Sentinel Monitoring for New and Legacy Pesticides in the Shallow Groundwater of Long Island

  The shallow groundwater of Nassau and Suffolk Counties is prone to contamination from current land-use, including agricultural, residential, and recreational. The aquifer system of Long Island is highly susceptible to human-derived contamination, in particular, because the soils and underlying sediments are generally composed of sandy, permeable materials that allow contaminants to move readily from the land surface into the groundwater below. Of increasing concern are the human-derived contaminants stemming from past and present uses of pesticides. In addition to the threat of direct runoff to adjacent waterbodies, pesticide transport through the shallow aquifer from inland sources may discharge to the surface waters of Long Island. Furthermore, increased demands on the aquifer system have accelerated the downward flow of younger groundwater into the deeper parts of the system from which most supply wells withdraw water. This has increased the potential for contaminants introduced at land surface to reach the deeper aquifers sooner than under normal groundwater-flow conditions. Early detection of groundwater contaminants before they discharge to local waters or are drawn into supply wells is critical to making informed decisions about best land- and pesticide-use practices related to groundwater quality and sustainability. Understanding the occurrence and persistence of current-use pesticides and their physical and chemical changes in the shallow aquifer system is complex and requires a consistent and systematic monitoring strategy that considers a variety of environmental and developmental factors. The proposed monitoring of shallow groundwater at sentinel locations throughout Long Island will provide critical information on the occurrence and persistence of pesticides and pesticide degradates. It will also develop a better understanding of co-occurrences of nutrients, major ions, and select metals associated with agricultural and residential pesticide use, and inform how these constituents may persist and travel through the aquifer system.   Background   The sole-source aquifer system of Long Island, which is relied on by the residents of Nassau and Suffolk Counties for abundant, potable drinking water, has increasingly been threatened by point and non-point sources of pollution. Precipitation that provides recharge to the aquifer system can transport pesticides applied to agricultural and residential lands to the water table. The rate and extent of pesticide transport depends on a variety of factors, including the solubility of a compound or mixture of compounds, ability of the vadose zone to filter (retain and [or] degrade) contaminants, the type of sediments with which the water is in contact, and the continual geochemical evolution of groundwater along flow paths. Drawdown of groundwater levels by supply wells can accelerate contaminant migration across and deeper into the aquifer system, in some cases against the natural flow gradient.   Agriculture on Long Island is continuously changing based on socioeconomic demands, with shifts in crops and loss of farmland to residential use and, in turn, changes in the types of pesticides applied to a given land area . However, pesticide application is not just limited to agricultural lands, and U.S. Geological Survey (USGS) studies of Long Island groundwater and streams have reported detections of dozens of pesticides used to treat residential, commercial, and recreational areas (Phillips and others, 1999; Reiser and O'Brien, 1999). Furthermore, the Suffolk County Department of Health Services (SCDHS) has been monitoring groundwater (mainly in eastern Suffolk County) for select pesticides to support the New York State Department of Environmental Conservation (NYSDEC) Pesticide Monitoring Program in accordance with the Pesticide Monitoring Law (Environmental Conservation Law Article 33, Title 12, 1996). In these studies, pesticides were detected in shallow wells located in every land-use classification assessed.   The USGS assesses water quality (and quantity) following nationally-consistent study designs and methods of sampling and analysis (for example, through the National Water-Quality Assessment [NAWQA] Program) and has performed this work in Nassau and Suffolk Counties, New York City, and throughout New York State. The assessments build local knowledge about water-quality issues and trends in a particular system while providing an understanding of the regional and national context of how and why water quality varies (Denver et al., 2009). Comprehensive studies conducted by the USGS have also considered U.S. Environmental Protection Agency and NYSDEC drinking water standards to inform decision- and policy-making across the State and nationally. These have led to the creation of USGS Health-Based Screening Levels for Evaluating Water-Quality Data ( http://cida.usgs.gov/hbsl/apex/f?p=104:1:0::NO), which currently provide information on 208 pesticides and 153 pesticide degradates analyzed by USGS laboratories.   Analytical capabilities developed by USGS laboratories can now detect over 200 pesticide-related compounds at the sub-parts-per-billion level, thereby aiding in their detection in groundwater as early as possible. Data collected by the USGS—available from a publicly-accessible database—compliment groundwater-quality samples collected and analyzed by the SCDHS, Suffolk County Water Authority, the water purveyors of Nassau County, and other stakeholders who provide valuable data that, when synthesized, can be used to analyze for and track pesticide trends in the Long Island aquifer system.   Approach   Observation wells from the 32-well NAWQA Land-Use Study (LUS) network (figure 1) will be evaluated and re- established for monitoring of pesticide contamination ( http://water.usgs.gov/nawqa/). The NAWQA well network is comprised of shallow (