Quantifying the fate of agricultural nitrogen in an unconfined aquifer: Stream-based observations at three measurement scales

Authors

Troy E. Gilmore, University of Nebraska-LincolnFollow
David P. Genereux, North Carolina State UniversityFollow
D. Kip Solomon, University of UtahFollow
John E. Solder, Utah Water Science Center
Briant A. Kimball, Utah Water Science Center
Helena Mitasova, North Carolina State University
Francois Birgand, North Carolina State University

Document Type

Article

Date of this Version

2016

Citation

Gilmore, T. E., D. P. Genereux, D. K. Solomon, J. E. Solder, B. A. Kimball, H. Mitasova, and F. Birgand (2016), Quantifying the fate of agricultural nitrogen in an unconfined aquifer: Stream-based observations at three measurement scales, Water Resour. Res., 52, 1961– 1983, doi:10.1002/2015WR017599.

Comments

U.S. government work.

Abstract

We compared three stream-based sampling methods to study the fate of nitrate in groundwater in a coastal plain watershed: point measurements beneath the streambed, seepage blankets (novel seepage-meter design), and reach mass-balance. The methods gave similar mean groundwater seepage rates into the stream (0.3–0.6 m/d) during two 3–4 day field campaigns despite an order of magnitude difference in stream discharge between the campaigns. At low flow, estimates of flowweighted mean nitrate concentrations in groundwater discharge ([NO^-₃ ]_FWM) and nitrate flux from groundwater to the stream decreased with increasing degree of channel influence and measurement scale, i.e., [NO^-₃ ]_FWM was 654, 561, and 451 mM for point, blanket, and reach mass-balance sampling, respectively. At high flow the trend was reversed, likely because reach mass-balance captured inputs from shallow transient high-nitrate flow paths while point and blanket measurements did not. Point sampling may be better suited to estimating aquifer discharge of nitrate, while reach mass-balance reflects full nitrate inputs into the channel (which at high flow may be more than aquifer discharge due to transient flow paths, and at low flow may be less than aquifer discharge due to channel-based nitrate removal). Modeling dissolved N₂ from streambed samples suggested (1) about half of groundwater nitrate was denitrified prior to discharge from the aquifer, and (2) both extent of denitrification and initial nitrate concentration in groundwater (700–1300 mM) were related to land use, suggesting these forms of streambed sampling for groundwater can reveal watershed spatial relations relevant to nitrate contamination and fate in the aquifer.