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Water-quality samples collected in an area prone to groundwater flooding in Wawarsing, New York, were analyzed and assessed to better understand the hydrologic system and to aid in the assessment of contributing water sources. Above average rainfall over the past decade, and the presence of a pressurized water tunnel that passes about 700 feet beneath Wawarsing, could both contribute to groundwater flooding. Water samples were collected from surface-water bodies, springs, and wells and analyzed for major and trace inorganic constituents, dissolved gases, age tracers, and stable isotopes. Distinct differences in chemistry exist between tunnel water and groundwater in unconsolidated deposits and in bedrock, and among groundwater samples collected from some bedrock wells during high head pressure and low head pressure of the Rondout–West Branch Tunnel. Samples from bedrock wells generally had relatively higher concentrations of sulfate (SO4 2-), strontium (Sr), barium (Ba), and lower concentrations of calcium (Ca) and bicarbonate (HCO3 -), as compared to unconsolidated wells. Differences in stable-isotope ratios among oxygen-18 to oxygen-16 (δ18O), hydrogen-2 to hydrogen-1 (δ2H), sulfur-34 to sulfur-32 (δ34S) of SO4 2-, Sr-87 to Sr-86 (87Sr/86Sr), and C-13 to C-12 (δ13C) of dissolved inorganic carbon (DIC) indicate a potential for distinguishing water in the Delaware–West Branch Tunnel from native groundwater. For example, 87Sr/86Sr ratios were more depleted in groundwater samples from most bedrock wells, as compared to samples from surface-water sources, springs, and wells screened in unconsolidated deposits in the study area. Age-tracer data provided useful information on pathways of the groundwater-flow system, but were limited by inherent problems with dissolved gases in bedrock wells. The sulfur hexafluoride (SF6) and (or) chlorofluorocarbons (CFCs) apparent recharge years of most water samples from wells screened in unconsolidated deposits and springs ranged from 2003 to 2010 (current) and indicate short flow paths from the point of groundwater recharge. All but three of the samples from bedrock wells had interference problems with dissolved gases, mainly caused by excess air from degassing of hydrogen sulfide and methane. The SF6 and (or) CFC apparent recharge years of samples from three of the bedrock wells ranged from the 1940s to the early 2000s; the sample with the early 2000s recharge year was from a flowing artesian well that was chemically similar to water samples collected at the influent to the tunnel at Rondout Reservoir and the most hydraulically responsive to water tunnel pressure compared to other bedrock wells. Data described in this report can be used, together with hydrogeologic data, to improve the understanding of source waters and groundwater-flow patterns and pathways, and to help assess the mixing of different source waters in water samples. Differences in stable isotope ratios, major and trace constituent concentrations, saturation indexes, tritium concentrations, and apparent groundwater ages will be used to estimate the proportion of water that originates from Rondout– West Branch Tunnel leakage. Introduction Groundwater flooding of streets and residential basements during fall 2008 in the Rondout Valley near the town of Wawarsing, Ulster County, New York, prompted the New York City Department of Environmental Protection (NYCDEP) to ask the U.S. Geological Survey (USGS) to characterize the groundwater-flow system of the area. Precipitation records indicate that above average rainfall over much of the past two decades, particularly over the past decade, has aggravated flooding problems (Stumm and others, 2012). Additionally, a segment of the Rondout–West Branch (RWB) Tunnel, which is an underground concrete-lined structure that passes about 700 feet (ft) beneath Wawarsing and carries pressurized water through from the Catskill Mountains to New York City (NYC), is leaking and also could be contributing to groundwater flooding (Stumm and others, 2012). This 44-mile (mi) long, 13.5-ft diameter water tunnel between the Rondout Reservoir and the Catskill Aqueduct carries about one-half of the NYC water supply and has a capacity of about 1.03 billion gallons per day (gal/d) (New York City Department of Environmental Protection, 2010). One of the two primary areas of leakage along the RWB Tunnel is beneath Wawarsing; during construction of the RWB Tunnel, this section of bedrock beneath the Rondout Valley was observed to be highly faulted and 2 Preliminary Assessment of Water Chemistry Related to Groundwater Flooding in Wawarsing, New York, 2009–11 weathered, had over 9,000 gallons per minute (gal/min) of inflow, and required over 20,000 bags of concrete and steel reinforcements to stabilize the bedrock. The total estimated leakage out of the RWB Tunnel is 15 to 35 million gallons per day (Mgal/d) (James Canale, New York City Department of Environmental Protection, written commun., 2011; DiNapoli, 2007). In 2003 and 2009, an autonomous underwater vehicle was used to inspect the interior of the RWB Tunnel and recorded 8,200 linear feet of cracking throughout the tunnel (James Canale, New York City Department of Environmental Protection, written commun., 2011). Most of the cracking was observed in the sections of the tunnel that are within the limestone units underlying Roseton and Wawarsing, N.Y. Water-chemistry data were interpreted with hydrogeologic data, which are detailed in a companion report (Stumm and others, 2012), to help identify and distinguish between water sources that may contribute to the groundwater flooding observed in Wawarsing. Inorganic constituents, stable isotopes, and age tracers in groundwater and springs are useful indicators of different sources of water, including surface water or anthropogenic sources, as well as geochemical processes in different parts of the aquifer system. Waterchemistry data were interpreted with hydrogeologic data, which are detailed in a companion report (Stumm and others, 2012), to help identify and distinguish between water sources that may contribute to the groundwater flooding observed in Wawarsing. In 2008, the USGS was tasked by the NYCDEP to characterize the hydrogeologic conditions in the Wawarsing area and investigate the possible relation between groundwater flooding and leakage from the adjacent RWB Tunnel. A companion report (Stumm and others, 2012) describes the hydrology of the Wawarsing study area. Together, the reports serve as a foundation for understanding the hydrologic system and how it is affected by precipitation and pressurized water leaking from the RWB Tunnel. Purpose and Scope The purposes of this report are to describe the collection and analysis of water samples and to present some general interpretation of water-chemistry data in order to better understand the hydrologic system and to aid in the assessment of sources of groundwater flooding in the town of Wawarsing, N.Y. The report presents information on the hydrogeologic setting, the occurrence and distribution of chemical constituents and characteristics, and groundwaterage estimates for groundwater samples in the study area. Hydrogeologic data from a companion report (Stumm and others, 2012) are used in this analysis. The report presented herein describes general water-chemistry characteristics, such as oxidation-reduction (redox) conditions, pH, and water types at the Rondout Reservoir inflow to the RWB Tunnel, Lippman Lake, 4 springs, and 19 wells, including 9 wells screened in unconsolidated deposits and 10 wells completed in bedrock. A description of the design of the monitoring network and methods of data collection and analysis of water-level fluctuations are included. The interpretive steps for estimating groundwater ages using atmospheric tracers are described, along with the distribution of the interpreted groundwater ages in selected wells. The level of interpretation in this report is limited because additional information is required for a detailed assessment. Information on the geology, hydrogeology, and additional water-quality sample analysis in the study area is required to assess the sources of water in the aquifer system related to flooding. Description of Study Area The study area encompasses a 12-square mile (mi2) area in Wawarsing, N.Y., that is prone to frequent groundwater flooding. A sampling site at Rondout Reservoir at Lackawack (station 01366400; fig. 1), where reservoir water enters the RWB Tunnel portion of the Delaware Aqueduct was used to represent water from the RWB Tunnel. The Rondout Valley near the town of Wawarsing, Ulster County, N.Y., is part of the Port Jervis Trough, which is a deep, glaciated valley that extends about 100 mi southwestward from Kingston through Kerhonkson, N.Y., and continues into Pennsylvania. The Port Jervis Trough separates the Shawangunk Mountains to the east from the Catskill Front (eastern side of the Catskill Mountains) to the west (Reynolds, 2007). The valley in the study area is underlain by gravel, sand, silt, and clay of Pleistocene and Holocene age (Cadwell and others, 1989) and bedrock sequences of limestone, sandstone, and shale with substantial fracturing and faulting (Dibbell, 1944). The population of Wawarsing was 13,157, according to the 2010 U.S. Census (U.S. Census Bureau, 2010), and land use is predominantly residential. Wawarsing lies to the west of the Shawangunk Mountains, which are the north-easternmost expression of the Valley and Ridge province of the central Appalachians. Land-surface altitudes range from about 270 ft in the flood-prone area in Wawarsing to about 730 ft at the “mixing chamber” from the Rondout Reservoir to the Delaware Aqueduct (in this report, altitudes are referenced to the North American Vertical Datum of 1988, unless otherwise specified). The sampling network in Wawarsing and the Delaware Aqueduct water sampled in Lackawack are herein described as the “

Preliminary assessment of water chemistry related to groundwater flooding in Wawarsing, New York, 2009-11