Science Center Objects

Groundwater Flow Under Current (1987) and Future Conditions, Potomac-Raritan-Magothy Aquifer System, Camden Area

By Anthony S. Navoy and Glen B. Carleton

 

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

AREA STUDIED: Camden, NJ and vicinity

AQUIFERS SIMULATED: Potomac-Raritan-Magothy aquifer system

MOST RECENT WITHDRAWALS SIMULATED: 1987 (original), 1996 (updated)

MODEL SIZE: 5 layers, 99 rows, 106 columns

MINIMUM GRID SPACING: 880 feet x 1,650 feet

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

 

The Potomac-Raritan-Magothy aquifer system, locally referred to as "the PRM," is composed of Cretaceous clastic deposits that are present at the base of the Coastal Plain sediments. These deposits extend southeast from the Fall Line and underlie southern New Jersey. The Delaware River flows across the outcrop of the aquifer system in the vicinity of Camden, New Jersey, and Philadelphia, Pennsylvania, and is, therefore, hydraulically connected to the aquifer system. The river is affected by tides throughout this reach, but is fresh most of the time.

The aquifer system provides most of the potable water supply for the Camden area. Groundwater withdrawals (pumpage), which began about 1900, currently (1987) total about 125 million gallons per day. The high rate of withdrawal has created a regional cone of depression in the aquifer system's potentiometric surface that extends more than 100 feet below sea level, reversing the natural hydraulic gradient between the aquifer system and the river. Under predevelopment conditions, groundwater discharged to the Delaware River. Now, the cone of depression provides the gradient to induce water to flow from the river into the aquifer system in many places. A significant amount of the recharge originates as precipitation on the local outcrop of the aquifer system. Groundwater also flows into the cone of depression from other parts of the aquifer system both laterally and vertically from overlying aquifers. The magnitude of the groundwater withdrawal has resulted in several potentially deleterious circumstances or threats to the potable supply from the aquifer: (1) deep cones of depression and continuing water-level decline, (2) movement of saline water from the downdip parts of the aquifer toward public-supply wells, (3) induced infiltration of saltwater from the Delaware River, and (4) induced infiltration of water containing contaminants from human-related activities on the aquifer system's outcrop area.

A finite-difference model was developed to simulate groundwater flow in the three aquifers of the Potomac-Raritan-Magothy aquifer system in the Camden area and adjacent parts of Pennsylvania. Results of the simulations were used to evaluate (1) the groundwater-flow system; (2) the sensitivity of the system to potential threats to groundwater potability; and (3) the effects of withdrawals, sea-level rise, and channel dredging on groundwater levels and on the flow budget of the aquifer system. The initial model input data and boundary flows between the modeled area and the parts of the aquifers outside the modeled area were derived from the New Jersey Coastal Plain Regional Aquifer System Analysis (RASA) model. Simulated results obtained with the calibrated model indicate that the most significant sources of water to the Potomac-Raritan-Magothy aquifer system are recharge from precipitation on the outcrop and flow from overlying aquifers. Induced flow from the Delaware River and related tributaries is simulated to be currently (1987) about 29 million gallons per day, which is about 25 percent of total withdrawals. Results of a particle-tracking analysis of the simulation show that about one-third of the water-supply withdrawals from the Potomac-Raritan-Magothy aquifer system in the Camden area are within the area influenced by induced recharge from the Delaware River and its tributaries. Lateral flow from outside the areas where water is withdrawn is about 12 million gallons per day. About 8 million gallons per day flows into the study area from the southeast (downdip). The induced movement of saline water from this direction could threaten the potability of the water supply.

The effects of future water-supply withdrawals on the Potomac-Raritan-Magothy aquifer system in the Camden area were evaluated by simulating three withdrawal scenarios with the groundwater flow model: withdrawals continued in an unconstrained manner (Scenario A), withdrawals maintained at current (1987) rates (Scenario B), and withdrawals reduced to 65 percent of 1983 rates (Scenario C). The distribution of withdrawals in each of the scenarios is identical. Withdrawals are simulated for the 30-year period from 1990 to 2020.

The rate of withdrawals in Scenario A was increased to 27 percent more than the current (1987) rate by the year 2020, when the regional cones of depression are predicted to extend to a maximum depth of about 140 feet below sea level. This is a decline of about 40 feet from present (1987) levels. Because the simulated increase in withdrawals was distributed linearly through time, the rate of decline was constant. Locally available recharge to the Potomac-Raritan-Magothy aquifer system would account for about 59 percent of the water withdrawn. Inflow, possibly containing saline water, from southeast (downdip) of the Camden area would account for about 6 percent of the water withdrawn.

The rate of withdrawals in Scenario B was the same as the current rate through the year 2020. The depths of the regional cones of depression are predicted to remain essentially at present (1987) levels because the withdrawals remained fixed. The simulated water-level stabilization would occur within a 5-year period. Locally available recharge to the Potomac-Raritan-Magothy aquifer system would account for about 63 percent of the water withdrawn. Inflow derived from downdip, possibly saline water from southeast of the Camden area would account for about 7 percent of the water withdrawn.

The rate of withdrawals in Scenario C was 35 percent less than the 1983 rate and was fixed at that rate until the year 2020. The regional cones of depression are predicted to extend to a maximum depth of about 60 feet below sea level. These levels, which are similar to those observed in the mid-1960's, represent a recovery of about 40 feet from present levels. The simulation results indicate that the majority of the recovery would take place over the initial 5-year period. With withdrawals fixed, water levels would remain essentially constant thereafter. Locally available recharge to the Potomac-Raritan-Magothy aquifer system would account for about 76 percent of the water withdrawn. Inflow derived from downdip, possibly saline water from southeast of the Camden area would account for about 9 percent of the water withdrawn. Although the inflow derived from saline water, when considered as a percentage, would be 6 percent in Scenario A and 9 percent in Scenario C, the actual volume of the flow decreased from Scenario A to Scenario C, as would be expected as a result of the lower withdrawal rates.

The significant difference among the three scenarios is the proportion of water derived from locally available recharge to water derived from distant sources when considered as components of the total amount of water withdrawn for public supply in the study area. As withdrawals increased from Scenario C rates to Scenario A rates, the proportion of water leaking from overlying aquifers and flowing from the Potomac-Raritan-Magothy aquifer system in areas distant from the Camden area increased from about 60 percent of the total water withdrawn to about 75 percent. The flow of water from the distant sources has contributed to the problems in this area. The minimization of dependency on the flow of water from distant sources may constitute a viable water-management objective for the Potomac-Raritan-Magothy aquifer system in the Camden area.