PRELIMINARY GEOCHEMICAL ASSESSMENT OF GROUNDWATER CONDITIONS NEAR THE CATSKILL AQUEDUCT, HIGH FALLS, NEW YORK

In cooperation with the New York City Department of Environmental Protection (NYCDEP), the U.S. Geological Survey is studying groundwater conditions to assess leakage effects from a segment of the Catskill Aqueduct near High Falls, NY. During construction of the pressure tunnel beneath the Rondout Valley in the early 1900s, fractured and karstic zones were identified in the Helderberg limestone, Binnewater sandstone, and the High Falls shale in the High Falls area that produced inflows to the tunnel ranging from 200 to 2,200 gallons per minute. A network of more than 60 monitoring wells and surface-water sites were established to characterize the hydrogeology and geochemistry of the bedrock and unconsolidated aquifers and to delineate the magnitude and extent of influence of the leaking aqueduct. During 2019 and 2020, 18 of these sites were sampled for water chemistry and three wells were instrumented with continuous water-quality monitors. Springs near the tunnel stop flowing when the tunnel is shut down for maintenance, suggesting that the tunnel is the source of flow to the springs. Water chemistry is interpreted with hydrogeologic data to help characterize water sources in the study area, including leakage from the tunnel that originates from the Ashokan Reservoir. This assessment will also provide context to help NYCDEP evaluate the effectiveness of planned repairs of the Catskill Aqueduct.

Most aqueduct leakage in the study area appears to be associated with a thrust fault identified during tunnel construction that runs across dipping beds of sandstone, shale, and limestone; several units are calcareous and soluble, including the Rondout Formation that hosts a nearly 4,000-foot cave. Water sampled from the aqueduct and wells affected hydraulically by leakage generally had enriched ^87/86Sr ratios and δ¹⁸O values, and lower calcite-saturation indices than water from wells less affected by tunnel leakage. Other indicators, including the stable isotopes δ³⁴S of sulfate and δ¹³C of dissolved inorganic carbon and chloride-bromide ratios, also were used to help understand geochemical differences and processes of the host rocks. Water-chemistry data and mixing models are being used to assess the relative contribution and the effects of aqueduct leakage on local groundwater.