Surface-Subsurface Exchange and Transient Storage in Relation to Riparian Corridor Land Cover in an Urban Watershed

A suite of methods is being utilized by the Baltimore Waters Test Bed to develop an understanding of the interaction between groundwater and surface water at multiple space and time scales. As part of this effort, bromide tracer experiments were conducted over two 10-day periods in August 2007 and May 2008 along two 900-m reaches of Dead Run, a small urban stream located in Baltimore County, Maryland, to investigate the influence of distinct zones of riparian land cover on surface-subsurface exchange and transient storage under low- and high-baseflow conditions. Riparian land cover varied by reach along a gradient of land use spanning parkland, suburban/residential, and commercial, and included wooded, meadow, turf grass, and impervious cover. Reach-average surface-subsurface exchange was estimated using tracer-dilution and velocity-gaging stream flow measurements. Transient storage parameters (active storage area and storage exchange rate coefficient) were estimated using the USGS-code OTIS. Bed sediment grain size distribution was found to be unrelated to watershed position and riparian cover; however, a trend of finer bed sediment under low baseflow conditions was observed at most stations. In the lower section of Dead Run, gross inflow, gross outflow and surface-subsurface exchange (defined as the absolute value of net inflow) tended to be higher under high baseflow conditions (May 2008) compared to low baseflow conditions (August 2007). Surface-subsurface exchange was highest in reaches with greater riparian pervious cover while transient storage area and exchange rate were highest in reaches with greater impervious cover. Results from the tracer test are being incorporated into a coupled watershed model of the region. When working in urban watersheds, many complexities not typically encountered in more pristine areas must be addressed. Among these are high ambient salt concentrations (> 200 mg/L chloride), unexpected (and transient) stream discharges and withdrawals, and potential interactions with the public and police.