Sources of Dissolved Carbonate in an Aquifer Free of Carbonate Minerals

Water from 14 wells in the Magothy aquifer, Long Island, New York, was sampled for chemical analysis and C¹³/C¹² ratio measurements of the dissolved carbonate. The Magothy is essentially free of carbonate minerals and contains water whose chemistry is controlled by the chemistry of the atmospheric precipitation that recharged the aquifer. Wells near the recharge area have δC¹³ values of −25‰ and total carbonate contents corresponding to a PCO₂ of about 4×10⁻³ atmosphere, showing that the carbonate here is derived from the soil zone. Downgradient, the total carbonate doubles, but the δC¹³ values remain more negative than −18‰. The dissolved carbonate increase is due in part to carbonate mineral solution but results largely from oxidation of lignite in the aquifer by oxygen dissolved in the water.