Problem
The Town of Riverhead in the northeastern Suffolk County includes rural farmland and suburbs and is bounded by the Long Island Sound to the north and Peconic Bay to the southeast. Riverhead’s close proximity to saline embayments and its location along the northeastern discharge area for Long Island’s groundwater flow system makes it vulnerable to saltwater intrusion. Several public-supply wells in Riverhead have experienced increased chloride concentrations recently due to the encroachment of saltwater. Riverhead is underlain by a sequence of unconsolidated deposits ranging in age from Pleistocene to Upper Cretaceous that overlie a basement complex of Precambrian bedrock. These deposits are part of Long Island's groundwater aquifer system and are the sole source of potable water supply for the Town of Riverhead. The importance of this reservoir to the local population and economy of the area is critical, and, therefore, needs to be studied carefully to avoid problems associated with excessive pumping and saltwater intrusion that will cause irreparable damage to the local ground-water system.
The Town of Riverhead has monitored ground-water levels and chloride concentrations at its public-supply wells and at a few observation wells. Due to the complex hydrogeology of the area and lack of strategically located observation wells, little insight has been gained concerning the source of the intruding saltwater from this monitoring. Previous studies by the USGS have delineated areas of saltwater intrusion along coastal communities (Stumm, 2001) that has been attributed to excessive ground-water withdrawals from public-supply and irrigation wells. The establishment of a ground-water data collection network is needed to address these concerns, and to provide the data necessary to delineate the saltwater-freshwater interface and thereby assist stakeholders to better manage this resource.
Objective
To delineate the location of the saltwater-freshwater interface within the Town of Riverhead and determine what effects local variations in the hydrogeologic framework have on the intrusion of saltwater, an observation-well network will be established to better monitor the saltwater-freshwater interface through chloride sampling and borehole geophysical logging and to permit the continuation of such monitoring into the future.
Background
At selected locations where observation wells do not exist the USGS proposes to collect TDEM soundings at selected sites. The TDEM sounding is a surface geophysical method that uses induced electric currents and electromagnetic fields to characterize subsurface resistivity. Electrical resistivity and its inverse conductivity is an intrinsic property of the chemistry of groundwater and commonly are closely correlated with the chloride concentrations in the aquifer. TDEM survey methods are well suited for measuring the saltwater-freshwater interface in coastal aquifers. TDEM soundings were used to map saltwater intrusion in south Florida (Stewart and Gay, 1986) (Fitterman and Prinos, 2011). A 40 or 100 meter square array of wire is placed on an open field and a small current is introduced. Measuring the secondary electromagnetic field during on and off periods provides data on the conductivity of the subsurface to a depth of 600 feet below land surface (fig. 1).
The depth to the saltwater interface on the North Fork of Long Island (eastern Riverhead) is controlled by the elevation of the water table. The Ghyben-Herzberg equation relates the elevation of the water table to the
elevation of the boundary of the interface between the freshwater and underlying saltwater zones of an unconfined aquifer (Reilly and Goodman, 1985). The lower the elevation above sea level of the water table, the
shallower the depth to the saltwater interface will be due to this relation. In Riverhead the saltwater interface will be delineated using surface geophysical methods and borehole electromagnetic induction (EM) logs at observation wells drilled by the Town of Riverhead based upon the results of the surface geophysical surveys. EM conductivity log methods have been used throughout Long Island to delineate and monitor saltwater intrusion Stumm (1993; 2001) and Stumm and others (2002; 2004) (fig. 2). The USGS will analyze the water quality sample data collected during sampling of observation wells to determine if the source of the elevated chlorides is from roadsalt or seawater. Analysis of the chloride-bromide ratios in groundwater can be used to determine if roadsalting or saltwater intrusion from seawater is the source of chlorides. All water quality samples will be processed by the USGS National Water Quality Lab (NWQL).
Approach
A. Surface Geophysics: The USGS will select at least 12 locations within the Town of Riverhead in the vicinity of supply wells to collect TDEM soundings of resistivity. These surface geophysical measurements have been used on Long Island by the USGS to delineate the depth to the saltwater-freshwater interface. Once the selected locations have been measured using the TDEM method several sections will be constructed of the depth to the saltwater underlying and surrounding the study area. These data will be used to select observation well drilling locations.
B. Network Establishment: Locations for drilling a network of 4 to 5 observation wells will be selected within the Town of Riverhead by reviewing historical well records, results of past hydrogeologic studies, and TDEM survey soundings of the saltwater-freshwater interface. Final well selection will be made by the USGS in consultation with the Riverhead Water District. The Town of Riverhead will hire a professional well drilling company and facilitate the drilling and installation of a groundwater well network within the Town based upon locations selected by the USGS.
C. Borehole Geophysics: The USGS will geophysically log selected observation wells to be drilled by the Town of Riverhead within the study area using natural gamma, resistivity, and electromagnetic induction (EM conductivity) to determine the thickness and concentration of saltwater underlying and surrounding the study area. These data will be compared with those from surface TDEM measurements. Once the observation well network is installed the USGS can relog these wells using EM conductivity to evaluate movement of the saltwater-freshwater interface.
D. Water-Level Monitoring: The USGS will install continuous ground-water-level recorders at four of the newly drilled observation wells. These wells will be in areas where there is concern about saltwater intrusion. Water-level elevations will be collected hourly (with monthly retrieval) at the four observation wells equipped with recorders, collected monthly at two non-instrumented observation wells, and collected annually at all other observation wells. Annual measurements will coincide with the USGS’s synoptic data collection effort conducted annually on Long Island in March-April. Water levels affected by tidal fluctuations will be measured within one hour of local high tide.
E. Water-Quality Monitoring: The USGS will collect water-quality samples (inorganic constituents including chloride and bromide) from eight wells (four pre-existing and four newly drilled wells) within the Town of Riverhead that will be analyzed for chloride and major inorganic constituent concentrations. The samples will be collected following USGS’s National Water Quality Assessment Program protocols (USGS, 1997), so that the results will be comparable to data collected nationally.
References Cited
Fitterman, D. V. and Prinos, S. T., 2011, Results of time-domain electromagnetic soundings in Miami-Dade and southern Broward Counties, Florida, U.S. Geological Society Open-File Report 2011-1299, 42 p.
Reilly, T.E. and Goodman, A.S., 1985, Quantitative analysis of saltwater-freshwater relationships in ground-water systems—a historical perspective: Journal of Hydrology, v. 80, no. 1/2, p. 125-160.
Stewart, M. and Gay, M. C., 1986, Evaluation of transient electromagnetic soundings for deep detection of conductive fluids, Ground Water, 24, p. 351–356.
Stumm, Frederick, 1993, Use of focused electromagnetic-induction borehole geophysics to delineate the saltwater-freshwater interface in Great Neck, Long Island, New York, in Bell, R., Lepper, C., eds. Symposium on the Applications of Geophysics to Engineering and Environmental Problems, Vol. 2: Environmental and Engineering Society, proceedings, p.513-525.
Stumm, Frederick, 2001, Hydrogeology and extent of saltwater intrusion of the Great Neck peninsula, Great Neck, Long Island, New York: U.S. Geological Survey Water Resources Investigations Report, 99-4280, 41p
Stumm, F., and Lange, A.D., 2002, Hydrogeology and extent of saltwater intrusion of the Manhasset Neck peninsula, Long Island, New York: U.S. Geological Survey Water Resources Investigations Report, 00-4193, 42 p.
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.
U.S. Geological Survey, 1997 to present, National field manual for the collection of water-quality data: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. A1-A9, 2 v., variously paged. [Also available online at http://pubs.water.usgs.gov/twri9A. Chapters originally were published from 1997-1999; updates and revisions are ongoing and are summarized at:
http://water.usgs.gov/owq/FieldManual /mastererrata.html].
Project
Location by County
Suffolk
County, NY
