ASSESSMENT OF GROUNDWATER FLOODING USING WATER CHEMISTRY AND GEOCHEMICAL MODELING, WAWARSING, NEW YORK

Groundwater flooding of streets and residential basements in the Rondout Valley near the town of Wawarsing, Ulster County, New York, was studied by the U.S. Geological Survey to characterize the groundwater-flow system of the area. Groundwater flooding in Wawarsing could be caused by above-average precipitation over much of the past two decades, and (or) a leaking segment of the Rondout–West Branch water tunnel, which passes about 700 feet (ft) beneath Wawarsing and carries pressurized water from the Catskill Mountains to New York City. Major and trace element chemistry, stable isotopes, and age tracers were interpreted with hydrogeologic data to help identify and distinguish between water sources that may contribute to the groundwater flooding observed in Wawarsing.

Water samples from bedrock wells in the study area generally had higher concentrations of SO₄^2- and Cl^- and lower concentrations of Ca, HCO₃^-, and CO₃^2- compared to samples from most wells screened in the unconsolidated aquifer and from springs. Differences in calcite and gypsum saturation indexes were evident in water samples among (1) most bedrock wells and wells screened in the unconsolidated aquifer, and (2) water samples collected from two bedrock wells before and during a tunnel shutdown. Stable-isotope ratios among δ¹⁸O, δD, δ³⁴S of SO₄^2-, and ⁸⁷Sr/⁸⁶Sr were used to help distinguish water in the tunnel from native groundwater. Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios, for example, are distinctly different among samples from most bedrock wells and samples from tunnel water. Age-tracer data for chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF₆) provided some useful information about pathways of the groundwater-flow system, but were limited by inherent problems with dissolved gas data from bedrock wells caused by excess air and degassing from high concentrations of H₂S and CH₄. Water chemistry data and mixing models were used together with hydrogeologic data to assess the relative contribution of tunnel water and native groundwater to local groundwater and springs related to flooding.