Transport of nitrogen rich groundwater to surface waters by riparian macropore flow in an agriculturally dominated watershed

Elaine Langone Center, Forum

Nitrogen (N) pollution continues to be a major concern in the Chesapeake Bay watershed due to its potential contributions to climate change, negative health effects, and the decline in quality of aquatic ecosystems. Groundwater in Pennsylvania can have elevated concentrations of dissolved N that can migrate to surface waters, either by diffuse discharge through the streambed or by macropore flow in the riparian zone. These riparian seeps show significant variability in both discharge and nutrient concentrations. In this study, we use stream measurements to differentiate and quantify contributions of groundwater discharge from matrix flow in the streambed and riparian seeps. A combination of differential stream gauging, streambed measurements of hydraulic head and conductivity, and water chemistry, were used to solve for riparian groundwater flux using a reach mass balance equation. A 175m stream reach was identified in a heavily cultivated 45 hectare watershed in east-central Pennsylvania in the headwaters of the Susquehanna River. Despite air-water manometers readings from piezometers installed in the shallow streambed (30 cm) indicating a losing reach, discharge substantially increased (36-66%) throughout the reach consistently throughout the study period. Using the mass balance approach, riparian groundwater fluxes contributed 85-206 m 3 d -1 of water, while transporting 1.48- 3.98 kg N d-1 through this fractured aquifer system. Throughout the winter/spring recharge period (Jan-May), there was a constant increase in both flow and N transport from these macropore riparian seeps. Chemical data for the stream, streambed, and shallow ground waters suggest that the stream is disconnected from the underlying aquifer and that groundwater riparian seeps supply essentially all of the water and N to the system. The results from this study, and water chemistry comparison between stream, shallow groundwater, deep groundwater, and riparian seeps, provide insight into the sources of these inputs and help determine the transport and fate of N in a fractured system. This information is important for planning mitigation techniques and best management practices. Further analysis of water isotopes and hydrometric data will be used to test our hypothesis of a perched stream disengaged from the aquifer below.