Application of a new Model for GroundWater age Distributions
Abstract
Modeled groundwater age distributions can aid in understanding groundwater flow systems because they provide a continuous indication of contributions from different source waters that span a range of ages. The groundwater age equation of Ginn (Ginn, T.R., 1999, On the distribution of multicomponent mixtures over generalized exposure time in groundwater flow and reactive transport: Foundations; formulations for groundwater age, geochemical heterogeneity, and biodegradation, Water Resources Research, 35(5):13951408) was solved numerically to obtain groundwater age distributions for a part of a realworld groundwater flow system. The results of the groundwater age model were compared with results from a particletracking model that accounts for dispersion. The means of the groundwater age distributions also were compared with isotopic (tritium and carbon14) ages of groundwater at selected locations. The groundwater age model consists of two physical dimensions (axial and vertical) and the exposuretime dimension. The twodimensional velocity field for the physical dimensions in the groundwater age model was extracted from a threedimensional groundwater flow model of the RialtoColton Basin, California. The velocity in the exposuretime dimension is unity. The numerical approximation to the initial conditions is zero everywhere in the groundwater age model domain. Source concentrations are located in the same cells where recharge occurs in the flow model. Dispersive mixing occurs in the physical dimensions in the groundwater age model whereas transport in the exposuretime dimension is through advection only. The resulting simulated groundwater age distributions showed the expected trend of a greater distribution of mass in older exposuretime cells with distance from the recharge cells and with depth in the groundwater age model domain. As a consequence of this trend, the calculated mean ages increased with distance downgradient from the recharge sources and with depth. Comparison of the groundwater age distributions and mean ages with the results of the particletracking model showed that the groundwater age distributions were similar to the frequency of particles of different ages at selected locations but the mean ages were younger for the groundwater age model. The computed mean ages from the groundwater age model compared favorably to the isotopic ages near the recharge sources; however, the difference between the isotopic ages and the computed mean ages increased with distance from the recharge sources.
 Publication:

AGU Spring Meeting Abstracts
 Pub Date:
 May 2005
 Bibcode:
 2005AGUSM.H51A..08W
 Keywords:

 1829 Groundwater hydrology;
 1832 Groundwater transport;
 1899 General or miscellaneous