Collaborative Research: Pleistocene Hydrogeology of the Atlantic Continental Shelf, New England

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0337634PersonIn many coastal settings around the world the distribution of freshwater within continental shelf sediments is not in equilibrium with modern sea level conditions. One of the most extreme examples of this can be found in New England where groundwater within a shallow Miocene sand about 150 km offshore Long Island, New York is remarkably fresh ( ~ 3000 mg/l salinity). Closer to the mainland, a 512m borehole on Nantucket Island, Massachusetts, contains freshwater ( 2000 mg/l salinity) in all permeable units. Thick confining units within this well have a parabolic salinity profile consistent with ongoing vertical diffusion. Analytical calculations suggest that diffusion began within the confining units as recently as 21ka. Heads within the Cretaceous section beneath Nantucket Island are overpressured by about 4 m above the local water table. We propose to use mathematical modeling and environmental isotope analysis to determine whether flushing of coastal plain sediments offshore New England was caused by: (1) meteoric recharge during Pleistocene sea-level low-stands; (2) sub- ice-sheet recharge during the last glacial maximum; (3) recharge from proglacial lakes; (4) clay dehydration reactions; (5) vertical infiltration of freshwater associated with local flow cells that may have developed on the continental shelf during sea level low stands. A second goal of this research is to determine whether the observed excess heads on Nantucket Island can be explained by solely by Pleistocene sediment loading or whether more exotic mechanisms such as clay dewatering reactions must be invoked. We will construct a three-dimensional, variable-density groundwater flow, heat, and solute mass transport model for the Atlantic continental shelf between Maine and New Jersey. The code will be run on the Indiana University SP Supercomputer. Direct simulation of isotope transport and groundwater residence times will be compared to environmental isotopic data (C-13, C-14, O-18, H-2) collected from wells tapping confined aquifers on the continental shelf. In order to constrain the isotopic composition of pro-glacial lake recharge, we will drill one well to extract pore fluids from lacustrine sediments 230 feet below land surface on Nantucket Island. Pleistocene ice sheet, climatic, and sea-level reconstructions will be used to constrain temporal variations in hydrologic, chemical, and isotopic boundary and initial conditions. Flexural adjustments of land surface topography during Wisconsin glaciations will be made to account for the effects of ice sheet loading on relative sea level.