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Hydrogeologic framework, availability of water supplies, and saltwater intrusion, Cape May County, New Jersey

By Pierre J. Lacombe and Glen B. Carleton

Water-Resources Investigations Report 01-4246

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ABSTRACT

During 1960-90, saltwater intrusion forced the abandonment of at least 10 public-supply wells, 3 industrial-supply wells, and more than 100 domestic-supply wells in Cape May County, N.J. Actual or imminent ground-water contamination caused by land-use practices and human activities has forced the closure of at least six shallow public-supply wells and many domestic-supply wells.

Freshwater in Cape May County flows in many small streams and is held in wetlands and natural and artificial ponds. More importantly, freshwater from precipitation passes through and is stored in five aquifers-- Holly Beach water-bearing zone, estuarine sand aquifer, Cohansey aquifer, Rio Grande water-bearing zone, and Atlantic City 800-foot sand.

Surface-water discharges were measured at 14 stream sites. The Tuckahoe River is the largest stream in Cape May County. The mean annual discharge for the Tuckahoe River at Head of River was 43.8 ft3/s (cubic feet per second) or 10,800 Mgal/yr (million gallons per year) during the period of record (1969-93). Mean daily discharge ranged from 25 ft3/s or 16 Mgal/d in September to 73 ft3/s or 47 Mgal/d in April. Mean daily discharge at the eight largest streams wholly within the county ranged from 15.9 to 3.05 ft3/s (3,750 to 720 Mgal/yr).

Total water use in the county was about 8,600 Mgal/yr in 1990, including about 25 Mgal/yr of surface water, 3,000 Mgal/yr from the Holly Beach water-bearing zone, 1,000 Mgal/yr from the estuarine sand aquifer, 2,200 Mgal/yr from the Cohansey aquifer, 200 Mgal/yr from the Rio Grande water-bearing zone, and 2,200 Mgal/yr from the Atlantic City 800-foot sand.

Water-level data collected during April 1991 for more than 200 wells show that in some locations ground-water flow directions and rates have changed when compared with those shown on historical potentiometric-surface maps. In 1991, water levels in the Holly Beach water-bearing zone were nearly identical to levels prior to development. A cone of depression has developed in the estuarine sand aquifer; the water-level altitude near the center of the cone was about -5 ft in 1991. An extensive cone of depression has developed in the Cohansey aquifer; the water-level altitude near the center of the cone was about -20 ft. A small cone of depression has developed in the Rio Grande water-bearing zone; the altitude near the center was -5 ft. An elongated cone of depression has developed in the Atlantic City 800-foot sand; the water-level altitude was about -70 ft in Ocean City and -20 ft in Stone Harbor. Waterlevel maps from predevelopment, 1958, 1978, 1983, and 1988 show that the cones of depression are getting deeper and are expanding in the Atlantic City 800-foot sand.

The 250-mg/L (milligram per liter) line of equal chloride concentration and 50 mg/L line of equal sodium concentration have moved inland, possibly since the early 1900's. Chloride concentrations have increased in many wells in the confined aquifers along the coastline in the southern part of the county. Nitrate concentrations greater than 1 mg/L were present in water samples collected from 10 wells that tap the Holly Beach water-bearing zone. Concentrations of nitrate greater than 10 mg/L in samples collected in Lower, Middle, Upper, and Dennis Townships may result from effluent from septic systems or from agricultural activities.

A water budget shows that the mean annual precipitation is about 42 in., and about 119,000 Mgal falls each year on uplands and freshwater wetlands in the county. About 63,600 Mgal/yr is evapotranspired, 8,200 Mgal/yr becomes overland flow, and 47,200 Mgal/yr recharges the Holly Beach water-bearing zone. In northern Cape MayCounty, most recharge ultimately is discharged to streams. In southern Cape May County, about 20 percent of recharge is diverted to withdrawal wells.

Because saltwater intrusion has occurred in the confined aquifers along the Atlantic and Delaware Bay coastlines, new supply wells placed along the axis of the peninsula would likely provide freshwater for the longest period of time. The supply wells could be coupled with observation wells placed at or near the saltwater front in the aquifers. The observation wells could be used to obtain water-quality samples and waterlevel measurements to observe any changes in the location of the saltwater front that might result from withdrawals.


CONTENTS

Abstract

Introduction

Purpose and scope

Location and extent of study area

Climate

Previous investigations

Numbering system for stream-flow gaging stations and wells, and well information

Methods of investigation

Acknowledgments

Surface water and wetlands

Freshwater wetlands, streams, and ponds

Tuckahoe River permanent gaging station

Small stream partial-record stations

Saltwater wetlands

Hydrostratigraphic framework

Shallow aquifers and confining units

Holly Beach water-bearing zone

Confining unit overlying the estuarine sand aquifer

Estuarine sand aquifer

Confining unit overlying the Cohansey aquifer

Cohansey aquifer

Deep aquifers and confining units

Confining unit overlying the Rio Grande water-bearing zone

Rio Grande water-bearing zone

Confining unit overlying the Atlantic City 800-foot sand

Atlantic City 800-foot sand

Confining unit underlying the Atlantic City 800-foot sand

Water withdrawal

Surface-water withdrawals

Ground-water withdrawals

Holly Beach water-bearing zone

Estuarine sand aquifer

Cohansey aquifer

Rio Grande water-bearing zone

Atlantic City 800-foot sand

Ground-water levels and flow directions

Holly Beach water-bearing zone

Estuarine sand aquifer

Cohansey aquifer

Rio Grande water-bearing zone

Atlantic City 800-foot sand

Water quality and saltwater intrusion

Holly Beach water-bearing zone

Chloride, sodium, and saltwater intrusion

Iron

Nitrate

Estuarine sand aquifer

Chloride, sodium, and saltwater intrusion

Iron

Cohansey aquifer

Chloride, sodium, and saltwater intrusion

Iron

Rio Grande water-bearing zone

Atlantic City 800-foot sand

Chloride, sodium, and saltwater intrusion

Iron

Water budgets

Shallow aquifer system prior to development

Shallow aquifer system during 1990

Deep aquifer system prior to development

Deep aquifer system during 1990

Availability of freshwater supplies

Surface-water supply

Shallow aquifer water supply

Holly Beach water-bearing zone

Estuarine sand aquifer

Cohansey aquifer

Deep aquifer water supply

Rio Grande water-bearing zone

Atlantic City 800-foot sand

Summary

Conclusions

References cited




U.S. Department of the Interior, U.S. Geological Survey
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