Problem
Perchlorate detected in a shallow supply well within the southern portion of the Locust Valley Water District (LVWD) has prompted interest in determining the possible existence of a deeper confined aquifer (North Shore Aquifer) that may be protected from shallow contamination (fig. 1). Previous USGS studies in this area indicate the northern part of Nassau County has a complex hydrogeologic framework (Stumm and others, 2004). A previously mapped buried glacial valley may extend and be present at this location. If such a buried valley exists, all Cretaceous age deposits (Magothy aquifer, Raritan clay, and Lloyd aquifer) may have been eroded and Pleistocene-aged deposits including the North Shore aquifer and North Shore confining unit may underlie the upper glacial aquifer. To delineate what aquifer and confining units are present and their relation with the surrounding hydrogeologic framework, a test well will be drilled to bedrock by the LVWD’s contracted driller. The test well is being drilled as part of LVWD’s plan to deepen the shallow supply well effected by elevated levels of perchlorate.
The LVWD has requested the technical assistance of the U.S. Geological Survey (USGS) for the collection and analyses of hydrogeologic, aquifer test, and water quality data, and geophysical logs from a deep test well drilled to bedrock in the area and analysis of associated data from another test well drilled last year. The data collected in this investigation is essential to define both the complex hydrogeology and water quality of groundwater in the area which will serve to protect and enhance the water resources in the region for human health. In Nassau County all of the public drinking water is derived from groundwater.
Objective
The objective of this project is to determine the hydrogeology and water quality of a potential deep confined aquifer in the southern portion of the LVWD supply area.
Approach
The USGS proposes a cooperative project with the LVWD to collect hydrogeologic, geophysical, and water-quality samples during the drilling of one test well using methods utilized in a previous study conducted by USGS (Stumm and others, 2004). The test well will be installed to determine the hydraulic properties of the North Shore aquifer (if present) at the site and using other Water District borehole well logs a regional analysis will be done to determine the extent and hydrogeologic features of a major buried valley. Water quality analyses will be used to determine if perchlorate is present in the deep aquifer system in the vicinity of the test well.
The USGS will have a hydrologist onsite during drilling and coring to collect and analyze the core samples to determine how these correlate to the previously mapped hydrogeology in the area (Stumm and others, 2004). The new test well will be pumped at a rate of 400 gallons per minute by the driller continuously for up to 72 hours. Water levels in the drilled well will be monitored by a USGS hydrologist during pumping and recovery. No other nearby observation wells are available for use in the aquifer test. Water-level recovery data will be utilized in the estimation of aquifer hydraulic properties. These water level data will be analyzed using the over-the-counter aquifer test analyses software AQTESOLV (Duffield, 2007) to estimate the hydraulic properties of the aquifer. The USGS will collect water-quality samples of the pumped discharge at the end of the 72 hour pumping period. The water-quality samples will be analyzed by the National Water Quality Laboratory and an outside laboratory (Pace Analytical Services, Inc.) to determine the concentrations of major inorganic ions, VOC’s, and perchlorate. Samples will be collected following the USGS National Field Manual for the Collection of Water-Quality Data (US Geological Survey, variously dated). Geophysical logs including gamma, single-point resistance, long and short-normal resistivity, and electromagnetic induction, will be collected in the open borehole after the well is drilled to bedrock. A polyvinyl chloride (PVC) casing will be installed with a screen zone within the confined aquifer at the site. The hydrogeologic data obtained from the test well will be integrated with data from the other supply wells in the area and the previously mapped hydrogeologic framework to determine the hydraulic interconnection of the North Shore aquifer and the shallow upper glacial aquifer in the area.
References
Duffield, G.M., 2007, AQTESOLV for Windows version 4.5 User’s Guide, HydroSOLVE, Inc, Reston, VA
Evenson, E.J., and others, 2012, Strategic Directions for U.S. Geological Survey Water Science, 2012–2022—Observing, Understanding, Predicting, and Delivering Water Science to the Nation—Public Review Release: U.S. Geological Survey Open-File Report 2012–1066, 42 p.
U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. A1-A9, available online at
http://pubs.water.usgs.gov/twri9A.
Stumm, F., Lange, A.D., and Candela, J.L., 2004, Hydrogeology and Extent of Saltwater Intrusion in the Northern Part of the Town of Oyster Bay, Nassau County, New York: U.S. Geological Survey Water Resources Investigations Report 03-4288, 55 p.
Project
Location by County
Nassau County, NY
