Topographic Models and Infiltration Rate Analysis: Evaluating Extent of Potential Groundwater Denitrification of Low-Relief Streams Draining Watersheds of the Atlantic Coastal Plain

Nitrate flux from agricultural watersheds via streams impairs coastal water quality on the Atlantic shoreline. On Virginia’s Eastern Shore, low-relief gaining streams drain 54 small agricultural watersheds (80-1100 ha) that extend from the center of the Delmarva Peninsula to coastal lagoons and the Atlantic Ocean. Microbially mediated denitrification in subsurface water can dramatically reduce the amount of nitrate flowing from watersheds. Stage hydrographs were developed for four streams using continuous measurements of water-surface elevation in stilling wells, and median and maximum stage was identified. Putting together cross sections derived from hand measurements in the field at each stream, a topographic model of the stream bank, floodplain, and highlands was developed using GIS software. Stage record was combined with stream geometry to illustrate the areal extent that stream water inundates the floodplain following rain events. The increases in inundation area between median and high stage varied greatly among the streams, from a high of 1247% to a low of 46%. Some streams had significant reaches of dry channel over the year, and their water-covered areas increased the most with rain events. With new insights into stream access to floodplain areas during high-flow events, the potential for bank infiltration in riparian-zone sediments was explored. Infiltrometer measurements were collected in a manner that supported quantification of the comparative potential of surface-water input to the subsurface under different conditions, including location of stream, type of landscape, and vegetative cover of soil. The average hydraulic conductivity of the soils at each stream differed, but the hydraulic conductivities were on average 60% greater in the floodplain compared to in the stream bank, and vegetation cover had no significant effect. These results suggest that the floodplain, established by the topography, has the highest potential for infiltration and subsequent potential for denitrification.