Stable Isotope Studies in Groundwater in the Northeastern United States and Associated Recharge of Glacial Meltwaters during the Last Glacial Maximum

Abstract

Freshwater has been observed in confined aquifers extending over 100 km offshore from the northeastern coastal U.S.A., much greater distances than can be explained by groundwater models using contemporary hydrologic boundary conditions. In order to investigate the origin and residence times of these groundwaters, a total of 25 groundwater samples were collected on Nantucket Island and Martha’s Vineyard, MA, Long Island, NY, and along the coastal plain of southern New Jersey. Sample sites were believed to contain remnant glacial meltwaters of the LGM. Samples were analyzed for stable isotopes (18O, D) and major and minor ion chemistry. A U.S.G.S. monitoring well on Sandy Hook, N.J., had a chloride concentration of 15,151 mg/l. This well was determined to be intruded by saltwater.

Observed groundwater δ18O and δD values were similar to those of modern meteoric waters in the recharge area. Other studies interested in LGM waters in Atlantic Coastal Plain aquifers have also found δ18O values representative of modern meteoric water. These studies used noble gases and 14C to confirm the presence of glacial age groundwater. Interestingly, their δ18O values did not correlate with colder recharge temperatures (~5°C cooler) as indicated by noble gas analyses. The absence of 18O-depleted groundwaters may be explained by (1) a poor correlation of δ18O to temperature along the Atlantic Coast, (2) conflicting effects of boundary conditions at the LGM (e.g. cooling accompanied by changes in atmospheric vapor origin or increases in evaporation), (3) sampling of modern Holocene waters believed to be LGM (4) inadequate sampling of aquifers.

Future work will involve noble gas and 14C analyses of the groundwater samples. This will allow for the determination of recharge temperatures and groundwater ages, further building an understanding of the behavior of stable isotope values in the Atlantic Coastal Plain aquifer system.