CROSS-SECTIONAL ANALYSIS AND INFILTRATION RATES 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 coast. 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. Nutrient flux is controlled by the magnitude of water flux, but the determination of water flux is confounded by inadequate rating curves. At 4 streams, continuous measurement (10-min intervals) of water-surface elevation was accomplished using submerged pressure transducers in stilling wells installed on the streambank. Rating curves developed from instantaneous discharge measurements using a hand-held flow meter failed to predict reasonable discharge at high stage, and a thorough examination of stream-channel morphology was undertaken to explore actual stream cross sections for various recorded stages. Cross sections were measured at regular intervals downstream of a culvert used as a control structure for gauging purposes with a laser level and tape and with a total station. Putting together the cross sections at each stream, a model of the stream that extended into the floodplain was developed. Using the stage record with the detailed stream geometry, the areal extent and depth as well as the length of time that streamwater inundates the floodplain were modeled. Microbially mediated denitrification in subsurface water can dramatically reduce the amount of nitrate flowing from watersheds. With new insights into stream access to floodplain areas during high-flow events, we explored the potential for bank infiltration and denitrification in riparian-zone sediments. Infiltration rates were measured using an infiltrometer to quantify the magnitude of surface-water input to the subsurface during high-stage events. Using the annual stage hydrograph, we demonstrate that bank infiltration during storm events and subsequent denitrification may contribute to overall nitrate reduction, although the magnitude of the effect on overall nitrate flux is small in these very flashy streams where the predominant environment for nitrate reduction is in the organic-rich streambed sediments where groundwater discharges to the stream.