Quantifying background nitrate removal mechanisms in an agricultural watershed with contrasting subcatchment baseflow concentrations

Journal of Environmental Quality
By: , and 

Links

Abstract

Numerous studies have documented the linkages between agricultural nitrogen loads and surface water degradation. In contrast, potential water quality improvements due to agricultural best management practices are difficult to detect because of the confounding effect of background nitrate removal rates, as well as the groundwater-driven delay between land surface action and stream response. To characterize background controls on nitrate removal in two agricultural catchments, we calibrated groundwater travel time distributions with subsurface environmental tracer data to quantify the lag time between historic agricultural inputs and measured baseflow nitrate. We then estimated spatially distributed loading to the water table from nitrate measurements at monitoring wells, using machine learning techniques to extrapolate the loading to unmonitored portions of the catchment to subsequently estimate catchment removal controls. Multiple models agree that in-stream processes remove as much as 75% of incoming loads for one subcatchment while removing <20% of incoming loads for the other. The use of a spatially variable loading field did not result in meaningfully different optimized parameter estimates or model performance when compared with spatially constant loading derived directly from a county-scale agricultural nitrogen budget. Although previous studies using individual well measurements have shown that subsurface denitrification due to contact with a reducing argillaceous confining unit plays an important role in nitrate removal, the catchment-scale contribution of this process is difficult to quantify given the available data. Nonetheless, the study provides a baseline characterization of nitrate transport timescales and removal mechanisms that will support future efforts to detect water quality benefits from ongoing best management practice implementation.

Study Area

Publication type Article
Publication Subtype Journal Article
Title Quantifying background nitrate removal mechanisms in an agricultural watershed with contrasting subcatchment baseflow concentrations
Series title Journal of Environmental Quality
DOI 10.1002/jeq2.20049
Volume 49
Issue 2
Year Published 2020
Language English
Publisher American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
Contributing office(s) WMA - Integrated Modeling and Prediction Division
Description 12 p.
First page 392
Last page 403
Country United States
State Maryland
Other Geospatial Upper Chester study area
Google Analytic Metrics Metrics page
Additional publication details