ABSTRACT In agricultural landscapes, increasing groundwater nitrate concentrations are common and reflect leaching from cultivated soils, often into adjacent riparian zones within stream corridors. High nitrate concentrations may be attenuated in riparian groundwater, where abundant organic matter and saturated anoxic soils and sediments (collectively “substrate”) support denitrifying activity. Variable substrate and the resulting residence time distribution in shallow groundwater can drive redox status and net nitrate removal, yet can be challenging to simulate in detail. This study explores how spatial variation in the texture of riparian aquifer substrate may influence groundwater residence times and biogeochemical behaviour of a riparian aquifer subject to chronic nitrate loading from non‐irrigated wheat production in the semiarid Northern Great Plains. This is addressed using a novel combination of physicochemical measurements, geophysical observation of a groundwater tracer injection, and a simplified groundwater mixing model analysis. Higher and more variable nitrate concentrations were documented in wells completed in coarser substrates, suggesting generally shorter residence times compared to finer substrates, which exhibited lower nitrate concentrations suggestive of longer residence times. Therefore, we hypothesised that net nitrate consumption could be captured with a simple simulation approach using (a) the proportion of finer‐textured riparian aquifer substrate to quantify redoximorphic processes that result in net nitrate consumption and (b) the proportion of coarser riparian substrate to quantify the groundwater residence time distribution. We tested this hypothesis by first exploring spatial patterns in groundwater chemistry and hydraulic characteristics at 16 shallow (< 1.5 m) wells and then by directly observing residence time of solutes in groundwater flow, using high frequency monitoring of groundwater specific conductivity and time‐lapse electrical resistivity tomography imaging of a cross‐section of a riparian groundwater flow path. Mixing models informed by geophysical imaging and tracer breakthrough constrain the potential influence of exchanges between fine and coarse substrates on the net nitrate transformation occurring along riparian groundwater flow paths. Dual‐textured groundwater mixing model simulations illustrate how the proportion of coarse textured material may dictate the total amount of flow through riparian substrate while the proportion of flow through the finer‐textured material that mixes with flow through coarse material may dictate the extent of net nitrate consumption processes. This work leverages novel geophysical observations to contribute a simple bimodal approach exploring how hydrologic complexity in riparian subsurface flow systems may influence the overall potential of riparian groundwaters to process nutrients before watershed export.
Poole et al. (Sun,) studied this question.