Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2014
Publication Date: 2/25/2014
Citation: Williams, M.R., Buda, A.R., Elliott, H., Hamlett, J., Boyer, E.W., Schmidt, J.P. 2014. Groundwater flow path dynamics and nitrogen transport potential in the riparian zone of an agricultural headwater catchment. Journal of Hydrology. 511:870-879. Interpretive Summary: Managing nitrogen losses from agricultural landscapes requires an understanding of how hydrology influences nitrogen delivery to streams. In this study, we evaluated surface and subsurface nitrogen flow pathways in the riparian zone of a headwater agricultural watershed. Nitrogen concentrations were greatest in areas of riparian groundwater seepage, which were hydrologically connected to upslope fields receiving fertilizers and manures. Findings illustrate the importance of targeting seepage areas for remedial practices that minimize nitrogen transfers to surface waters.
Technical Abstract: Stream riparian zones are often thought of as areas that provide natural remediation for groundwater contaminants, especially agricultural nitrogen (N). While denitrification and vegetative uptake tend to be efficient N removal processes in slow moving shallow groundwater, these mechanisms decrease in effectiveness as faster flows through soil macropores and other preferential flow pathways become dominant. The objective of our study was to characterize N concentration variability and hydrologic transport pathways through shallow groundwater draining areas of the riparian zone with and without emergent groundwater seeps. The study was conducted within FD36, an agricultural headwater catchment in the Ridge and Valley physiographic region of central Pennsylvania, USA. Three seep and adjacent non-seep areas were instrumented with a field of 40 piezometers installed in a grid pattern (1.5-m spacing) at both 20- and 60-cm depths. The piezometers were monitored seasonally for a period of approximately two years (Oct. 2010 – May 2012). Results showed that water table depths within seep areas were variable and some regions in the seep areas exhibited positive vertical hydraulic gradients of 5 to 10 cm. Non-seep areas were characterized by a uniform water table surface and were relatively hydrostatic. Discharge of groundwater onto the land surface was also common in seep areas, but was not observed in non-seep areas. Ammonium-N concentrations were mostly low (less than 0.1 mg L-1) and relatively similar between seep and non-seep areas at each of the three study sites. In contrast, nitrate-N (NO3-N) concentrations in seep areas were significantly greater than those measured in the non-seep areas at two of the study sites. A two-component mixing model using chloride (Cl-) as a conservative tracer indicated that shallow groundwater in seep areas of FD36 was primarily (53 – 75%) comprised of water from a fractured aquifer (less than 6-m deep), which had elevated NO3-N concentrations (mean: 5.7 mg L-1). Shallow groundwater in non-seep areas, however, were comprised (58 – 82%) of perched water on top of the fragipan (less than 1-m deep) that was likely recharged locally in the riparian zone and had low NO3-N concentrations (mean: 0.6 mg L-1). Higher NO3-N concentrations, variable water table depth, and groundwater emergence onto the land surface in seep areas provided evidence that preferential flow paths were an important conduit for water and N movement in these areas of the riparian zone. As a result, we concluded that the potential for N delivery to the stream in FD36 was much greater from seep areas compared to non-seep areas. Targeted management of seeps should be seen as a priority in future efforts to reduce NO3-N loss from headwater agricultural catchments.