SOURCES OF SALINITY IN A BRACKISH COASTAL-ZONE BORROW-PIT LAKE, PORTSMOUTH, VIRGINIA

Lake Ballard is a stratified oligotrophic pond in a former borrow pit dug close to a large tidal creek and the Chesapeake Bay. Now up to 13 m deep, the bottom of Lake Ballard penetrates the sandy surficial aquifer and shelly beds in the semi-confined Yorktown aquifer. Dewatering pulled salty water through these aquifers during pit excavations that ceased in the late 1980's. Occasional measurements of salinity taken since then indicate the surface of Lake Ballard remains consistently brackish despite flushing by groundwater and rainfall. To identify possible sources of the continuing salinity, water samples were collected from pore water of borehole samples, wells drilled into the surficial and the Yorktown aquifers, the tidal creek, and the deepest portions of Lake Ballard. Vertical profiles taken along transects across the lake include data on depth, temperature, salinity, and dissolved oxygen as well as water chemistry. Water samples were analyzed for alkalinity, chloride, sulfate, and chlorophyll a. These profiles (May-June 2006) indicate the presence of a metalimnetic oxygen maximum at 8 m depth and a chlorophyll a maximum at 10 m depth. Bottom waters have higher salinity and are markedly cooler, anoxic, sulfidic, and strongly alkaline below approximately 10 m depth. The decreased sulfate and increased alkalinity values with depth and the sulfuric odor of bottom waters suggest sulfate reduction in the waters, sediments and shelly strata along the lake bottom. On alkalinity-sulfate bivariate plots, surface lake waters lie in a mixing region between lines connecting tidal creek water, surficial aquifer water, and Yorktown aquifer water; bottom waters fall along a different trend outside of this mixing zone controlled by sulfate reduction. On salinity-chloride plots, lake waters also lie along mixing lines between ground water and tidal creek water. In the surficial aquifer, the salinity in shallow groundwater does not appear to come directly from the tidal creek, but from seepage exiting the lake, made saline by overturning of bottom water. Initial measures of the salinity in the lower aquifer system suggest that rates of lateral diffusion of chloride from the tidal creek through the aquifer into the lake bottom and beyond may exceed the ability of groundwater advection to flush the Yorktown beds.