Nitrogen Storage and Removal in Catchments on the Eastern Shore of Virginia

In this dissertation, I developed several approaches to understand how nitrogen is stored and removed from coupled groundwater-stream systems in the Mid-Atlantic Coastal Plain. I found that there is substantial storage of nitrogen in groundwater and that this storage results in lag times between nitrogen inputs and outputs ranging from decades to centuries. A major implication of this finding is that many of these catchments will experience large increases in groundwater nitrate concentrations in the coming decades, even if anthropogenic nitrogen inputs remain constant. Fortunately, not all nitrate in groundwater discharges to streams. Rather, a portion of nitrate is removed by microorganisms that naturally inhabit streambed and riparian zone sediments. The extent of nitrate removal that occurred in the streambed was a function of groundwater travel time, which varied widely in space. Greater removal occurred where groundwater moved more slowly through nitrate removal zones. Taking the stream as a whole, the extent of nitrate removal was describable by a single rate coefficient applied to high-flow regions of the streambed. Locations of lower flow were not important since they did not contribute significant amounts of nitrate to the stream. But even at a single location in the streambed, the rate of groundwater flow is not constant through time. I found that evapotranspiration can act as a pump in the riparian zone, drawing down the water table and driving patterns in groundwater travel time through the streambed. These patterns in groundwater travel time affected the extent of nitrate removal in the streambed and ultimately stream nitrate concentrations. Thus, the physical interactions between the climate, vegetation, groundwater, and streams determine the fate of nitrogen deposited on the land surface.