Limitations of the use of environmental tracers to infer groundwater age

Apparent ages obtained from the measured concentrations of environmental tracers have the potential to inform recharge rates, flow rates, and assist in the calibration of groundwater models. A number of studies have investigated sources of error in the relationships between the apparent ages, and the age assumed by models to relate this quantity to an aquifer property (e.g., recharge). These studies have also provided a number of techniques for correcting the known biases of apparent ages. In this paper, we review some of the concepts of age bias. We then demonstrate this bias through the use on four numerical examples, and assess the accuracy of correction methods in overcoming this bias. We examine this for CFCs, SF6, 3H/3He, 39Ar, and 14C. We demonstrate that in our four simple steady-state aquifer examples, bias occurs for a wide range of environmental tracers and flow configurations. When applying correction methods, we found that the values obtained are limited by the model assumptions. Models accounting for exchange with aquitards represent whole mobile zones and not discrete well screens. Mean transit times (comparable to mean ages) obtained from lumped parameter models deviate from actual values as the assumed distribution varies from the actual distribution. Methods that use multiple tracer ages are limited to ranges where both tracers report apparent ages. Our findings suggest that the incorporation of environmental tracer data into the understanding of groundwater systems requires approaches such as the direct use of concentrations, or the simulation of full age distributions.