Age dating of shallow groundwater with chlorofluorocarbons, tritium/helium 3, and flow path analysis, southern New Jersey coastal plain

Groundwater age dating through the combination of transient tracer methods (chlorofluorocarbons (CFCs) and tritium/helium 3 (3H/3He)) and groundwater flow path analysis is useful for investigating groundwater travel times, flow patterns, and recharge rates, as demonstrated by this study of the homogeneous shallow, unconfined Kirkwood-Cohansey aquifer system in the southern New Jersey coastal plain. Water samples for age dating were collected from three sets of nested observation wells (10 wells) with 1.5-m-long screens located near groundwater divides. Three steady state finite difference groundwater flow models were calibrated by adjusting horizontal and vertical hydraulic conductivities to match measured heads and head differences (range, 0.002-0.23 m) among the nested wells, with a uniform recharge rate of 0.46 m per year and porosities of 0.35 (sand) and 0.45 (silt) that were assumed constant for all model simulations and travel time calculations. The simulated groundwater travel times increase with depth in the aquifer, ranging from about 1.5 to 6.5 years for the shallow wells (screen bottoms 3-4 m below the water table), from about 10 to 25 years for the medium-depth wells (screen bottoms 8-19 m below the water table), and from about 30 to more than 40 years for the deep wells (screen bottoms 24-26 m below the water table). Apparent groundwater ages based on CFC- and 3H/3He-dating techniques and model-based travel times could not be statistically differentiated, and all were strongly correlated with depth. Confinement of 3He was high because of the rapid vertical flow velocity (of the order of 1 m/yr), resulting in clear delineation of groundwater travel times based on the 3H/3He-dating technique. The correspondence between the 3H/3He and CFC ages indicates that dispersion has had a minimal effect on the tracer-based ages of water in this aquifer. Differences between the tracer-based apparent ages for seven of the 10 samples were smaller than the error values. A slight bias toward older apparent ages, found not to be statistically significant, was noted for the 3H/3He-dating technique relative to the CFC-dating technique. This result may be caused by enrichment of local air in CFC-11 and CFC-12 from urban and industrial sources in the northeastern United States and minor contamination from sampling equipment. The demonstrated validity of the combined tracer-dating techniques to determine the age of water in the Kirkwood-Cohansey aquifer system indicates that groundwater flow models can be refined when apparent ages based on 3H/3He- and CFC-dating are used as calibration targets.