|Singha, Kamini - Colorado School Of Mines|
|Elliott, Herschel - Pennsylvania State University|
|Schmidt, John - Dupont Company|
Submitted to: Groundwater
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/13/2016
Publication Date: 9/12/2016
Citation: Williams, M.R., Buda, A.R., Singha, K., Folmar, G.J., Elliott, H.A., Schmidt, J.P. 2016. Imaging hydrological processes in headwater riparian seeps with time-lapse electrical resistivity. Groundwater. 55(1):136-148. doi:10.1111/gwat.12461.
Interpretive Summary: Identifying areas of the landscape responsible for the greatest nitrogen loss to surface waters is essential to protecting water quality in agricultural watersheds. We investigated the hydrology and nitrogen transport potential of two contrasting areas within the riparian zone of a headwater stream. Areas with emergent groundwater seepage were directly connected to a regional aquifer with elevated nitrogen concentrations derived from fertilizer and manure inputs, whereas areas without groundwater seepage lacked such a connection. Findings demonstrate the potential to target areas of riparian zone seepage in order to reduce nitrogen losses from agricultural landscapes.
Technical Abstract: The activation of subsurface seepage in response to precipitation events represents a potentially important pathway of nitrogen (N) delivery to streams in agricultural catchments. We used electrical resistivity imaging (ERI) and shallow piezometers to elucidate how seep and non-seep areas within the riparian zone responded to a series of three precipitation events that occurred over a one-month period. Two riparian study areas were established on soil possessing a fragipan in FD36, an agricultural catchment (40 ha) in central Pennsylvania. On six occasions, ERI data were collected across 32 electrodes in both areas. Forty piezometers in each area were also monitored for water table depth and N chemistry. While soil conditions prior to the first rainfall event were dry, the rainfall amount (41 mm) was sufficient to activate subsurface discharge within the seep. In contrast, the non-seep area showed no such response. ERI data showed localized decreases in resistivity (30-40%) within the seep area, suggesting groundwater was upwelling through the fragipan. Following two additional rainfall events (100 mm), both seep and non-seep areas were saturated, with overland flow visible within the seep. Seep NO3-N concentrations averaged 1.4 mg L**-1 when the subsurface initially became hydrologically active, but increased to 5.0 mg L**-1 as upwelling groundwater expanded the seep zone. Mean NO3-N concentration in the non-seep area was 0.2 mg L**-1. Results from ERI and hydrometric monitoring show that seep areas are dynamic and responsive to rainfall events compared to non-seep areas, and that subsurface hydrologic processes in seeps can significantly affect N losses from the landscape to the stream.