PLEISTOCENE HYDROGEOLOGY OF THE ATLANTIC CONTINENTAL SHELF IN NEW ENGLAND: THE ROLES OF ICE SHEETS, SEA LEVEL LOW STANDS, AND SUBMARINE CANYONS

Fresh to brackish water represents an important resource in coastal areas with large urban population centers. On the Atlantic continental shelf in New England, we estimate that up to 2800 cubic kilometers of freshwater are sequestered in Cretaceous, Tertiary, and Pleistocene sands below sea level. Scientific drilling during the 1970's revealed that TDS concentrations on the Atlantic continental shelf displays significant lateral variability. Off Long Island and New Jersey, for example, relatively low salinity groundwaters (less than 5000 ppm) extends over 150 km offshore. However, low salinity groundwater along the continental shelf off North Carolina, Georgia, and Florida are restricted to a few tens of km from the modern coastline at most. Two mechanisms have been proposed to explain the occurrence of offshore freshwater in New England: direct meteoric recharge from precipitation and ice sheet recharge beneath the Laurentide ice which extended as far south as Long Island.

To study the Pleistocene hydrogeology of the continental shelf in New England, we have recently developed a new, parallel paleohydrogeologic model (PGEOFE) capable of representing Pleistocene sea level fluctuations, the waxing and waning of ice sheets, lithosphere flexure, permafrost generation, variable-density groundwater flow, heat, and solute transport. To honor the geology and morphology of the continental shelf, we have constructed a numerical model which extends from Maine to New Jersey using 5.8 million tetrahedral elements. To simulate millions of years of the hydrologic system, this parallel code was run on 256 processors using the NSF Teragrid Network. We found that sea level low stands were incapable of driving saltwater far offshore due to their low hydraulic gradients during sea level low stands. Ice sheet loading was a far more effective mechanism in driving freshwater seaward. However, this mechanism is limited to relatively short time periods (about 4000 years) when ice sheets overrun the continental shelf. We found that the presence of Hudson submarine canyons, a site where Pliocene sands crop out on the continental slope, plays and important role in focusing groundwater discharge during Pleistocene sea level low stands. This goes a long way towards explaining why New Jersey has so much freshwater so far offshore.