Quantifying an aquifer nitrate budget and future nitrate discharge using field data from streambeds and well nests

Authors

Troy E. Gilmore, University of Nebraska-LincolnFollow
David P. Genereux, North Carolina State UniversityFollow
Kathleen M. Farrell, North Carolina Geological SurveyFollow
Helena Mitasova, North Carolina State UniversityFollow

Document Type

Article

Date of this Version

11-26-2016

Citation

Gilmore, T. E., D. P. Genereux, D. K. Solomon, K. M. Farrell, and H. Mitasova (2016), Quantifying an aquifer nitrate budget and future nitrate discharge using field data from streambeds and well nests, Water Resour. Res., 52, 9046–9065, doi:10.1002/2016WR018976.

Comments

Copyright 2016. American Geophysical Union. Used by permission.

Abstract

Novel groundwater sampling (age, flux, and nitrate) carried out beneath a streambed and in wells was used to estimate (1) the current rate of change of nitrate storage, dS_NO3 /dt, in a contaminated unconfined aquifer, and (2) future [NO₃^–]_FWM (the flow-weighted mean nitrate concentration in groundwater discharge) and f_NO3 (the nitrate flux from aquifer to stream). Estimates of dS_NO3 /dt suggested that at the time of sampling (2013) the nitrate storage in the aquifer was decreasing at an annual rate (mean = –9 mmol/m²yr) equal to about one-tenth the rate of nitrate input by recharge. This is consistent with data showing a slow decrease in the [NO₃^–] of groundwater recharge in recent years. Regarding future [NO₃^–]_FWM and f_NO3 , predictions based on well data show an immediate decrease that becomes more rapid after ~5 years before leveling out in the early 2040s. Predictions based on streambed data generally show an increase in future [NO₃^–]_FWM and f_NO3 until the late 2020s, followed by a decrease before leveling out in the 2040s. Differences show the potential value of using information directly from the groundwater—surface water interface to quantify the future impact of groundwater nitrate on surface water quality. The choice of denitrification kinetics was similarly important; compared to zero-order kinetics, a first-order rate law levels out estimates of future [NO₃^–]_FWM and f_NO3 (lower peak, higher minimum) as legacy nitrate is flushed from the aquifer. Major fundamental questions about nonpoint-source aquifer contamination can be answered without a complex numerical model or long-term monitoring program.