EXTENSIVE AQUITARD IN HYBLA VALLEY (NORTHERN VIRGINIA) SEPARATES NEAR-SURFACE WETLAND HYDROLOGY FROM FLOW IN DEEPER AQUIFER

Huntley Meadows Park, situated within the Hybla Valley of the northern Virginia Coastal Plain, is bounded by steep margins of an apparent Pleistocene meander scar along the Potomac River. The 1,557 acre park contains more than 900 acres of freshwater emergent and forested wetlands. Deterioration of the wetlands led to a restoration-and-expansion project completed by Wetland Studies and Solutions Inc. during 2014. Pre-construction analyses did not evaluate the strength of hydrologic connection between regional groundwater flow and surficial wetland dynamics. Thus a major focus of the present research is to determine the groundwater recharge-discharge patterns across the Hybla Valley and understand how they affect wetland hydrology within Huntley Meadows Park. Building on previous studies, our geomorphic and stratigraphic analyses document a complex suite of aquitards and aquifers, produced by terrestrial and estuarine processes over several glacial-interglacial cycles. Hydrograph data and modeling results gathered to date indicate there is an extensive aquitard effectively separating the wetland from a deeper confined aquifer. Slug tests suggest that the hydraulic conductivity of the confined aquifer is much greater than that of the unconfined aquifer. Additional research aims to measure changes in hydrologic and vegetative interaction produced by the wetland expansion. Data from an array of shallow wells placed in different hydrologic and vegetative settings, analyzed using White’s Diurnal Flux Method (1932), reveal spatial variations in evapotranspiration. Comparisons of these evapotranspiration data with those from a Bowen’s Ratio meteorological station and rates calculated using Penman-Monteith and Thornthwaite equations will demonstrate the effectiveness of these different methods. The results of this study will be used to test the Wetbud wetland water-budget model.