Groundwater flow patterns in extensive shallow aquifers with gentle relief: Theory and application to the Galena/Locust Grove region of eastern Maryland

Abstract

The Delmarva Peninsula between the Chesapeake Bay and the Atlantic Ocean comprises an irregular sequence of almost horizontal sand and silt or clay layers [Shedlock et al., 1999; Böhlke and Denver, 1995] whose connectivity and detailed geometry are not well known. A mosaic of groundwater basins with areas generally less than 30 km2 provide the water supply for the region. The concentration of dissolved nitrate has increased greatly since 1950 [Shedlock et al., 1999], and a series of test wells, drilled in the Locust Grove area for chemical analyses of the groundwater, provide useful data on water table levels at various locations. A new depth‐integrated hydraulic model of the aquifer flow is developed to handle the complex and poorly known structure of the region. It requires as inputs (1) historical rainfall data, (2) the elevation profiles along potential discharge streambeds, and (3) the depth‐integrated hydraulic conductivity, inferred from a few individual well water levels. The analysis predicts quantitatively and without arbitrary parameters, the distribution of groundwater heights above mean sea level and the patterns of total groundwater flow throughout the study region. Throughout the test area the predicted and measured groundwater levels generally agree to within 1 meter. The groundwater flow patterns show two distinctive types of domain with little intermingling, one type originating near the groundwater divide and discharging into the semipermanent streams and the other kind arising between the lower stream sections and discharging directly along the estuary shoreline. The measured distribution of water age with depth [Böhlke and Denver, 1995] agrees well with a simple formula provided by the theory.