Delineation of Saltwater Intrusion Through a Coastal Borrow Pit by Resistivity Survey

Abstract

Dewatering of a borrow pit in Portsmouth, VA, induced localized saltwater intrusion into a sandy coastal aquifer during excavation of the surficial deposit. More than a decade after mining operations ceased, brackish water still contaminates residential wells and the pit lake despite the expectation that recharge would lower the salinity of the wells and the lake over time. Twenty-seven Schlumberger resistivity soundings and two Wenner profiles were taken around the lake and salinity measurements of surface and well water were collected at 27 locations in order to delineate the extent of brackish groundwater in the surficial aquifer. Interpretation of these data, when combined with information from prior limnological and geological studies, indicates the presence of a relatively saline, confined aquifer beneath the lake. This aquifer is connected to the lake only where a deep depression on the lake bottom penetrates through the confining bed overlying the aquifer. The only continuous source of chloride for the lake is brackish water in that depression. Fresh groundwater in the shallow aquifer becomes brackish as it passes through the lake, eventually reentering the aquifer, where it contaminates the residential wells.