INTERACTION OF GROUNDWATER AND CREEK WATER BENEATH A TIDAL MARSH SYSTEM, PORTSMOUTH, VIRGINIA

Groundwater discharge into Hoffler Creek, a tidal stream draining to the James River, passes below and through a salt marsh as it travels from the upland to the creek channel. In order to evaluate the physical and chemical interactions between the marsh ecosystem, groundwater, and saline creek water, we characterized aquifer sediments and constructed monitoring wells and piezometers along a transect between the upland and the creek. Under both the high marsh and low marsh surfaces lies less than 50 cm of anoxic organic mud overlying more than a meter of a sand and gravel unit. The coarse beds, interpreted as a probable braided stream deposit, contain armored mud balls and a few thin mud layers, as well as gravelly zones weakly cemented with iron oxides. Head, nitrate and salinity data from wells indicate that groundwater flowing at the base of the upland aquifer passes beneath the high marsh through the coarse gravelly sand and then seeps upward to pass through the low marsh mud. Nitrate concentrations, derived in part from rainwater and present in the upland aquifer, decrease markedly as groundwater seeps upwards through the organic mud of the low marsh. Salinity values from wells suggest that mud lenses in the coarse aquifer below the high marsh hydraulically isolate that portion of the aquifer, preventing upland groundwater from flushing brackish creek water out of sediments immediately below the high marsh surface. Along another transect of wells nearby, where Hoffler Creek directly interacts with the upland aquifer, head fluctuations driven by creek water oscillations pass laterally to wells 30 m from the creek; at the site studied, due to the buffering effects of the high marsh, almost no tidal head change occurs in upland wells. Estimated groundwater flux discharging from the upland aquifer into the marsh system, calculated with both water budget values and Darcy’s Law, is approximately 250 m³/yr per meter of shoreline.