Princeton University, under contract to the Department of Energy , maintains a Tokamak fusion test reactor in New Jersey. The U.S. Geological Survey investigated groundwater flow and estimated the effects of a hypothetical spill of radioactive water at the site on the local groundwater system. The study included test drilling; aquifer testing; measurement of water levels, infiltration capacity, and stream discharge; and a simulation of the hypothetical spill. The Triassic Stockton Formation-a water supply aquifer composed primarily of jointed siltstone and sandstone-underlies the site. The aquifer is confined by overlying weathered bedrock and underlying unjointed rock. Weathered bedrock is overlain by unconsolidated, partially saturated material which ranges from 6 to 39 ft in thickness. Groundwater recharge is by lateral flow into the study area, stream leakage, and precipitation. Discharge is by pumpage, evapotranspiration, stream inflow, and lateral flow out of the study area. Transmissivity of the aquifer is about 1,740 sq ft/day, and the storage coefficient is about 0.0002. The average linear velocity of groundwater at the site ranges from 100 to 270 ft/yr depending on location and time of year. The velocity over a large part of the site is controlled by on-site pumpage. Groundwater samples were collected and analyzed for common ions, trace metals, and tritium. The analyses reported no concentrations of common ions or trace metals which exceeded the criteria for drinking water standards recommended by the EPA, except for some instances of moderately high concentrations of iron and manganese. Iron and manganese are common in groundwater and surface water in the area and are not indicative of an on-site source of contamination. Tritium concentrations in the collected samples were also considered representative of background levels and were well below the maximum concentration permitted by the EPA. The fate of spilled radioactive water after a hypothetical accident would depend on the nature of the accident and weather. If the on-site pumpage continued after a spill, groundwater leaving the site probably would have only minimal tritium concentrations, but stream water leaving the site could contain significant tritium concentrations.