Integrating Geophysical, Hydrochemical, and Hydrologic Data to Understand the Freshwater Resources on Nantucket Island, Massachusetts

Abstract

In this study we integrate geophysical, hydrologic, and salinity data to understand the present-day and paleo-hydrology of the Continental Shelf near Nantucket Island, Massachusetts. Time-domain electromagnetic (TDEM) soundings collected across Nantucket and observed salinity profiles from wells indicate that the saltwater/freshwater interface is at least 120 m below sea level in the northern and central portions of the island, far deeper than predicted (80 m) by modern sea level conditions. TDEM soundings also indicate that higher salinity conditions exist on the southern end of the island. These findings suggest a relatively high-permeability environment. Paradoxically, a deep, scientific borehole (USGS 6001) on Nantucket Island, sampling Tertiary and Cretaceous aquifers, is over-pressured by about 0.08 MPa (8 m excess head), which is suggestive of a relatively low-permeability environment. We constructed a series of two-dimensional, cross-sectional models of the paleohydrology of the Atlantic Continental Shelf near Nantucket to understand the flushing history and source of overpressure within this marine environment. We considered two mechanisms for the emplacement of freshwater: (1) meteoric recharge during sea level low stands; and (2) sub-ice-sheet and glacial-lake recharge during the last glacial maximum. Results indicate the sub-ice-sheet recharge from the Laurentide Ice Sheet was needed to account for the observed salinity/resistivity conditions and overpressures. Both TDEM soundings and model results indicate that a lateral transition from fresh to saltwater occurs near the southern terminus of the island due to ice sheet recharge. We also conclude that the overpressure beneath Nantucket represents, in part, “fossil pressure” associated with the last glacial maximum.