The origin of high sulfate concentrations in a coastal plain aquifer,Long Island,New York

Ion-exchange batch experiments were run on Cretaceous (Magothy aquifer) clay cores from a nearshore borehole and an inland borehole on Long Island, NY, to determine the origin of high SO₄^2? concentrations in ground water. Desorption batch tests indicate that the amounts of SO₄^2? released from the core samples are much greater (980–4700 μg/g of sediment) than the concentrations in ground-water samples. The locally high SO₄^2? concentrations in pore water extracted from cores are consistent with the overall increase in SO₄^2? concentrations in ground water along Magothy flow paths. Results of the sorption batch tests indicate that SO₄^2? sorption onto clay is small but significant (40–120 μg/g of sediment) in the low-pH (<5) pore water of clays, and a significant part of the SO₄^2? in Magothy pore water may result from the oxidation of FeS₂ by dissolved Fe(III). The acidic conditions that result from FeS₂ oxidation in acidic pore water should result in greater sorption of SO₄^2? and other anions onto protonated surfaces than in neutral-pH pore water. Comparison of the amounts of Cl^? released from a clay core sample in desorption batch tests (4 μg/g of sediment) with the amounts of Cl^? sorbed to the same clay in sorption tests (3.7–5 μg/g) indicates that the high concentrations of Cl^? in pore water did not originate from connate seawater but were desorbed from sediment that was previously in contact with seawater. Furthermore, a hypothetical seawater transgression in the past is consistent with the observed pattern of sorbed cation complexes in the Magothy cores and could be a significant source of high SO₄^2? concentrations in Magothy ground water.