PLEISTOCENE HYDROGEOLOGY OF THE ATLANTIC CONTINENTAL SHELF, NEW ENGLAND

Groundwater resources of Nantucket Island are critical to the residents and seasonal visitors (Person, Taylor, Dingman, 1998). The distribution of freshwater on Nantucket Island, Massachusetts is far out of equilibrium with modern day sea level conditions. Here we assess the spatial distribution of the freshwater on Nantucket Island and try to resolve the mechanisms of Pleistocene freshwater emplacement using integrated geophysical, sedimentological, geochemical, isotopic and hydrologic data sets combined with mathematical modeling. We collected time-domain electromagnetism soundings (TDEM), sediment, and pore water geochemical samples from a Wisconsin-age glacio-lacustrine sediment from three boreholes. TDEM collected along a north-south transect across Nantucket indicate that the saltwater-freshwater interface is at least 120 m below the surface in the northern and central portion of the island. Sediment analysis of the glacio-lacustrine deposit show that the glacio-lacustrine deposit consists of interbedded silts, clays and fine sands, with a permeability measuring ~10-15 m2. Constant-rate-of-strain consolidation experiments indicate that the pore pressures within the glacio-lacustrine sediment are overpressured. The diffusion coefficient of the sediment is estimated to be 2x10-10m2/s. Pore fluids squeezed from glacio-lacustrine sediment have a near uniform isotopic composition which appears to be modern (-7‰). Since the connate pore fluids were probably isotopically light (-20‰) glacial melt waters, the glacial melt water must have been flushed or diffused vertically since the time of its deposition 17,000 years ago. This suggests that the glacio-lacustrine deposit did not act as a tight confining unit and may have allowed significant quantities of Holocene freshwater to infiltrate into deeper aquifers. Cross sectional model results suggest that sub-ice sheet recharge and input from pro-glacial lakes was the most likely mechanisms for freshwater emplacement beneath Nantucket Island.