Application of a new model for ground-water age distributions

Ground-water age distributions can aid in understanding ground-water flow systems because they provide a complete picture of the way in which different waters contribute to ground-water flow. This dissertation presents the first model of the ground-water age equation of Ginn (1999) and its application to part of a real-world ground-water flow system. An existing transport model is used to solve the ground-water age equation. The ground-water age model consists of two physical dimensions (axial and vertical) and the exposure-time dimension. The two-dimensional velocity field that is input for the physical dimensions in the ground-water age model was extracted from a three dimensional ground-water flow model of the Rialto-Colton Basin, California. The numerical approximation to the initial conditions is zero everywhere in the ground-water age model domain. Boundary conditions are represented by injection wells with source concentration of 1 kilogram per liter and are located in the same recharge cells as in the flow model. Dispersive mixing occurs in the physical dimensions while transport in the exposure-time dimension is through advection only. The resulting ground-water age distributions showed the expected trend of a greater distribution of mass to older exposure-time cells with distance from the recharge cells and with depth. As a result of this trend, the calculated mean ages increased with distance downgradient and with depth. Comparison of the ground-water age distributions and mean ages with the results of a particle-tracking model and isotopic data showed that the ground-water age distributions were similar to the histograms from the particle-tracking model but the mean ages were younger for the ground-water age model. The computed mean ages compared favorably to the isotopic data near the recharge sources; however, the difference between the isotopic ages and age components in the computed age distributions increased with distance from the recharge sources.