DISSOLVED NITROGEN CONCENTRATION GRADIENTS IN KARST WATERS AT CLOVER HOLLOW, GILES COUNTY, VA

Runoff intercepting synthetic fertilizers and manure contributes excess nutrients (nitrogen and phosphorous) to streams and shallow groundwater. This can lead to eutrophication of surface water bodies, altering available oxygen and nutrient supply to aquatic ecosystems. As a result, there are many studies regarding processes controlling nitrogen fate through freshwater systems. Aquifers in karst landscapes are particularly susceptible to nutrient pollution from both direct infiltration and concentrated surface runoff as soil water and streams flow quickly into the subsurface through tertiary porosity within the bedrock. In this study, we investigate dissolved nitrogen concentration gradients in three different sinking streams that flow through and underneath farmland in Clover Hollow, a karst valley in southwest Virginia (Giles County, VA). The streams selected for sampling have dominantly forested headwaters and flow through and in cave systems underlying farmland for several kilometers to a defined spring emerging from Smokehole Cave. Grab samples were collected along multiple transects within surface and subsurface streams in October, 2018 for the analysis of Total Dissolved Nitrogen (TDN).

Results show an increasing gradient in TDN concentrations as streams flow from the forested headwaters, then through and beneath the farmlands. Samples collected from the upper reaches of the watershed yielded the lowest concentrations (0.1-0.6 mg/L), and concentrations increase as surface streams enter farmland (0.4-0.8 mg/L). Samples collected from within Smokehole Cave, closer to the outlet of the watershed, yielded the highest TDN concentrations (1.6-2.2 mg/L). Although the absolute concentrations are low there is an order of magnitude increase in TDN concentrations from the headwaters to the watershed outlet. These preliminary results show that in the karst of the study area nutrient contamination from agricultural runoff can be transmitted directly from the surface into the subsurface via both sinking streams and diffuse infiltration through the epikarst. Future work will incorporate a temporal component through an event-based sampling regime to characterize nitrogen flux under different flow conditions when the subsurface is expected to receive substantial runoff from the surface.