The Potomac-Raritan-Magothy aquifer system is hydraulically connected to the Delaware River in parts of Camden and Gloucester Counties, New Jersey, and has more limited contact with the river in Salem County, New Jersey. The aquifer system is used widely for water supply, and 122 production wells that are permitted by the New Jersey Department of Environmental Protection to pump more than 100,000 gallons per year in the three counties are within 2 miles of the river. During drought, saltwater may encroach upstream from the Atlantic Ocean and Delaware Bay to areas where the aquifer system is recharged by induced infiltration through the Delaware River streambed. During the drought of the mid-1960's, water with a chloride concentration in excess of potability standards (250 mg/L (milligrams per liter)) encroached into the reach of the river that recharges the aquifer system. The vulnerability of the major production wells in the area to similar saltwater encroachment in the future is a concern to water managers. This vulnerability was evaluated by investigating two scenarios: (1) a one-time recurrence of the conditions approximating those that occurred in the1960's, and (2) the recurrence of those same conditions on an annual basis. Results of ground-water-flow simulation in conjunction with particle tracking and one-dimensional transport analysis indicate that the wells that are most vulnerable to saltwater intrusion are those in the Morris and Delair well fields in Camden County. A single 30-day event during which the concentration of dissolved chloride or sodium exceeds 2,098 mg/L or 407 mg/L, respectively, in the Delaware River would threaten the potability of water from these wells, given New Jersey drinking-water standards of 250 mg/L for dissolved chloride and 50 mg/L for dissolved sodium. This chloride concentration is about six times that observed in the river during the 1960's drought. An annually occurring 1-month event during which the concentrations of dissolved chloride or sodium in the river exceeds 1,818 mg/L or 358 mg/L, respectively, would threaten the potability of water from these wells. Wells outside the Morris and Delair well fields are substantially less vulnerable to the intermittent saltwater intrusion that was simulated.