An Investigation of Focused Aquifer Recharge in the Jornada Experimental Range Using Chloride as a Tracer

Abstract

Recently published research has suggested that up to 20% of precipitation in a small (0.05 km2) piedmont watershed in the Jornada Experimental Range of southern New Mexico becomes deep percolation that leads to aquifer recharge. This finding, based on a watershed mass balance approach measuring precipitation, runoff, evapotranspiration, and soil water storage change through time, challenges a prevalent understanding of Chihuahuan Desert ecohydrology that creosote shrubland prevents recharge. The potential management implications of this finding are consequential. Here I report on research designed to test for the hypothesized focused recharge in small, first-order channels by coring the sediment under two such channels and analyzing the chloride distribution with depth. If abundant recharge occurs then chloride should be flushed through the sediment to the groundwater, but if it does not then an accumulation, or bulge, of chloride should form at the most frequent depth of infiltration. To test the methodology, I also took two cores from flat interfluvial areas far from channels and one core from a 4-m-wide cobble-bed channel that almost certainly causes focused recharge. In addition to measuring chloride in the cores, I also measured gravimetric water content, soil water potential, and chloride to bromide ratios.

The interfluve cores showed chloride concentrations that rose rapidly within70 cm of the ground surface and remained high (8,000 mg/kg of pore water)through the remainder of the core, which confirms that chloride has accumulated in these non-recharging portions of the landscape. Water content was extremely low, with typical matric potential values of 6 MPa and a minimum value of 8.83MPa, equivalent to 900 m of head. Collection of the large channel core was inhibited by the coarse nature of the channel sediment and most material was lost from the core barrel in retrieval, but the sediment that was obtained had an average chloride concentration of 100 mg/kg and had a matric potential of 0 MPa, which is effectively saturated.

One channel core, Channel-C, was generally similar to the interfluve cores, with average chloride concentrations of 8,000 mg/kg of pore water. However, the chloride was displaced downward, beginning to accumulate at 1.0 m depth, and below this the concentration oscillated from 1,000 to 13,000 mg/kg for 1.5 m before stabilizing. Soil water potential also oscillated in unison with the chloride concentration, from 0 to -7 MPa. The other channel core, Channel-A, had 0MPa water potential and low chloride concentration down to 3 m depth. Below this, chloride concentration rose to 3000 mg/kg and water potential decreased to -2 MPa and remained at these approximate values to the end of the core at5 m depth. Core Channel-A was taken at a unique position on the landscape, where a road dammed the channel and caused ponding during runoff events. This road was built 48-77 years before the study, based on time-bracketing air photos. My interpretation is that while Channel-C is in steady state, with root extraction of soil water in balance with infiltration, Channel-A exhibits a transient wetting front, with plants still expanding to catch up with the newly available water supply. Yet even in this unique location, the augmented infiltration has not removed the chloride accumulation from the underlying soil, suggesting that percolation has not yet infiltrated beyond the root zone.

These observations are limited to single points on the landscape, and do not preclude the possibility of lateral or preferential flows. However, the extremely negative matric potentials I observed have been associated with limited preferential flow by previous workers. I suggest that the chloride profiles are a more robust indicator of infiltration processes than the watershed mass balance approach. In particular, the location of the evapotranspiration measuring equipment appears to have produced a sampling footprint that excludes the enhanced transpiration along the banks of the small channels within the watershed. I recommend that water planners refrain from incorporating small-catchment focused recharge from Chihuahuan Desert piedmont slopes into their resource assessments.