Pleistocene hydrogeology of the Atlantic continental shelf, New England

Salinity data from the Atlantic continental shelf off New England indicate that the freshwater/saltwater interface is far out of equilibrium with modern sea-level conditions. More than 150 km offshore of Long Island, New York, aquifer salinity levels are less than 5 parts per thousand (5 ppt). Salinity levels within confining units beneath Nantucket Island, Massachusetts, are 30%-70% of seawater levels and exhibit a parabolic profile consistent with ongoing vertical diffusion. Here, we evaluate two fluid-flow-inducing mechanisms that could explain the apparent flushing of these coastal-plain aquifers: (1) meteoric recharge during Pleistocene sea-level low-stands, and (2) subglacial recharge from the Laurentide Ice Sheet. Analytical models of vertical solute diffusion for the Nantucket confining units suggest that flushing of aquifers beneath Nantucket began in the late Pleistocene between ca. 195 and 21 ka; the models assume a diffusion coefficient of 3.0 × 10^-11 m²/s. Cross-sectional numerical models of variable-density groundwater flow, heat, and solute transport could not reproduce the relatively low-salinity groundwaters observed off Long Island by applying boundary conditions consistent with Pleistocene sea-level fluctuations. Observed salinity conditions were most closely matched in the models by also including the effects of subglacial recharge from the Laurentide Ice Sheet and allowing groundwater to discharge from Miocene aquifers along submarine canyons near the continental slope. Simulated recharge induced by Laurentide Ice Sheet meltwater was probably short lived but, on average, about two to ten times greater than modern subaerial levels. A sensitivity analysis performed using our cross-sectional model suggests that a narrow range of hydrologic conditions can drive fresh water long distance offshore across the continental shelf.