GROUNDWATER-SURFACE WATER INTERACTIONS AFFECTING WATER AND NITROGEN FLUXES IN AN AGRICULTURAL WATERSHED

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 45-hectare 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. Seeps were identified using IR thermal imaging. Calculated matrix flow from hydraulic head and conductivity measurements paired with differential stream gauging and in-stream injections of nitrate and chloride, 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. Air-water manometer readings indicated losing head gradients from the stream to groundwater; despite substantial (36-66%) increases in stream flow. Hydrochemical data suggest the stream is mostly disconnected from the underlying aquifer and seeps supply essentially all water and N to the system, with minor in-stream denitrification and plant uptake. Water isotope data reinforced the notion of seeps composing most of stream water and having shorter residence times than shallow groundwater. Seeps are likely sourced with N at nearby agricultural fields, and accelerated through the system with shorter residence times than shallow groundwater. 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.