C.: Analysis of riverbed temperatures to determine the geometry of subsurface water ow around in-stream geomorphological structures, J. Hydrol., 539, 7487, 2016.

Mulholland, P. J., Marzolf, E. R., Webster, J. R., Hart,D. R., and Hendricks, S. P.: Evidence that hyporheic zones increase heterotrophic metabolism and phosphorus up-take in forest streams, Limnol. Oceanogr., 42, 443451, doi:http://dx.doi.org/10.4319/lo.1997.42.3.0443

Web End =10.4319/lo.1997.42.3.0443 http://dx.doi.org/10.4319/lo.1997.42.3.0443

Web End = , 1997.

Nimick, D. A., Gammons, C. H., and Parker, S. R.: Diel biogeo-chemical processes and their effect on the aqueous chemistry of streams: A review, Chem. Geol., 283, 317, 2011.

OConnor, B. and Hondzo, M.: Enhancement and Inhibiton of Denitrication by Fluid-Flow and Dissolved Oxygen Flux to Stream Sediments, Environ. Sci. Technol., 42, 119125, 2008.

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017

648 J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM

OConnor, B., Hondzo, M., and Harvey, J. W.: Predictive modeling of transient storage and nutrient uptake: Implications for stream restoration, J. Hydraul. Eng.-ASCE, 136, 10181032, 2010.

Patsch, J. and Radach, G.: Long-term simulation of the eutrophication of the North Sea: temporal development of nutrients, chlorophyll and primary production in comparison to observations, J.Sea Res., 38, 275310, 1997.

Pellerin, B. A., Downing, B. D., Kendall, C., Dahlgren, R. A., Kraus, T. E. C., Saraceno, J., Spencer, R. G. M., and Bergamaschi, B. A.: Assessing the sources and magnitude of diurnal nitrate variability in the San Joaquin River (California) with an in situ optical nitrate sensor and dual nitrate isotopes, Freshwater Biol., 54, 376387, 2009.

Rode, M., Hartwig, M., Wagenstein, D., Kebede, T., and Borchardt,D.: The importance of hyporheic zone processes on ecological functioning and solute transport of streams and rivers, in: River Basin Ecohydrology, Springer Science&Business Media, Dordrecht, 2015.

Rode, M., Halbedel, S., Anis, M. R., Borchardt, D., and Weitere, M.: Continuous in-stream assimilatory nitrate uptake from high frequency sensor measurements, Environ. Sci. Technol., 50, 5685 5694, 2016a.

Rode, M., Wade, A. J., Cohen, M. J., Hensley, R. T., Bowes, M.J., Kirchner, J. W., Arhonditsis, G. B., Jordan, P., Kronvang, B., Halliday, S. J., Skefngton, R. A., Rozemeijer, J. C., Aubert, A.H., Rinke, K., and Jomaa, S.: Sensors in the stream: the high-frequency wave of the present, Environ. Sci. Technol. 50, 10297 10307, 2016b.

Ruehl, C. R., Fisher, A. T., Los Huertos, M., Wankel, S. D., Wheat,C. G., Kendall, C., Hatch, C. E., and Shennan, C.: Nitrate dynamics within the Pajaro River, a nutrient-rich, losing stream, J.N. Am. Benthol. Soc., 26, 191206, 2007.

Runkel, R. L.: Toward a transport-based analysis of nutrient spiraling and uptake in streams, Limnol. Oceanogr.-Meth., 5, 5062, 2007.

Saenger, N. and Zanke, U. C. E.: A depth-oriented view of hydraulic exchange patterns between surface water and the hyporheic zone: analysis of eld experiments at the River Lahn, Germany, Adv.Limnol., 61, 927, 2009.

Schmidt, C., Buettner, O., Musolff, A., and Fleckenstein, J.H.: A method for automated, daily, temperature-based vertical streambed water-uxes, Fundamental and Applied Limnology, 184, 173181, doi:http://dx.doi.org/10.1127/1863-9135/2014/0548

Web End =10.1127/1863-9135/2014/0548 http://dx.doi.org/10.1127/1863-9135/2014/0548

Web End = , 2014.Seitzinger, S., Harrison, J. A., Bohlke, J. K., Bouwman, A. F.,

Lowrance, R., Peterson, B., Tobias, C., and Van Drecht, G.: Denitrication across landscapes and waterscapes: A synthesis, Ecol.Appl., 16, 20642090, 2006.

Seitzinger, S. P., Styles, R. V., Boyer, E. W., Alexander, R. B., Billen, G., Howarth, R. W., Mayer, B., and Van Breemen, N.: Nitrogen retention in rivers: model development and application to watersheds in the northeastern USA, Biogeochemistry, 57, 199 237, doi:http://dx.doi.org/10.1023/a:1015745629794

Web End =10.1023/a:1015745629794 http://dx.doi.org/10.1023/a:1015745629794

Web End = , 2002.

Skogen, M. D., Eilola, K., Hansen, J. L. S., Meier, H. E. M., Molchanov, M. S., and Ryabchenko, V. A.: Eutrophication status of the North Sea, Skagerrak, Kattegat and the Baltic Sea in present and future climates: A model study, J. Marine Syst., 132, 174184, 2014.

Smith, V. H., Joye, S. B., and Howarth, R. W.: Eutrophication of freshwater and marine ecosystems, Limnol. Oceanogr.-Meth., 51, 351355, doi:http://dx.doi.org/10.4319/lo.2006.51.1_part_2.0351

Web End =10.4319/lo.2006.51.1_part_2.0351 http://dx.doi.org/10.4319/lo.2006.51.1_part_2.0351

Web End = , 2006.

Smith, L., Watzin, M. C., and Druschel, G.: Relating sediment phosphorus mobility to seasonal and diel redox uctuations at the sedimentwater interface in a eutrophic freshwater lake, Limnol. Oceanogr., 56, 22512264, doi:http://dx.doi.org/10.4319/lo.2011.56.6.2251

Web End =10.4319/lo.2011.56.6.2251 http://dx.doi.org/10.4319/lo.2011.56.6.2251

Web End = , 2011.

Stewart, R. J., Wollheim, W. M., Gooseff, M. N., Briggs, M. A., Jacobs, J. M., Peterson, B. J., and Hopkinson, C. S.: Separation of river networkscale nitrogen removal among the main channel and two transient storage compartments, Water Resour. Res., 47, W00J10, doi:http://dx.doi.org/10.1029/2010WR009896

Web End =10.1029/2010WR009896 http://dx.doi.org/10.1029/2010WR009896

Web End = , 2011.

Strack, O. D. L. and Haitjema, H. M.: Modeling double aquifer ow using a comprehensive potential and distributed singularities: 2. Solution for inhomogeneous permeabilities, Water Resour. Res., 17, 15511560, 1981.

Teasdale, P. R., Batley, G. E., Apte, S. C., and Webster, I. T.: Pore water sampling with sediment peepers, Trend. Anal. Chem., 14, 250256, 1995.

Trauth, N., Schmidt, C., Vieweg, M., Maier, U., and Fleckenstein, J. H.: Hyporheic transport and biogeochemical reactions in pool-rife systems under varying ambient groundwater ow conditions, J. Geophys. Res.-Biogeo., 119, 910928, doi:http://dx.doi.org/10.1002/2013jg002586

Web End =10.1002/2013jg002586 http://dx.doi.org/10.1002/2013jg002586

Web End = , 2014.

Trauth, N., Schmidt, C., Vieweg, M., Oswald, S. E., and Fleckenstein, J. H.: Hydraulic controls of in-stream gravel bar hyporheic exchange and reactions, Water Resour. Res., 51, 2243 2263, doi:http://dx.doi.org/10.1002/2014wr015857

Web End =10.1002/2014wr015857 http://dx.doi.org/10.1002/2014wr015857

Web End = , 2015.

USEP: Technical Manual: Methods for Collection, Storage and Manipulation of Sediments for Chemical and Toxicological Analyses-Contaminated Sediments in Water, Chapter 6: Collection of Interstital water, in: United States Environmental Protection Agency, available at: https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =https://nepis.epa.gov/Exe/ZyNET. https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =exe/20003PLT.TXT?ZyActionD=ZyDocument&Client= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =EPA&Index=2000+Thru+2005&Docs=&Query=&Time= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =&EndTime=&SearchMethod=1&TocRestrict=n&Toc= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =&TocEntry=&QField=&QFieldYear=&QFieldMonth= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =&QFieldDay=&IntQFieldOp=0&ExtQFieldOp= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =0&XmlQuery=&File=D:%5Czyles%5CIndexData% https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =ANONYMOUS&Password=anonymous&SortMethod=h| https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =Resultspage&MaximumPages=1&ZyEntry=1&SeekPage= https://nepis.epa.gov/Exe/ZyNET.exe/20003PLT.TXT?ZyActionD=ZyDocument&Client=EPA&Index=2000+Thru+2005&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D:%5Czyfiles%5CIndex Data%5C00thru05%5CTxt%5C00000004%5C20003PLT.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h|-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL#

Web End =x&ZyPURL# (last access: October 2015), 2013.

Valett, H. M., Hakenkamp, C. C., and Boulton, A. J.: Perspectives on the Hyporheic Zone: Integrating Hydrology and Biology. Introduction, J. N. Am. Benthol. Soc., 12, 4043, doi:http://dx.doi.org/10.2307/1467683

Web End =10.2307/1467683 http://dx.doi.org/10.2307/1467683

Web End = , 1993.

Verreydt, G., Annable, M. D., Kaskassian, S., Van Keer, I., Bronders, J., Diels, L., and Vanderauwera, P.: Field demonstration and evaluation of the Passive Flux Meter on a CAH groundwater plume, Environ. Sci. Pollut. R., 20, 46214634, doi:http://dx.doi.org/10.1007/s11356-012-1417-8

Web End =10.1007/s11356-012-1417-8 http://dx.doi.org/10.1007/s11356-012-1417-8

Web End = , 2013.

Biogeosciences, 14, 631649, 2017 www.biogeosciences.net/14/631/2017/

J. V. Kunz et al.: Quantifying nutrient uxes a new HPFM 649

Wagenschein, D. and Rode, M.: Modelling the impact of river morphology on nitrogen retention A case study of the Weisse Elster River (Germany), Ecol. Model., 211, 224232, doi:http://dx.doi.org/10.1016/j.ecolmodel.2007.09.009

Web End =10.1016/j.ecolmodel.2007.09.009 http://dx.doi.org/10.1016/j.ecolmodel.2007.09.009

Web End = , 2008.

Triska, F. J., Kennedy, V. C., Avanzino, R. J., Zellweger, G. W., and Bencala, K. E.: Retention and Transport of Nutrients in a Third-Order Stream in Northwestern California: Hyporheic Processes, Ecology, 70, 18931905, 1989.

Ward, A. S., Gooseff, M. N., and Johnson, P. A.: How can sub-surface modications to hydraulic conductivity be designed as stream restoration structures? Analysis of Vauxs conceptual models to enhance hyporheic exchange, Water Resour. Res., 47, W08512, doi:http://dx.doi.org/10.1029/2010WR010028

Web End =10.1029/2010WR010028 http://dx.doi.org/10.1029/2010WR010028

Web End = , 2011.

Zarnetske, J. P., Haggerty, R., Wondzell, S. M., and Baker, M. A.: Dynamics of nitrate production and removal as a function of residence time in the hyporheic zone, J. Geophys. Res.-Biogeo., 116, G01025, doi:http://dx.doi.org/10.1029/2010jg001356

Web End =10.1029/2010jg001356 http://dx.doi.org/10.1029/2010jg001356

Web End = , 2011b.

Zarnetske, J. P., Haggerty, R., Wondzell, S. M., Bokil, V. A., and Gonzalez-Pinzon, R.: Coupled transport and reaction kinetics control the nitrate source-sink function of hyporheic zones, Water Resour. Res., 48, W11508, doi:http://dx.doi.org/10.1029/2012wr011894

Web End =10.1029/2012wr011894 http://dx.doi.org/10.1029/2012wr011894

Web End = , 2012.

www.biogeosciences.net/14/631/2017/ Biogeosciences, 14, 631649, 2017