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The hydrologic impacts of continuous ground-water withdrawals at 0.75, 1.0, and 1.25 Mgal/d (million gallons per day) from a test-well site in the Cape Cod National Seashore, Truro, Massachusetts, were evaluated with a three-dimensional finite-difference steady-state-flow digital model. The digital model was prepared during an earlier study and is only briefly described in this report. Continuous withdrawal of more than 1.0 Mgal/d from a well screened from 10 to 40 feet below sea level at the test site will result in upward movement of the freshwater-saltwater interface and most likely saltwater will eventually contaminate the well. Pumping from a shallower well will decrease the potential for the movement of saltwater into the well, but the water table may be drawn down to the well screen. It is unlikely that movement of the freshwater-saltwater interface in response to pumping from the test site at the simulated rates will result in saltwater contamination of the shallow domestic supply wells in Truro. For the simulated pumping schemes, the water-table decline below average (1963-76) levels did not exceed 0.6 foot except near the pumping wells. Continuous withdrawal at the average year-round rate and the average summer rate will decrease freshwater discharge to the wetland and ocean along the northeastern boundary of the aquifer. INTRODUCTION In July 1978, one of the two well fields in the town of Truro that supply drinking water to the town of Provincetown was closed as a precautionary measure after an estimated 3,000 gallons of gasoline leaked from a service station storage tank located about 600 feet from the well field. The gasoline reportedly had been detected in several nearby private wells. The National Park Service granted a permit to Provincetown to install and pump a temporary supply well within the boundaries of the Cape Cod National Seashore in Truro while the contaminated part of the aquifer is being reclaimed. The well was used in 1979 to supply 32 percent of the town's water needs. At the request of the National Park Service, the U.S. Geological Survey has estimated the potential impacts of withdrawal from the temporary well at pumping rates of 0.75, 1.0, and 1.25 Mgal/d. A digital model of the Truro aquifer previously prepared, calibrated, and documented by the Geological Survey (Guswa and LeBlanc, 1981) during an earlier study of Cape Cod's ground-water resources was used for this evaluation. The original computer code and input data, except for the pumping rates, were not modified during this evaluation. Therefore, this report only briefly describes the digital model. The reader is referred to Guswa and LeBlanc (1981) for a detailed description of the computer code, the hydrology of Cape Cod, the preparation of the model input data, and the model calibration. PROVINCETOWN WATER-SUPPLY WELLS Provincetown is located at the outer end of the Cape Cod peninsula (fig. I). Groundwater quality within the town is poor (Frimpter and Gay, 1979, p. 7), and Provincetown has obtained its drinking water from wells in the adjacent town of Truro since 1908. Before 1978, Provincetown obtained its drinking water from the Knowles Crossing and South Hollow well fields (fig. 2). Characteristics of the two well fields are included in table i. Monthly pumpage from the two well fields from 1975 to 1979 is shown in table 2. Each field supplied about half of the water used by Provincetown during 1975-77. Gasoline Leak In December 1977, a leak was discovered in a gasoline storage tank located about 600 feet southwest of the nearest well in the South Hollow well field. Approximately 3,000 gallons of gasoline had leaked into the ground, and much of it reached the water table (R. A. Weimar, Camp, Dresser, and McKee, Inc., oral commun., I960). The South Hollow well field was shut down in July 1978 to avoid inducing movement of the gasoline from the contaminated area toward the wells. Efforts to remove the gasoline from the unsaturated zone and the aquifer at the site of the leak began in March 1979 and were continuing in May I960 (R. A. Weimar, oral commun., I960). Detailed information on the gasoline leak, the testing program to determine the nature and extent of the contamination, and efforts to recover the gasoline and reclaim the aquifer is given in a report by Camp, Dresser, and McKee (I978b). Supplemental Water Sources The town of Provincetown cannot supply all of its requirements for drinking water from the Knowles Crossing well field. The three wells in this field are located approximately 1,500 feet from Cape Cod Bay, and pumping for extended periods at the field's rated capacity, 0.86 Mgal/d, results in increased sodium and chloride concentrations owing to seawater intrusion (Frimpter and Gay, 1979, p. 6). The chloride concentration rose from 105 mg/L in November 1978 to 225 mg/L in March 1979 after the well field was pumped continuously for 4 months at 0.6 to 0.7 Mgal/d (J. E. Smith, Provincetown Water Department, oral commun., 1980). To supplement water pumped from the Knowles Crossing well field during 1978-79, Provincetown also obtained water from two wells within the boundaries of the Cape Cod National Seashore (Camp, Dresser, and McKee, Inc., I978a; National Park Service, 1979). In July 1978, a temporary supply well was completed at Test Site No. 4 (fig. 2). The site had been identified previously by Whitman and Howard, Inc., (1966, 1969) as a potential source of drinking water for Provincetown. The Test Site No. 4 well supplied water to the town from July to November 1978 and May to November 1979 (table 2). During 1978 and 1979, the pumping rate was restricted to 0.75 Mgal/d or less by the Park Service. The second source of supplemental water, a well that supplies the North Truro Air Force Station (fig. 2), supplied water to Provincetown from June to December 1978 and April to November 1979 (table 2). The characteristics of these wells are summarized in table i. The South Hollow well field remained closed during the summer of I960 while efforts to rehabilitate the aquifer at the site of the gasoline leak continued. Provincetown was granted permission by the National Park Service and the U.S. Air Force to pump water from the emergency supply wells in I960. Pumping from the Air Force well resumed in April 1980; pumping from the Test Site No. 4 well resumed in May I960. THE TRURO AQUIFER The aquifer in Truro is Pleistocene glacial outwash composed of unconsolidated sand and gravel with some silt and clay (Oldale, 1976; Koteff and others, 1967). Fresh ground water is contained in the unconsolidated sediments under unconfirmed conditions. The sediments generally are very permeable and yield water readily to wells. The ground-water hydrology of the Truro area has been described by many investigators (Delaney and Cotton, 1972$ Strahler, 1972; Burns, Frimpter, and Willey, 1975; Guswa and Londquist, 1976; Guswa and LeBlanc, 1981) and is briefly reviewed below.

Potential hydrologic impacts of ground-water withdrawal from the Cape Cod National Seashore, Truro, Massachusetts