Determination of Groundwater Residence Times Using a 2-Parameter Mixing Model

This study presents a lumped parameter mixing model for the calculation of the mean residence time in groundwater and surface-water systems. The model follows the 2-parameter gamma distribution and simulates dispersive mixing using the concept of mixing efficiency, which varies from 0 to 1. The extreme cases of no mixing (piston or plug flow) and complete (perfect) mixing can be obtained from the model as special cases when the mixing efficiency = 0 and 1, respectively. Dispersive mixing occurs when 0 < mixing efficiency < 1. The two parameters of the model are the mean residence time and the mixing efficiency.

The model is applied to tritium data collected from a fractured aquifer. Application of the model resulted in good agreement between the predicted and observed tritium values, thus indicating reliable estimates of the mean residence time and nature of dispersive mixing in the aquifer.