Science Center Objects

Groundwater Flow and Quality in the Atlantic City 800- Foot Sand

by Steven D. McAuley, Julia L. Barringer, Gary N. Paulachok, Jeffrey S. Clark, and Otto S. Zapecza

 

MODEL VERSION/TYPE: MODFLOW-2005, steady state and transient

AREA STUDIED: Parts of Atlantic, Burlington, Cumberland, Cape May, and Ocean Counties

AQUIFERS SIMULATED: Atlantic City 800-foot sand

MOST RECENT WITHDRAWALS SIMULATED: 1986

MODEL SIZE: 5 layers, 33 rows, 63 columns

MINIMUM GRID SPACING:3,300 feet x 3,300 feet

MODEL ARCHIVE is available by email request at gs-nj-model-request@usgs.gov

NOTE: The Great Egg-Mullica model uses an updated model that covers approximately the same area as the Atlantic City model, and is now available and should be used in lieu of the Atlantic City model. However, the Atlantic City model provides a simple dataset that may be useful for educational or training purposes.

 

REPORT ABSTRACT:

The regional, confined Atlantic City 800-foot sand is the principal source of water supply for coastal communities of southern New Jersey. In response to extensive use of the aquifer—nearly 21 million gallons per day in 1986—water levels have declined to about 100 feet below sea level near Atlantic City and remain below sea level throughout the coastal areas of southern New Jersey, raising concerns about the potential for saltwater intrusion into well fields.

Water levels in the Atlantic City 800-foot sand have declined in response to pumping from the aquifer since the 1890's. Water levels in the first wells drilled into the Atlantic City 800-foot sand were above land surface, and water flowed continuously from the wells. By 1986, water levels were below sea level throughout most of the coastal areas. Under current conditions, wells near the coast derive most of their supply from lateral flow contributed from the unconfined part of the aquifer northwest of the updip limit of the confining unit that overlies the Atlantic City 800-foot sand. Groundwater also flows laterally from offshore areas and leaks vertically through the overlying and underlying confining units into the Atlantic City 800-foot sand. The decline in water levels upsets the historical equilibrium between freshwater and ancient saltwater in offshore parts of the aquifer and permits the lateral movement of saltwater toward pumping centers. The rate of movement is accelerated as the decline in water levels increases. The chloride concentration of aquifer water 5.3 miles offshore of Atlantic City was measured as 77 mg/L (milligrams per liter) in 1985 at a U.S. Geological Survey observation well. Salty water has also moved toward wells in Cape May County. The confined, regional nature of the Atlantic City 800-foot sand permits water levels in Cape May County to decline in response to pumping in Atlantic County and vice versa. Historically, chloride concentrations as great as 1,510 mg/L have been reported for water in a former supply well in southern Cape May County. These data indicate that salty water has moved inland in Cape May County. Analysis of the chloride-concentration data indicates that groundwater with a chloride concentration of 250 mg/L is within 4 miles of supply wells in Stone Harbor, Cape May County, and is about 10 miles offshore of supply wells near Atlantic City.

Results of numerical simulations of groundwater flow were analyzed to determine the effects of four water-supply alternatives on water levels, the flow budget, and potential saltwater movement toward pumping centers during 1986-2040. In the supply alternatives, pumpage is (1) held constant at 1986 rates of pumpage; (2) increased by 35 percent at 1986 locations; (3) increased by 35 percent, but with relocation of some supply wells further inland; and (4) increased by 35 percent but with some of the increase derived from inland wells tapping the Kirkwood-Cohansey aquifer system rather than the Atlantic City 800-foot sand. Inland relocation of supply wells closer to the updip limit of the overlying confining unit results in the smallest decline in water levels and the smallest rate of groundwater flow between the offshore location of salty water and coastal supply wells. Increased pumpage from coastal supply wells results in the greatest water-level declines and the greatest increase in the rate of groundwater flow from offshore to coastal wells.

Flow of undesirable salty groundwater from offshore locations remains nearly the same as for current (1986) conditions when pumping rates do not change, and the flow-rate increase is smallest for the relocated pumpage (fourth) alternative. In comparing the two conditions of a 35-percent increase in pumpage, the flow from undesirable salty water positions is lessened and flow from the unconfined aquifer is increased when some of the pumping centers are relocated farther inland. Groundwater from the 250-mg/L isochlor position does not reach supply wells during any simulated conditions predicted for 1986-2040. The analysis of the simulation, however, includes only advective freshwater flow from an estimated 250-mg/L isochlor position and does not include density effects. A chloride concentration data-collection network could be designed to monitor for saltwater intrusion and serve as an early warning system for the communities of southern Cape May County and coastal communities near Atlantic City. Data from existing offshore wells could continue to serve as an early warning system for the Atlantic City area; however, observation wells south of Stone Harbor, in the Wildwood area, would be useful as an early warning system for southern Cape May County.