DELINEATION OF SALINE GROUNDWATER INTRUSION THROUGH A COASTAL BORROW PIT BY RESISTIVITY SURVEY

Hoffler Lake in Portsmouth, Virginia fills a borrow pit located 350 m from a brackish estuary. Pumping during excavation of the pit induced localized saltwater intrusion into the surficial (Columbia) aquifer and the shallow confined (Yorktown) aquifer. More than a decade after mining operations ceased, brackish water still contaminates the pit lake and a few residential wells pumping in the upper aquifer. Data from well water analyses and interpretation of Wenner resistivity profiles and Schumberger soundings around the pit lake permit delineation of the extent of brackish groundwater in the surficial aquifer. Fresh water fills most of the surficial aquifer but brackish water exists in the surficial aquifer immediately below and next to the lake, and in a broader area between the western end of the lake and nearby estuaries. Only the lowermost portions of the surficial aquifer in this area contain brackish water. Resistivity data for the top of the Yorktown formation suggest that brackish water fills those strata under most, if not all, of the study area. The only continuing source of chloride for the lake seems to water in an unusually deep depression on the lake bottom, the only portion that penetrates into the Yorktown aquifer. Wolny’s (1999) bathymetric and limnologic data indicate the lake is stratified with the highest salinity waters residing in the depression. Although the entire lake is brackish, water below -9 m depth is consistently anoxic, colder, and more brackish. Apparently brackish water seeping into the lower portions of the lake diffuses upward and mixes laterally to disperse salts through the lake. Fresh groundwater enters the lake from the shallow aquifer along the eastern margin, becomes brackish as it passes through the lake, and seeps along the bottom of the surficial aquifer as it moves westward.