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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.
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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.
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).
These different sources of He to the atmosphere (Wuest et al., 1992), and will similarly lead to underestimates of ground-water ages (Schlosser et al., 1989).
Analytical uncertainties usually result in errors in age estimates of less than 10% (Solomon et al., 1993).
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.
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.
measurable) amounts of radiogenic He content of ground-water samples.
The solid-to-liquid mass transfer rate can be determined directly in the laboratory or by calibration using ground-water age data obtained using other (i.e. Once the solid-to-liquid mass transfer rate has been estimated, ground-water ages are determined by measuring total He and Ne in ground-water samples, and computing the component of radiogenic Please contact Carol Kendall ([email protected]) for questions and comments regarding this page.
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