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1. Introduction

The Danube River Basin has the largest catchment area, spanning more than 817,000 km² within the territory of the European Union (EU) [1]. Since the Danube River crosses 10 countries, and its waters collect tributaries from 9 other countries, international agreements were signed to preserve the riverine ecosystems’ navigability, landscape, and good status. The International Commission for the Protection of the Danube River (ICPDR) was established in 1994, the Transnational Monitoring Network (TNMN) in 1996, and the Danube River Protection Convention was signed and entered into force in 1998 [2]. One of its primary objectives was to implement measures to prevent inadvertent releases of hazardous substances and to minimize the contamination of water and sediments that ultimately reach the Black Sea. The pollution of the river was a significant concern due to the high concentration of heavy metals (referred to in this work as trace elements), polycyclic aromatic hydrocarbons (PAHs), pesticides, cosmetics, medicines, and other chemicals in the river water. These substances were either dissolved, transported as suspended particulate matter, trapped as bottom sediments, or deposited during floods on the riverbanks.

The quality of the Danube River water and its sediment can impact the health of humans and ecosystems in the region. A substantial body of research has been conducted to gain insight into the river’s geomorphology [3,4] and erosion and sedimentation processes [5], as well as the impact of hydropower dams [6,7], navigation [8,9], agriculture [10,11], industry [12], mining [13,14], waste disposal [15,16], and other anthropogenic activities on the river [2,17].

The Third Joint Danube Survey (JDS3), conducted by the ICPDR along the entire length of the Danube River, revealed that contamination with trace elements was the primary source of acute and chronic toxicity in benthic invertebrates at the sampled stations, and industrial waste was the second-most significant contributor to this phenomenon [18,19]. The analysis identified several chemical substances, including insecticides, herbicides, fungicides, pharmaceuticals, home and personal care products, and other such chemicals, which were found to manifest chronic toxicity with low or no detectable toxic effects (in the latter case, the substances were not detected).

Trace elements have been demonstrated to possess carcinogenic properties when introduced into the human body [20]. As a result, there is a clear rationale for monitoring their concentration, identifying areas with elevated and potentially harmful concentrations, and implementing remediation strategies. Various risk assessment methods have been employed to accurately assess the degree of contamination with trace elements in riverine and estuarine systems. Among these, pollution indices have demonstrated efficacy as suitable tools [21]. A review of 18 of the most used indices in various studies worldwide [21] revealed that almost all were based on comparing the measured concentrations of trace elements against a set of reference values. These reference values are defined as geochemical background [22], pre-industrial values [23], average content in the Earth’s upper continental crust [24], and average content in soil surface horizons worldwide [25]. Proportionally, a smaller number of indices are based on toxicological studies. These employ the tolerable levels in soil [26], meaning the concentrations at which there are almost no effects on benthic organisms (Effects Range Low, ERL) and at which probable effects occur (Effects Range Median, ERM) [27] or they are based on the ranks given to substances by the Agency for Toxic Substances and Disease Registry (ATSDR) [28].

The environmental regulations establish upper limits (threshold, alert, and intervention values) for heavy metal concentrations in water, soil, and sediment to preserve the local ecosystems. It is evident that contamination assessment based on pollution indices computed using mean upper crust values determined at the global level is inadequate for providing accurate results. This is because local conditions, dependent on geology, are not considered. Furthermore, implementing unnecessary remediation measures can even destroy ecosystem equilibrium [29,30]. The substitution of mean upper crust values with local background values from a deep soil layer does not solve the problem, as the distribution of the analyzed metal is not homogeneous in soil layers due to weathering and pedogenesis processes, among other factors. This may result in an erroneous indication of contamination. It was recommended that raw data collected over a larger region be used instead to estimate the geochemical background range and identify geochemical anomaly patterns [31,32].

The geochemical background is determined based on the median ± 2 MAD (median absolute deviation) after outliers are removed with the aid of box-plot charts. It has been demonstrated that the median ± 2 MAD is a superior method to the mean ± 2 standard deviations for this purpose [33]. This method has been used in recent studies for the evaluation of data and the estimation of background intervals [34,35,36,37].

In several studies conducted in Romania [38,39], the values presented in [40,41] have been used as background values for calculating indices in assessing trace element concentrations in soil and sediment. These values were employed in conjunction with comparing the limits set forth by Romanian legislation [42]. Additionally, the values of the Canadian sediment quality guidelines (SQGs), precisely the threshold effect level (TEL) and probable effect level (PEL) described in [43], were also used in a study [38]. The values from the Romanian legislation were employed as a geochemical background for trace elements (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) in [44,45,46,47] to assess the sediment’s quality. It is noteworthy that Romanian legislation [42] also specifies limits for water quality evaluation. These limits were used to compute indices for trace elements [48,49], nutrients, and other physicochemical parameters in surface waters [2,50]. Other studies on soil pollution [51] employed the natural abundances of chemical elements, designated as Clarks [52], and the average values determined in various soil types [53] for the calculation of specifically developed indices. The Clarks represent the average content of an element in the lithosphere, and they are widely used in mineral geochemical prospecting across Eastern Europe and Russia [54]. In other studies [55], the values indicated for soils in [26,53] and Romanian legislation on soil were employed to calculate contamination/pollution indices. Romanian legislation on soil differentiates between normal, alert, and intervention values for sensitive and less sensitive categories of land use [56]. The values indicated in [26], as well as the maximum allowable concentrations for various European countries and the Dutch standard [57], were used in [58] to assess soil pollution due to mining in the Baia Mare area.

A study in the eastern Black Sea [59] assessed the presence of minor and trace elements in soils, bottom sediments, and biota. The study also compared the values to the average upper continental crust values [24] and soil trace element levels in the Netherlands, Germany, Finland, the USA, and Russia.

There is considerable variability [22,23,24,25,52,53] in how geochemical background values for trace elements are defined and referenced regarding various media. These include averages of the continental upper crust or soil surface. These reference values are used in a multitude of publications to facilitate straightforward comparisons or the calculation of pollution indices. This study acknowledges that there are no universally true background values [29,30,31,32,33,34]. In the authors’ opinion, assessing the geochemical influences on a regional level is more valuable and accurate, considering the sediment transport in each major river catchment area. This approach would yield geochemical background values contingent on the superimposition of geochemical characteristics across countries and regions. In exceptional instances, the sediment transported from a geologically significant region to another via major rivers and tributaries may exert an influence on the background values that have to be established for the specific watercourse.

A study conducted between 1994 and 2006 by the Geological Institute of Romania (IGR) in collaboration with the Federal Institute for Geosciences and Natural Resources of Hannover, Germany (Bundesanstalt für Geowissenschaften und Rohstoffe—BGR), had the objective of determining the geochemical background values for chemical elements in Romanian stream water and sediment. The sampling was conducted in areas unaffected by industrial activity or other anthropogenic effects. The specific locations were selected to represent the dominant geological unit in the respective sampling zone. The aforementioned study culminated in creating a geochemical atlas at a scale of 1:3,000,000 [60].

The present study established the geochemical background relevant in Romania for the trace elements As, Cd, Cr, Cu, Ni, Pb, and Zn using data from the 2006 Geochemical Atlas of Romania. The data set comprises analysis results generated by sediment samples from 167 locations distributed relatively uniformly throughout the country and distant from known local contamination sources. Box-plots were constructed and employed from the data set for each selected trace element to facilitate the removal of outliers. Subsequently, the median and its corresponding median absolute deviation (MAD) were extracted from the filtered data sets. The geochemical background for Romania was established as the median ± 2 MAD extracted from the data sets after removing outliers for each of the selected trace elements from the Geochemical Atlas of Romania. As mercury was not included in the Geochemical Atlas of Romania, the value measured in 2020 in an excavation located in the Danube River floodplain sampled at 3.3 m below ground surface was considered the geochemical threshold of this element for the present work, in lieu of more relevant data.

Except for Cr and Ni, the geochemical threshold obtained was below the quality standard stipulated in Romanian legislation for sediments [42]. This quality standard was based on the Dutch standard, with only two minor modifications. As previously stated, the thresholds used in this study are deemed more suitable for Romania, as they reflect the country’s distinctive geology. The pollution indices were calculated for samples collected from the same locations as those used in previous studies [61,62], although the comparison was made with different reference values, and the analyses were conducted in a different laboratory.

The elevated threshold levels of Cr and Ni in Romania, which are higher than the Dutch standard and the country’s legislative requirements, have been subjected to detailed scrutiny. This investigation has sought to corroborate information derived from various geochemical prospecting studies. The findings indicate that these elevated threshold levels are consistent with the geology of the Carpathian-Balkan region. Considering these observations, a proposal for updating the reference values in the specific legislation should be submitted to the Romanian Ministry of Environment, which issued Order 161/2006 [42].

The novelty of this study consists of establishing the level of anthropogenic pollution of sediments (evaluated through different indices) by starting from the regional geochemical background and not from a set of normal reference values (sometimes, these are established only considering the toxicological potential of substances).

2. Materials and Methods

2.1. Study Area

Approximately one-third of the Danube basin area (29%) is situated within the borders of Romania. Consequently, the river plays an integral role in the lives and well-being of the people who inhabit this region. The Lower Danube Basin represents the third portion of the Danube catchment area, where the river is compelled to traverse the mountains once more, forming a remarkable gorge. The Lower Danube begins at the Iron Gates, not far after the Danube enters Romania (west of Baziaș), forming a natural border with Serbia and then with Bulgaria, and ends at Ceatal Izmail [63]. Not far from this point, the Danube River divides into two branches (Chilia and Tulcea), marking the beginning of the Danube Delta [64,65].

From a geomorphological perspective, Vogel (2001) delineates the Danube River into nine reaches [66], with the Romanian reaches being represented by the following numbers: Reach 6 (from the Sava River at Belgrade up to the Iron Gate Dam, between 1202 km and 943 km), Reach 7 (from the Iron Gate Dam up to the confluence with the Jantra River, between 943 km and 537 km), Reach 8 (from the Jantra River up to Reni, between 537 km and 132 km), and Reach 9 (from Reni up to the Black Sea, between 132 km and 12 km). The aforementioned reaches exhibit a range of distinctive features. Reach 6 encompasses the Southern Carpathians, characterized by mountainous terrain. In contrast, Reach 7 encompasses the Romanian Plain, comprising loess, Aeolian sediments, and a steep, 150 m high Danube embankment on the right side. Reach 8 features a low relief with alluvial islands, while Reach 9 encompasses the Delta, which is home to a unique ecosystem.

From a geological perspective, the tributaries from both orogens of the Romanian territory, the Carpathian orogen and the North Dobrogea orogen [67], which exhibit significant differences in terms of geological age and characteristics, contribute their waters to the Danube River, together with the tributaries from the Moesic Platform. In the last sector, the Danube Delta corresponds geologically with the Pre-Dobrogea Depression [67]. The Carpathian Orogen is part of the extensive Alpine belt, which encompasses the Pyrenees, the Betic Cordillera with the Balearic Islands, the Rif, the Tell-Atlas, the Apennines, the Alps, the Carpathians, the Balkans, the Dinaric Mountains, and the mountainous regions of Anatolia, Iran, Crimea, the Caucasus, and the Turkmen-Khorasan Mountains, continuing further eastwards [68]. Several mineral deposits have been exploited over time within the Banatitic Magmatic and Metallogenetic Belt (BMMB), as described in [69,70]. The belt extends for 1500 km from the Apuseni Mountains (Western Romanian Carpathians) in Romania, towards the Southern Carpathians (in Romania and Serbia) and further to the southeast (in Bulgaria), hosting the only significant porphyry-copper deposits of Europe [70], along with very diverse deposits of Cu, Au, Mo, Zn, Pb, and Fe, which are of magmatic-hydrothermal origin.

Mining these deposits has contributed to the anthropogenic impact of trace elements on the environment in the Lower Danube, along with other contamination from industry in Romania. This has been added to the contamination already transported by the Danube waters from upstream cities and capitals. Another source of contamination with trace elements (Cd, for example) comes from fertilizers used in agriculture.

2.2. Sampling

Nine sampling locations were selected for the study area, encompassing over 1000 km along the Danube River, from Baziaș (km 1075) to Sulina (near km 0—Figure 1). Seven sampling stations were situated on transverse profiles of the Danube River, with two supplementary transverse profiles located in the lower reaches of the Jiu and Olt tributaries. The selection of the locations was based on the criteria established in [71], which stipulates that the sampling location should be correlated with a Trans-National Monitoring Network station or a national monitoring station for water/sediments. Additionally, the location should be representative for the water body, have a different geological background, be easily accessible, and have information on potential sources of contamination. The final station coordinates were established in collaboration with the National Water Authority in Romania (ANAR).

The collection of suspended and bottom sediments from the transverse profiles was conducted in accordance with the ISO sampling guidelines for sediments [72,73]. Three verticals were established on each transverse profile, designated as “Left” (at 50 m from the Danube River’s left bank), “Centre” (in the center of the river), and “Right” (at a distance of 50 m from the Danube River’s right bank).

At each vertical, 30 L of water was collected using a submersible pump from the following depths: 0.5 m below the water surface, the middle depth, and 0.5 m above the bottom of the river. A composite water sample with suspended sediment, comprising 90 L, was obtained by using this method and deposited in a plastic barrel. After 24 h, the supernatant was removed, and 8 L of water with settled suspended sediment was collected and refrigerated (4 °C) in a plastic bottle.

To collect the bottom sediments, a Van Veen grab with a volume of 250 cm³ was deployed from a manual-driven winch at each vertical (left, center, and right). Samples of the riverbed sediments were obtained from the uppermost 5 cm. A bottom sediment sample, a replicate, and a duplicate were collected and stored in 750 mL glass jars, also kept in low-temperature conditions. In the case of the Jiu and Olt tributaries, the bottom and suspended sediments were sampled from a single vertical profile situated at the center of the river.

Active floodplain sediment (overbank) was collected close to the river transverse profiles, with appropriate consideration given to the extent of the flood hazard maps [74]. Floodplain sediment samples were collected from two depths using spades and shovels: the topsoil or top layer (0–10 cm) and the bottom layer (40–50 cm). This approach was undertaken to attempt to separate recent contamination from older contamination and potentially identify a pre-industrial, non-contaminated state [75]. A composite sample was created by combining the sediment collected from five sampling locations along a linear riverbank profile of 250–500 m in accordance with the methodology outlined in the Geochemical Atlas of Europe [76].

The sampling campaign was conducted in the autumn of 2020 (active floodplain sediments) and, for logistical reasons, in the late spring of 2021 (suspended and bottom sediments). Given the difficult sampling conditions, 54 unique samples were collected and transported under refrigerated conditions to the laboratory.

The floodplain samples and the bottom sediments were subjected to a sieving process, during which each sample was divided into two distinct grain sizes: <0.063 mm and <2 mm. This procedure was necessary because the distribution of trace elements is directly dependent on their granulometry [77,78,79,80]. The suspended sediments with finer grain size were analyzed exclusively in the <0.063 mm fraction because larger particles have a tendency to settle, becoming part of the bottom and floodplain sediments. In total, 78 samples were analyzed.

An accredited laboratory (ALS Laboratory Group) performed all the chemical analyses. The samples were processed in the laboratory to determine the total dry matter (ISO 11465 [81], EN 12880 [82], EN 14346 [83]) and were mineralized with aqua regia (EN 13657 [84], ISO 11466 [85]). The mineralized samples were analyzed for mercury using atomic fluorescence spectrometry (EN ISO 17852 [86], ISO 16772 [87], and PSA APP025 [88]). For the remaining elemental constituents, the mineralized samples were analyzed using inductively coupled plasma mass spectrometry (US EPA 200.8 [89], EN ISO 17294-2 [90], US EPA 6020A [91]).

2.3. Establishing the Geochemical Background

This step is of great importance in assessing the contamination of sediments by trace elements, as most pollution indices consider the geochemical background, reference, or “normal” value, specifically the threshold, which is the actual value used in the calculation. Considering the significant discrepancy in the geochemical background values used in various studies conducted in the Lower Danube (as previously outlined in the introduction), this study employs values extracted from the data provided by the Geochemical Atlas of Romania (scale 1:3,000,000) [60]. This is the first geochemical atlas comprising samples from the entirety of Romania, despite preceding efforts to create atlases for selected regions or for particular objectives.

While developing the Atlas, samples of stream water and sediments were collected from locations distributed throughout the country following a grid of 0.5 degrees of longitude and latitude. The samples were obtained from sites located at a considerable distance from areas of anthropogenic activity, such as mines, industrial facilities, human settlements, railways, and major roads. The field team conducted the sampling between 1994 and 1996. The water samples were collected in 125 mL plastic flasks and were preserved with 1 mL of HNO₃ to prevent the adsorption of cations. The water samples were analyzed in the German laboratory for the concentration of 48 elements (Ag, Al, As, B, Ba, Be, Bi, Br, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, I, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr). On-site measurements were conducted to determine the temperature, electrical conductivity, and pH levels.

A total of 167 stream sediment samples were collected from minor riverbeds in 0.5 L plastic recipients, dried at 40 °C, and sieved to obtain fractions of <2 mm and <0.063 mm. The sediment analyses were conducted in the BGR laboratory utilizing X-ray fluorescence spectrometry (XRF) for the determination of major constituents and inductively coupled plasma mass spectrometry (ICP-MS) for the analysis of minor and trace elements. No further information is available on the specific manufacturer and model of the analytical equipment used. The analyzed major constituents were SiO₂, TiO₂, Al₂O₃, Fe₂O₃, MnO, MgO, CaO, Na₂O, K₂O, P₂O₅, SO₃, Cl, F, and LOI, and the analyzed minor/trace elements were Ag, As, B, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Ga, Ge, Hf, In, La, Li, Mn, Mo, Nb, Ni, Pb, Rb, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr.

This study employs the trace elements (As, Cd, Cr, Cu, Ni, Pb, and Zn) values for stream sediments across the entire Romanian territory, as presented in the Geochemical Atlas of Romania [60], to ascertain the geochemical background at the national level. This approach aligns with the methodology proposed in [31,32]. Figure 2 illustrates the distribution and range of values for the aforementioned trace elements.

The statistical data describing the trace element data sets from the Geochemical Atlas of Romania are provided in Table 1. Among the statistical parameters calculated are the geochemical background and the threshold used as a reference value for the calculation of pollution indices in this study. The threshold was calculated based on the median of measured concentration values [33,34] from the Geochemical Atlas of Romania:

(1) $T h r e s h o l d = M e d i a n + 2 M A D$

where MAD is the median absolute deviation, calculated as the median, M_i, of the deviations (in absolute value) between each value, x_i, of the series of n observations and their median, M_j [92]:

(2) $M A D = M_{i} (| x_{i} - M_{j} |)$

The aforementioned thresholds or reference values (in mg/kg) are as follows (Table 1): 11 (As), 0.41 (Cd), 172.5 (Cr), 34 (Cu), 52 (Ni), 29 (Pb), and 111.5 (Zn). The trace element concentrations measured in the Lower Danube Basin in this study were obtained using the same analytical method (ICP-MS) and are reported as “total metal” in accordance with the procedures outlined in the Geochemical Atlas of Romania [60], thereby suggesting data compatibility between the analyzed samples and the reference set.

The mercury concentration in a sediment sample collected from an excavation at a depth of 3.3 m below the ground surface was used as the reference value for calculating pollution indices, as data for this element were not available in the Geochemical Atlas of Romania [60]. The excavation was situated close to the floodplain sediment sampling profile at Hârșova. Five samples were collected from the excavation (Figure 3) at varying depths and were subsequently analyzed in the laboratory for two fractions (i.e., <2 mm and <0.063 mm), together with the other floodplain sediment samples. The mercury concentration at the bottom of the hole was determined to be 0.081 mg/kg for the < 0.063 mm fraction and 0.073 mg/kg for the <2 mm fraction. The value corresponding to the finer fraction, 0.081 mg/kg, was selected as the threshold and was considered to be the “pre-industrial level” for the purposes of this study.

Figure 4 illustrates the evolution of the box-plots for the nine sampling locations in the Danube area. The complete set of results can be found in the Supplementary Material, Table S1. The box-plots show the first and third quartiles, the median of the values, the mean, and the outliers for each trace element. The presence of outliers indicates how much some individual concentrations of trace elements are greater than 3/2 of the third quartile. No lower outliers were identified.

2.4. The Calculation of Pollution Indices

The threshold values established based on the Geochemical Atlas of Romania were used to compute pollution indices. Among the simple indices calculated for each trace element, the Geoaccumulation Index (I_geo) [22] is a widely employed, readily interpretable, and precisely scaled metric [21]. It is defined as

(3) $I_{g e o} = \log_{2} (\frac{C_{n}}{1.5 \cdot B_{n}})$

where

C_{n}

is the measured concentration of the investigated element,

B_{n}

is the geochemical background, and the constant

1.5

has the role of reducing the variations of lithology. Seven classes were defined (Table 2) for the Geoaccumulation Index [93]:

From the category of complex indices, which take into account all analyzed trace elements, the Nemerow Pollution Index (PI_Nemerow), developed for the Environmental Protection Agency (EPA) of the United States in [94], was selected due to its wide use in a multitude of regions across the globe [95,96,97,98,99]. This index features a precise scale focused on the most contaminated element and is designed to evaluate the overall quality of the sediment/soil. The Pollution Index PI has a relatively straightforward calculation based on the ratio between the concentration of a specific element and a reference value, which is typically the geochemical background or a threshold concentration:

(4) $P I = \frac{C_{n}}{B_{n}}$

(5) ${P I}_{N e m e r o w} = \sqrt{\frac{({(\frac{\sum_{i = 1}^{n} P I}{n})}^{2}) + {P I}_{m a x}^{2}}{2}}$

The quality classes based on PI_Nemerow values are listed in Table 3 [100,101]:

Another index that was employed in this study was the Potential Ecological Risk (RI), which was introduced in [8] and can be calculated using the following formula:

(6) $R I = \sum_{i = 1}^{n} E_{r}^{i}$

(7) $E_{r}^{i} = T_{r}^{i} \cdot P I$

where

n

is the number of trace elements,

E_{r}^{i}

is the index of ecological risk factor [21,23,36],

P I

is the simple Pollution Index (2), and

T_{r}^{i}

is the toxicity response coefficient of a trace element. These toxicity responses are [23,36] Cr = 2; Zn = 1; Ni = Cu = Pb = 5; As = 10; Cd = 30; and Hg = 40. The ecological risk is classified into four categories (Table 4). The PI was calculated using the regional geochemical background values, as indicated in Table 1. Studies conducted in other regions cannot use the pre-industrial values observed in Scandinavian lakes as a basis for comparison. Research conducted in [102] employed regional Chinese background values, and recent studies worldwide are increasingly adopting the median of the regional values ± 2 MAD.

In addition to the indices based on the geochemical background (threshold value), an index was also computed for comparison purposes that does not include the geochemical background in the formula and, instead, is based on the toxicity of the trace elements. This index, developed in [28], is the Environmental Toxicity Quotient (ETQ). It employs the Total Scores (TSs) determined by the United States Agency for Toxic Substances and Disease Registry (ATSDR), Division of Toxicology and Environmental Medicine, Atlanta. The ETQ is calculated as follows:

(8) $E T Q = \frac{\sum_{i = 1}^{n} (\frac{{T S}_{i}}{{T S}_{A s}} \cdot C_{i})}{n}$

where

C_{i}

is the measured concentration of the

i

trace element,

{T S}_{i}

is the total score of the respective trace element, and

{T S}_{A s}

is the total score of As, considered the highest in the ATSDR list [103]. The TS of the trace elements included in this study are As = 1675, Pb = 1531, Hg = 1455, Cd = 1317, Ni = 994, Zn = 916, Cr = 892, and Cu = 807.

In accordance with the established ranges of values, the ETQ index is divided (Table 5) into five distinct classes [28,104].

3. Results

3.1. Geoaccumulation Index I_geo

The results of the Geoaccumulation index are presented in Figure 5.

3.1.1. I_geo for As

A slight level of arsenic contamination was detected in 12 samples, specifically from Baziaș (4 samples), Pristol (2 samples), Svishtov-Zimnicea (2 samples), Silistra-Călărași (2 samples), and Hârșova (2 samples). In contrast, moderate-to-high levels of arsenic contamination were found in three samples only from Pristol.

Regarding sediment type, contamination was observed in the bottom sediment fraction (<0.063 mm) in four samples, in the bottom sediment fraction (<2 mm) in one sample, and in suspended sediments in the remaining ten samples (predominantly from the right bank). These suspended sediments did not correlate with bottom sediments from the same location, suggesting momentary contamination. No contamination was detected in floodplain sediments.

3.1.2. I_geo for Cd

Slight cadmium contamination was detected in 15 samples, specifically from Baziaș (4 samples), Pristol (2 samples), Svishtov-Zimnicea (2 samples), Oltenița (1 sample), Silistra-Călărași (3 samples), and Hârșova (3 samples). Moderate contamination was found in three samples (Baziaș, Silistra-Călărași, and Hârșova), while three samples exhibited extreme pollution (Pristol).

One-third of the contaminated samples were bottom sediments, while the remaining two-thirds were suspended sediments, likely influenced by the Danube flow regime in the area. Extreme contamination was observed in the Pristol region, specifically in suspended sediments collected from the right bank and central areas. In the bottom sediments, contamination predominantly occurred in the fine fraction, with five out of seven samples exhibiting pollution. No contamination was observed in the floodplain sediments.

3.1.3. I_geo for Cr

The chromium concentration was below the threshold in all the analyzed samples.

3.1.4. I_geo for Cu

Slight copper contamination was identified in 19 samples, including those from Baziaș (4 samples), Zăval (1 sample), Svishtov-Zimnicea (2 samples), Oltenița (2 samples), Silistra-Călărași (4 samples), Hârșova (4 samples), and Sulina (2 samples). Moderate contamination was recorded in one sample (Hârșova), while three samples showed extreme contamination (Pristol).

The distribution of contamination across sediment types was as follows: a total of 7 samples of bottom sediment, 13 samples of suspended sediment, and 3 samples of floodplain sediment. Notably, extreme contamination was concentrated in the Pristol region, specifically in suspended sediments collected from the right bank and central areas. Within the bottom sediments, contamination was predominantly associated with the fine fraction, with five out of seven samples displaying elevated concentrations.

3.1.5. I_geo for Hg

A total of 27 samples exhibited slight mercury contamination, including those from Baziaș (3 samples), Pristol (4 samples), Islaz (4 samples), Svishtov-Zimnicea (3 samples), Zimnicea (3 samples), Oltenița (1 sample), Silistra-Călărași (4 samples), Hârșova (2 samples), and Sulina (3 samples). Five samples demonstrated moderate mercury contamination, specifically from Pristol (one sample), Zăval (three samples), and Zimnicea (one sample). Additionally, four samples showed moderate to strong pollution, including those from Zăval (two samples), Hârșova (one sample), and Sulina (one sample).

Pollution distribution across sediment types was as follows: a total of 6 samples of bottom sediment, 15 samples of suspended sediment, and 15 samples of floodplain sediment. In the floodplain sediments, contamination was observed in eight samples from the upper layer and seven samples from the lower layer. Within the bottom sediments, pollution was predominantly concentrated in the fine fraction, with five out of six samples showing elevated contamination levels.

3.1.6. I_geo for Ni

The distribution of this contamination across sediment types is as follows: a total of four samples were collected from bottom sediments, while two samples were obtained from suspended sediments. Notably, no contamination was detected in the floodplain sediment samples analyzed.

3.1.7. I_geo for Pb

A total of 12 samples, specifically from Baziaș (3 samples), Pristol (5 samples), Silistra-Călărași (1 sample), and Hârșova (3 samples), exhibited slight lead contamination.

The distribution of contamination across sediment types is as follows: five samples were collected from bottom sediments, while seven samples were obtained from suspended sediments. No lead contamination was observed in the floodplain sediment samples.

3.1.8. I_geo for Zn

Slight zinc contamination was detected in 11 samples, specifically from Baziaș (4 samples), Pristol (1 sample), Svishtov-Zimnicea (1 sample), Silistra-Călărași (2 samples), and Hârșova (3 samples). Three samples from Pristol exhibited moderate contamination, while 1 sample from Pristol showed moderate-to-strong contamination.

The distribution of contamination across sediment types is as follows: a total of 5 bottom sediment samples and 10 suspended sediment samples. No contamination was observed in the floodplain sediments. Of the contaminated bottom sediment samples, 80% were from the fine fraction.

Significant concentrations of As, Cd, Cu, and Zn were identified in the suspended and bottom sediment samples from the Pristol sampling locations, likely attributed to mining activities in the Bor region. Notably, exceptionally high copper concentrations were observed in the suspended sediments, with 2570 mg/kg of sediment recorded in the right bank, likely due to the transport of trace elements by the Timok River, which discharges into the Danube upstream of the sampling location. In the bottom sediments, copper concentrations reached 2090 mg/kg in the central section of the transversal profile and 1690 mg/kg on the right bank of the transversal section. These values are detailed in the Supplementary Materials (Table S1: Trace elements concentration in different types of sediment samples from the Lower Danube Basin).

3.2. Nemerow Pollution Index PI_Nemerow

According to the Nemerow index, the analyzed samples have the following distribution: a total of 29 samples belong to the precaution domain, 27 samples are slightly polluted, 14 are moderately polluted, and 8 samples are seriously polluted (Figure 6). The percentage of the safe domain (class 1) is 0 because of the low threshold value for Hg (0.081 mg/kg sediment) that negatively influenced the value of the Nemerow index for the entire set of samples; this is because, for every sample, it considers all the measured metals.

The samples classified as seriously polluted according to PI_Nemerow (class 5) were located at Pristol (in bottom and suspended sediments) and at Hârșova (in bottom sediments) mainly because of high concentrations of As, Cd, Cu, Ni, Pb, and Zn. The samples classified as moderately polluted (class 4) were found at Baziaș (in the bottom and suspended sediments), Zăval, Svishtov-Zimnicea, Silistra-Călărași and Hârșova (in the suspended sediments) and Sulina (in the floodplain sediments) mainly because of As, Cd, Cu, Pb, and Zn.

The three samples from class 5 (two at Zăval and one at Sulina) and four samples from class 4 (two at Zăval and two at Zimnicea) belong to the respective classes exclusively due to mercury, which is why they were not mentioned in the previous listing.

3.3. Potential Ecological Risk RI

According to the Potential Ecological Risk (RI) values (Figure 6), the analyzed samples have the following distribution: a total of 3 samples belong to class 4 (very high potential ecological risk) and are located in Pristol (bottom sediments for the center and right and suspended sediments for the right part of the transverse section); 4 samples belong to class 3 (considerable potential ecological risk) and are located in Zăval (top layer of the floodplain sediment—both fractions), Hârșova (bottom sediments and fine fraction), and Sulina (suspended sediment in the left part of the transverse section); 22 samples are in class 2 (moderate potential ecological risk), and 49 samples are in class 1 (low potential ecological risk). The very high potential ecological risk comes from the As, Cd, Cu, Ni, Pb, and Zn concentrations.

3.4. Environmental Toxicity Quotient (ETQ)

According to the Environmental Toxicity Quotient (ETQ) (Figure 6), the analyzed samples are mostly in the second class (60 samples). Only one sample falls in class 3 (high ETQ), and three samples in class 4 (very high ETQ). These are the same samples from Pristol, which have very high potential ecological risk and are seriously polluted, according to PI_Nemerow.

The threshold value 0.081 mg/kg, computed for Hg, although close to the natural abundance or Clark of this element, seems too low, influencing both PI_Nemerow and RI, leading to the idea that a great number of samples show some pollution or a moderate potential ecological risk. By removing mercury from the computations, the new class statistics (Figure 7) dramatically changed, with the number of samples in the “precaution domain” increasing from 29 to 43, the low RI class increasing to 73 instead of 49 samples, and the moderate RI class decreasing to only 2 instead of 22 samples.

Nevertheless, a comparison of Figure 6 and Figure 7 reveals that the Hg threshold is not the sole factor determining the number of samples reported as polluted or having a moderate potential ecological risk. Furthermore, the ecological risk may be attributed to outlier values (e.g., Cd and Cu), as illustrated in Figure 4, which depicts the box-plots for the trace elements.

4. Discussion

In calculating the pollution indices in this study, the threshold values determined based on [60] were used, which are lower than the Romanian legislation for sediments [42] for As, Cd, Cu, Hg, Pb, and Zn but higher in the case of Cr and Ni. Table 6 shows the values for these eight trace elements compared with those in the Romanian legislation for sediments and soils [42,56], other values widely used in the literature for the Danube, or those determined in different parts of Europe and worldwide.

Table 6 shows that Cr and Ni determined using [60] are, however, lower than the Dutch intervention values (380 for Cr and 210 for Ni) and lower than the Romanian intervention values for soils (300 for Cr and 150 for Ni) in the case of more sensitive land use categories [56]. It is worth noting that the in-force Romanian legislation [42] has limit values very similar to those of the Dutch standard [57].

Geochemical prospecting works carried out in Romania during the 1970s and 1980s for mineral exploration [106,107,108,109,110,111] revealed high contents in Cr and Ni in some areas with mafic and ultramafic rocks in the Western Romanian Carpathians and Southern Carpathians. In the Western Carpathians, the range of these contents is 140–5500 ppm for Cr and 440–2600 ppm for Ni in core drill serpentinites from exploration wells in the Poiana Ruscă Mts., and 160–3000 ppm (Cr) and 110–3800 ppm (Ni) in the <1 mm fraction of the pan samples [106]. Towards the north, in the Drocea Mts., part of the Apuseni Mts., in ophiolites and other basic igneous rocks [107], the ranges are 2–565 ppm (Cr) and 3–73 ppm (Ni), while towards the southeast, in the Mehedinți Plateau [108], the ranges in the ultrabasic igneous rocks are 1800–2900 ppm (Cr) and 1500–2400 ppm (Ni). The mean values of 2200 ppm (Cr) and 1990 ppm (Ni) for this area are within the ranges of the mean values established in [112], at 1600–2400 ppm for Cr and 1500–2000 ppm for Ni, respectively. The Cr/Ni ratio is >1, with a mean of 1.15 and a range of 0.95–1.46. In the Făgăraș Mts., part of the Southern Carpathians, the contents in Cr and Ni of the metamorphic rocks vary significantly in different areas of these mountains; for Cr, the range of the minimum values is 31–100 ppm, and the maximum is 1200–6500 ppm; for Ni, these ranges are 14–1000 ppm for the minimum and 300–16,000 ppm (1.6%) for the maximum. The Cr values are constantly lower than those for Ni in the transformed ultrabasic rocks [109]. In the southwestern part of the Banat Mts. (Western Romanian Carpathians) in the Almaj Mts., after analyzing 5258 samples of soil from basic and ultrabasic rocks parental material, it was established (using the parabola method [110]) that the geochemical background value was 400 ppm for Cr and 600 pm for Ni. In contrast, the threshold for geochemical anomalies was 1000 ppm (Cr) and 1500 ppm (Ni). In the same mountains, for the serpentinized basic and ultrabasic igneous rocks [111], the Ni content can reach a mean of over 2000 ppm up to 7300 ppm; the Cr content in the samples with massive chromite is 165,500–282,900 ppm (16.55–28.29%).

The results of the geochemical explorations described above are in accordance with the outputs of geochemical analyses at the pan-European level carried out via FOREGS (Forum of European Geological Surveys) and GEMAS (Geochemical mapping of agricultural and grazing land soil) projects; although these did not include data and maps for Romania, the intense geochemical anomalies of Cr and Ni found in the area of the Balkans are in close relationship with the values found in Romania. The highest values of Cr and Ni in the Romanian geochemical explorations for areas in the Western Romanian Carpathians are related to the Vardar Ocean [70] and its chemically basic crust. Figure 8 shows the extension of the geological formations of the Carpathian chain in Romania to the south in the Balkan Peninsula. Chromium is predominantly found as chromite in magnesian igneous rocks, where it is commonly associated with minerals containing nickel, typically olivine [52]. The alteration of these rocks during the subduction of the Vardar oceanic plate [70], in conjunction with weathering and erosion [113], resulted in the enrichment of sediments and, subsequently, of soils’ parent material regarding these elements.

Pan-European geochemical studies carried out in the 1990s to establish the geochemical background values for soils and sediments in Europe, such as the FOREGS project (Forum of the European Geological Surveys)—initiated by the International Union of Geological Sciences and the International Association of Geochemistry and Cosmochemistry [76]—analyzed over 2000 samples in soils (top layer (or topsoil), bottom layer, and humus), stream sediments, and floodplain sediments for a series of elements; this produced the Geochemical Atlas of Europe. Maps have been drawn up covering 26 European countries, but without Romania, Bulgaria, and some other countries from the Balkan peninsula (except for Greece and Albania). A geostatistical analysis carried out on FOREGS data [114] for the trace critical elements (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) highlighted a high correlation for Cr and Ni, related to ultramafic rocks and polymetallic sulfide minerals but also some sedimentary rocks and lateritic deposits, as well as soils enriched in these elements due to the weathering processes. The Geochemical Atlas of Europe shows an intense anomaly of Cr and Ni in Greece and Albania due to mineralized ophiolitic and sedimentary rocks derived from them. An important Cr-Ni mineral deposit is known at Vourinos, in northern Greece, and [115] mentions Greece, Albania, and Serbia amongst the Ni-producing countries in Europe. High values of Cr and Ni in soils and sediments are considered [116] to be of a geogenic nature, reflecting the composition of the substratum.

In this context, as shown in [34], when considering the whole European continent [117], using the threshold values for trace elements established in the legislation of one northern European country (Finland) suggested severe contamination with Cr and Ni in the Balkan peninsula, if the regional geological conditions were not considered in the interpretation. Moreover, [118] showed that it is impossible to establish a unique threshold value for Ni for the entire European continent due to the actual geological conditions.

The GEMAS project, which investigated more than 2000 samples in agricultural plowed soils and grazing land across the European continent (but not including data and maps for Romania), led to similar conclusions; the highest Ni concentrations were found in Greece and Serbia, and the highest Cr concentrations were found in northwestern Italy, the southern Balkans, and Greece due to the nature of soil parental materials made up of detritus originating from ophiolites complexes or sedimentary rocks formed by their weathering [119]. These authors [119] propose that guidelines for managing productive land at the European level should be defined for regions with common geological history and common lithologic settings.

Studies performed in Hungary [120] for establishing environmental quality standards in the Danube River sediments were based on the solids-water partition coefficient K_sw, similar to the methodology employed in [121], which was later developed and led to the sets of values [122] that were taken and considered as background values in the Romanian legislation [42]. The study [121] warned, however, that the suspended particles and the sediments in the Netherlands have fractions of lutum and organic matter, which are specific to this region and which could be different for other rivers in Europe.

Moreover, a recent report on the state of European soils, which included an analysis of the trace elements concentrations in EU agricultural soils based on the LUCAS 2009 database [123], found that if the European Lower Limit Values for trace elements proposed [124] for the Sewage Sludge Directive were to be used all over Europe, one-third of the European agricultural land would exceed these limit values for at least for one trace element, and no sludge could be spread thereafter. Therefore, care should be taken by European countries, including Romania, to adopt reasonable limit values both for sediments and soils, in line with their geological background and, at the same time, aiming to protect human health and preserve water- and soil-related ecosystem services.

5. Conclusions

The threshold values for trace elements proposed in the in-force Romanian legislation [42] have limit values very similar to those of the Dutch standard, which have been developed for an area with a very different geology. Thus, it would be advisable to refine them in accordance with Romania’s existing geochemical background values.

This study proposes more adequate thresholds (upper geochemical background values) as follows:

-. As: 15 mg/kg;
-. Cd: 0.5 mg/kg;
-. Cr: 175 mg/kg;
-. Cu: 35 mg/kg;
-. Hg: 0.3 mg/kg (the same value as the Romanian legislation [42]);
-. Ni: 55 mg/kg;
-. Pb: 30 mg/kg;
-. Zn: 120 mg/kg.

Seven of the eight investigated trace elements showed an important difference regarding the threshold value, the most prominent being Cr and Ni. The proposed threshold values reflect the geological background in Romania and, thus, should be included in the new national legislation regarding environmental protection.

In order to establish more accurate threshold values for the eight critical trace elements examined in this study, as well as for other elements of interest, it is imperative to conduct a geochemical survey of the entire Romanian territory. This investigation can be facilitated within the framework of the recently proposed soil legislation at the European level. The investigation should follow the protocol outlined in the Geochemical Atlas of Romania, using a more extensive sampling grid [60] and performing the analyses in a single accredited laboratory or under appropriate inter-comparison conditions. This approach will provide more accurate and up-to-date values for the geochemical background of Romania, facilitating environmental studies and mineral exploration. Consequently, this study may initiate the development of a more accurate geochemical survey of sediments in Romania, with the potential for replication in other regions.

Author Contributions

Conceptualization, A.-M.V.; methodology, A.-M.V.; software, A.A.I.; validation, I.P., A.A.I. and V.E.A.; formal analysis, A.-M.V., I.P. and A.A.I.; investigation, A.-M.V., I.P. and A.A.I.; resources, L.L.B.; data curation, A.A.I.; writing—original draft preparation, A.-M.V.; writing—review and editing, I.P., A.A.I., V.E.A. and N.-S.A.; visualization, A.A.I.; supervision, I.P.; project administration, I.P.; funding acquisition, I.P. and L.L.B. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

We are grateful to Albert Baltres for his contributions to the SIMONA project, particularly regarding the sampling methodology and for his involvement in the suspended, bottom, and floodplain sediments collection campaigns.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the study’s design; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the result.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

View Image - Figure 1. Sample locations: (a) Baziaș; (b) Pristol; (c) Zăval; (d) Islaz; (e) Svishtov—Zimnicea; (f) Oltenița—Tutrakan; (g) Silistra—Chiciu; (h) Hârșova; (i) Sulina; (j) distribution of the sampling sites in Romania. Background map credentials: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Kong Kong), Esri Korea, Esri (Thailand), NGCC © OpenStreetMap contributors, and the GIS User Community.

Figure 1. Sample locations: (a) Baziaș; (b) Pristol; (c) Zăval; (d) Islaz; (e) Svishtov—Zimnicea; (f) Oltenița—Tutrakan; (g) Silistra—Chiciu; (h) Hârșova; (i) Sulina; (j) distribution of the sampling sites in Romania. Background map credentials: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Kong Kong), Esri Korea, Esri (Thailand), NGCC © OpenStreetMap contributors, and the GIS User Community.

View Image - Figure 2. Sample location and concentration distribution for trace elements: (a) As, (b) Cd, (c) Cr, (d) Cu, (e) Ni, (f) Pb, and (g) Zn—after [60]. Background map credentials: Esri; USGS; NOAA.

Figure 2. Sample location and concentration distribution for trace elements: (a) As, (b) Cd, (c) Cr, (d) Cu, (e) Ni, (f) Pb, and (g) Zn—after [60]. Background map credentials: Esri; USGS; NOAA.

Figure 3. The excavation at the Hârșova floodplain, which provided the sample used to establish the mercury threshold (authors’ photos).

View Image - Figure 4. Box-plots for trace elements: (a) As, (b) Cd, (c) Cr, (d) Cu, (e) Hg, (f) Ni, (g) Pb, and (h) Zn from the Danube data set. The mean is represented by the cyan-colored “□” symbol, “×” represents the 99% and 1% percentiles, “–” represents the maximum and minimum values from the data set, and the magenta-colored circles represent outliers.

Figure 4. Box-plots for trace elements: (a) As, (b) Cd, (c) Cr, (d) Cu, (e) Hg, (f) Ni, (g) Pb, and (h) Zn from the Danube data set. The mean is represented by the cyan-colored “□” symbol, “×” represents the 99% and 1% percentiles, “–” represents the maximum and minimum values from the data set, and the magenta-colored circles represent outliers.

View Image - Figure 5. Results of Geoaccumulation Index, Igeo, for the investigated trace elements. The blue background segments’ lengths are proportional to the values, ranging from the highest value possible, which occupies 100% of the column width, to the lowest value, which occupies 0% of the column width.

Figure 5. Results of Geoaccumulation Index, Igeo, for the investigated trace elements. The blue background segments’ lengths are proportional to the values, ranging from the highest value possible, which occupies 100% of the column width, to the lowest value, which occupies 0% of the column width.

Figure 6. Results of complex indices: PINemerow, RI, and ETQ.

Figure 7. Statistics of PINemerow and RI without mercury.

View Image - Figure 8. Geological map of SE Europe. Red lines delimit geological provinces: 1103—Dobrogea foreland; 4048—Pannonian Basin; 4051—Alps; 4057—Transylvania; 4061—Carpathian-Balkanian Basin; 4063—Dobrogea Orogen; 4064—West Black Sea Basin; 4071—Dinaric Alps; 4075—Aegean. Used with permission [Eric Gaba, Wikimedia Commons user Sting, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11171117, accessed 4 December 2024].

Figure 8. Geological map of SE Europe. Red lines delimit geological provinces: 1103—Dobrogea foreland; 4048—Pannonian Basin; 4051—Alps; 4057—Transylvania; 4061—Carpathian-Balkanian Basin; 4063—Dobrogea Orogen; 4064—West Black Sea Basin; 4071—Dinaric Alps; 4075—Aegean. Used with permission [Eric Gaba, Wikimedia Commons user Sting, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11171117, accessed 4 December 2024].

Table 1

Statistics of the measured concentrations [mg/kg] for 167 stream sediment samples [60].

Statistical Parameter	As	Cd	Cr	Cu	Ni	Pb	Zn
Count	167	167	167	167	167	167	167
Maximum	48	30.5	607	177	147	273	3649
Minimum	1	0.05	22	0.1	2	3	27
Mean	8.35	0.53	137.67	19.61	32.52	27.15	108.88
Median	7	0.28	122	18	31	20	79
Mode	6	0.29	97	0.1	19	14	80
St. dev.	0.45	2.37	80.63	21.81	19.33	27.69	281.44
MAD	2	0.07	28	8	12	6	18
Kurtosis	18.43	155.22	14.36	22.49	7.50	41.75	151.56
Skewness	3.29	12.29	3.20	3.87	1.81	5.61	12.08
Background	3–11	0.14–0.42	66–178	2–34	7–55	8–32	43–115
Filtered Count *	161	159	160	164	163	153	158
Mean *	7.58	0.29	125.03	17.29	30.72	21	78.2
Median *	7	0.27	117.5	18	30	19	77.5
MAD *	2	0.07	27.5	8	11	5	17
Background *	3–11	0.13–0.41	62.5–172.5	2–34	8–52	9–29	43.5–111.5
Threshold *	11	0.41	172.5	34	52	29	111.5

* Statistics determined after outlier removal.

Table 2

Soil/sediment quality classes according to I_geo values.

Class	Description	I_geo
0	unpolluted	I_geo ≤ 0
1	slightly polluted	0 < I_geo ≤ 1
2	moderately polluted	1 < I_geo ≤ 2
3	moderately to strongly polluted	2 < I_geo ≤ 3
4	strongly polluted	3 < I_geo ≤ 4
5	strongly to extremely polluted	4 < I_geo ≤ 5
6	extremely polluted	I_geo > 5

Table 3

Soil/sediment quality classes according to PI_Nemerow values.

Class	Description	PI_Nemerow
1	safety domain/clean	PI_Nemerow ≤ 0.7
2	precaution domain/warning	0.7 < PI_Nemerow ≤ 1
3	slightly polluted	1 < PI_Nemerow ≤ 2
4	moderately polluted	2 < PI_Nemerow ≤ 3
5	seriously polluted	3 < PI_Nemerow

Table 4

Ecological risk classes according to RI values [23].

Class	Description	RI
1	low risk	RI < 150
2	moderate risk	150 ≤ RI < 300
3	considerable risk	300 ≤ RI < 600
4	very high risk	600 ≤ RI

Table 5

Environmental Toxicity Quotient classes based on ETQ values.

Class	Description	ETQ
1	low	ETQ ≤ 10
2	moderate	10 < ETQ ≤ 50
3	high	50 < ETQ ≤ 100
4	very high	100 < ETQ ≤ 300
5	extremely high	300 < ETQ

Table 6

Values (in mg/kg sediment) for trace elements according to various sources. ROM SV—Romania’s sediment; JDS—Joint Danube Survey values; TV—Dutch target values for soils; IV—Dutch intervention values for soils; UCC—Upper Continental Crust; Pre-ind.—pre-industrial; Soils TV—soils typical values in RO legislation; Soils AV—soils alert values in RO legislation for more sensitive land use categories; Soils IV—soils intervention values in RO legislation for more sensitive land use categories; FIN Thr.—Finland’s threshold values; FIN LGV—Finland’s lower guideline values; AUS EIL—Australia’s ecological investigation levels; AUS HIL—Australia’s health investigation levels.

Trace Element	As	Cd	Cr	Cu	Hg	Ni	Pb	Zn
This study	11	0.41	172.5	34	0.081	52	29	111.5
ROM SV [42]	29	0.8	100	40	0.3	35	85	150
JDS [40]	10	0.25	10–50	35	0.2	10	25	130
Dutch TV [57]	29	0.8	100	36	0.3	35	85	140
Dutch IV [57]	55	12	380	190	10	210	530	720
UCC [24]	4.8	0.09	92	28	0.05	47	17	67
Pre-ind. [23]	15	1	90	50	0.25	-	70	175
Clark [105]	5	0.5	200	100	0.07	80	16	50
Soils TV [56]	5	1	30	20	0.1	20	20	100
Soils AV [56]	15	3	100	100	1	75	50	300
Soils IV [56]	25	5	300	200	2	150	100	600
FIN Thr. [34]	5	1	100	100	0.5	50	60	200
FIN LGV [34]	50	10	200	150	2	100	200	250
AUS EIL [34]	20	3	400	100	1	60	600	200
AUS HIL [34]	200	40	200	2000	30	600	600	14,000

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/app15052458/s1: Table S1: Trace elements concentration in different types of sediment samples from the Lower Danube Basin; Box-plots of nine sections from the Lower Danube; Box-plots of geochemical atlas 1:3,000,000; Archive containing larger maps for Figure 1; Archive containing larger maps for Figure 2; and The complete Geological Map of Europe for Figure 8.

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Abstract

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The contamination of the Lower Danube Basin with trace elements has been the subject of intense study in recent years, with several pollution indices being employed for the assessment of water, soil, and sediment quality. Most pollution indices are calculated by comparing the measured concentration to a reference value, with different values from various sources being used. In this study, the threshold values for trace elements (As, Cd, Cu, Hg, Ni, Pb, and Zn) in Romania were selected based on the median of their concentration in stream sediment from 167 locations, which were carefully selected to be far from contamination sources. The threshold values were established based on a calculation starting from the geochemical concentration presented in the Geochemical Atlas of Romania (scale: 1:3,000,000, developed in 2006 by a collaborative Romanian-German team). By using these thresholds, pollution indices were calculated for nine sampling locations, from which suspended sediment, bottom sediment, and active floodplain sediment samples were collected and analyzed in an accredited laboratory. Most of the selected thresholds were found to be below the limits for sediments established by Romanian legislation, except for Cr and Ni, the higher values of which are consistent with the specific geological background of the country and the broader Balkan region.

Details

Title

Geochemical Background Values for the Characterization of Sediment Contamination with Trace Elements in Romania: A Case Study in the Lower Danube Basin

Author

Anca-Marina Vîjdea; Popescu, Iustina

; Alexandru Anton Ivanov

; Alexe, Veronica Elena; Natalia-Silvia Asimopolos; Bălan, Lidia Lenuța

First page

2458

Publication year

2025

Publication date

2025

Publisher

MDPI AG

e-ISSN

20763417

Source type

Scholarly Journal

Language of publication

English

DOI

https://doi.org/10.3390/app15052458

ProQuest document ID

3176310249

Geochemical Background Values for the Characterization of Sediment Contamination with Trace Elements in Romania: A Case Study in the Lower Danube Basin

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Full Text

1. Introduction

2. Materials and Methods

2.1. Study Area

2.2. Sampling

2.3. Establishing the Geochemical Background

2.4. The Calculation of Pollution Indices

3. Results

3.1. Geoaccumulation Index I_geo

3.1.1. I_geo for As

3.1.2. I_geo for Cd

3.1.3. I_geo for Cr

3.1.4. I_geo for Cu

3.1.5. I_geo for Hg

3.1.6. I_geo for Ni

3.1.7. I_geo for Pb

3.1.8. I_geo for Zn

3.2. Nemerow Pollution Index PI_Nemerow

3.3. Potential Ecological Risk RI

3.4. Environmental Toxicity Quotient (ETQ)

4. Discussion

5. Conclusions

Abstract

Details

Suggested sources

Geochemical Background Values for the Characterization of Sediment Contamination with Trace Elements in Romania: A Case Study in the Lower Danube Basin

Jump to:

Full Text

1. Introduction

2. Materials and Methods

2.1. Study Area

2.2. Sampling

2.3. Establishing the Geochemical Background

2.4. The Calculation of Pollution Indices

3. Results

3.1. Geoaccumulation Index Igeo

3.1.1. Igeo for As

3.1.2. Igeo for Cd

3.1.3. Igeo for Cr

3.1.4. Igeo for Cu

3.1.5. Igeo for Hg

3.1.6. Igeo for Ni

3.1.7. Igeo for Pb

3.1.8. Igeo for Zn

3.2. Nemerow Pollution Index PINemerow

3.3. Potential Ecological Risk RI

3.4. Environmental Toxicity Quotient (ETQ)

4. Discussion

5. Conclusions

Abstract

Details

3.1. Geoaccumulation Index I_geo

3.1.1. I_geo for As

3.1.2. I_geo for Cd

3.1.3. I_geo for Cr

3.1.4. I_geo for Cu

3.1.5. I_geo for Hg

3.1.6. I_geo for Ni

3.1.7. I_geo for Pb

3.1.8. I_geo for Zn

3.2. Nemerow Pollution Index PI_Nemerow