Groundwater contributions of flow and nitrogen in a headwater agricultural watershed

Authors: REDDER, BRIAN - Pennsylvania State University; Kennedy, Casey; Buda, Anthony; Folmar, Gordon; BOYER, ELIZABETH - Pennsylvania State University

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/6/2021
Publication Date: 4/16/2021
Citation: Redder, B., Kennedy, C.D., Buda, A.R., Folmar, G.J., Boyer, E.W. 2021. Groundwater contributions of flow and nitrogen in a headwater agricultural watershed. Hydrological Processes. 35(5):e14179. https://doi.org/10.1002/hyp.14179.
DOI: https://doi.org/10.1002/hyp.14179

Interpretive Summary: Nitrogen (N) pollution is a longstanding issue having negative environmental consequences within the Chesapeake Bay watershed. A team of researchers from Pennsylvania State University and USDA-ARS sought to quantify N transport from groundwater seeps, a poorly understood flowpath that may represent the largest N input to headwater streams in the Chesapeake Bay watershed. Multiple lines of evidence suggest that seeps supply essentially all water and N to headwater streams that are disconnected from the underlying aquifer. Identifying source areas that are causing seep discharges is beneficial for developing N pollution mitigation strategies and implementing management practices that aim to reduce nutrient loads to the Chesapeake Bay.

Technical Abstract: Nitrogen (N) pollution is a longstanding issue having negative environmental consequences within the Chesapeake Bay watershed and in other basins around the world. Human activities associated with agricultural practices account for a large percentage of the N pollution delivered to the Bay. This work aims to improve understanding of N transport from ground to surface waters, quantifying the principal hydrological processes driving water and N fluxes into and out of a stream reach. The study site is a 175-m stream reach in a heavily cultivated 40-ha watershed in east-central Pennsylvania. This subwatershed receives most of its base flow from groundwater, either by diffuse matrix discharge through the streambed or by localized discharge through riparian seeps. Samples of stream, seep, and shallow groundwater were collected approximately monthly under base flow conditions in 2017. Calculated matrix flow from hydraulic head and conductivity measurements paired with differential stream gauging were used to solve for the riparian seep flux using a mass balance approach. Riparian seep fluxes ranged from 45-216 m3 d-1, transporting 0.6-4.1 kg N d-1 of nitrate-N from the fractured bedrock aquifer to the stream. Hydrochemical data suggest the stream is mostly disconnected from the underlying aquifer and seeps supply essentially all water and N to the system. Seeps are likely sourced with N in nearby agricultural fields and accelerated through the system with shorter residence times than shallow groundwater. Water isotope data reinforced this notion. This study underscores the importance of agriculture as a source of N to ground and surface waters. Identifying source areas that are causing groundwater enrichment of N and seep areas where N discharges to streams is beneficial for developing N pollution mitigation strategies and implementing management practices that aim to reduce nutrient loads to the Chesapeake Bay.