LOW NITRATE AND RAPID HYDROCHEMICAL EVOLUTION OF RIVER AND GROUNDWATER DURING 2012 DROUGHT CONDITIONS, PLATTE RIVER, NEBRASKA

Hydrochemistry of shallow groundwater in the hyporheic zone (HZ) is affected by a complex range of hydrologic and biogeochemical interactions. Drought conditions can affect water quality by impacting any number of these interrelated processes. This study investigated the hydrochemical evolution of river water and shallow groundwater beneath the Platter River (near Grand Island, NE) over a three-month low-flow period in 2012. Seven riverbed piezometers monitored water levels and were sampled weekly in order to examine the evolution of major ions and stable isotopes through the drought. Results were compared with river water and ambient groundwater.

Nitrate concentrations remained primarily < 1 ppm throughout the monitoring period, in contrast to average concentrations ~4 ppm over the past 10 years. Redox indicators including ORP, dissolved oxygen, and dissolved organic carbon indicated reducing conditions favorable to denitrification in both riverbed and deeper sediments. As river and groundwater levels declined through the summer, temperature, specific conductance, SO₄^2-, total alkalinity, and ORP generally increased, while Cl^- and pH exhibited no clear trend. Values for δ¹⁸O and δ²H that were displaced from the meteoric water line indicate at least a portion of the solute enrichment is due to evapoconcentration, and that HZ water is a mixture of river and groundwater end members. The persistence of unusually low nitrate concentrations suggests that denitrification has removed nearly all of the river’s nitrate load during this low-flow period. Reduced groundwater discharge in low-flow conditions decreases freshening from groundwater influx, allows sufficient time for streambed denitrification to reduce nitrate loads, and amplifies the effect of convective heat flow on shallow groundwater temperatures.