[1] Interpretation of groundwater ages typically rests on assumptions of minimal mixing of different water ages in the water samples. The effects of three-dimensional, geologic heterogeneity on groundwater mixing and tracer concentrations, however, have not been evaluated. In this study, we use a series of 10 detailed geostatistical realizations along with high-resolution numerical groundwater flow and contaminant transport simulation to model distributions of groundwater ages and chlorofluorocarbon (CFC) ages at wells within a heterogeneous stream-dominated alluvial fan aquifer system. Results show that groundwater reaching a well in the heterogeneous aquifer system typically consists of a wide distribution of groundwater ages (often spanning >50 years), even over short (<1.5 m) screened intervals. Additionally, simulated arithmetic mean groundwater ages do not correspond to mean ages estimated from simulated CFC concentrations. Results emphasize the potential ambiguity of ''mean'' groundwater ages estimated from environmental tracer concentrations in typically heterogeneous geologic systems. The significant dispersion of groundwater ages also implies that ultimate, maximum effects of nonpoint source, anthropogenic contamination of groundwater may not be reached until after many decades or centuries of gradual decline in groundwater quality.

The random-walk method for simulating solute transport in porous media is typically based on the assumption that the velocity and velocity-dependent dispersion tensor vary smoothly in space. However, in cases where sharp interfaces separate materials with contrasting hydraulic properties, these quantities may be discontinuous. Normally, velocities are interpolated to arbitrary particle locations when finite difference or finite element methods are used to solve the flow equation. The use of interpolation schemes that preserve discontinuities in velocity at material contacts can result in a random-walk model that does not locally conserve mass unless a correction is applied at these contacts. Test simulations of random-walk particle tracking with and without special treatment of material contacts demonstrate the problem. Techniques for resolving the problem, including interpolation schemes and a reflection principle, are reviewed and tested. Results from simulations of transport in porous media with discontinuities in the dispersion tensor show which methods satisfy continuity. Simulations of transport in twodimensional heterogeneous porous media demonstrate the potentially significant effect of using a nonconservative model to compute spatial moments and breakthrough of a solute plume. Introduction The random-walk particle method (RWPM) has been used successfully for years to simulate conservative and reactive transport in porous media [Ahlstrom et al., 1977; Prickerr et al., 1981; Uffink, 1985; Tompson e! al., 1987; Tompson, 1993]. This method is computationally appealing because it is grid independent and therefore, given the proper conditions, will require little computer storage relative to finite element, finite difference, and method of characteristic models. In addition, this method does not Suffer from numerical dispersion in problems dominated by advection. Traditional finite element and finite difference models generally perform poorly under such conditions unless the computational grid is highly resolved. As a result, a random walk is often the method of choice for simulating transport in large, heterogeneous flow systems [Tompson and Gelhat, 1990; Tompson, 1993; Tompson et al., 1994]. Global mass conservation is compulsory with the RWPM because particles cannot disappear. This distinct advantage of the RWPM, however, is often overstated; accurate solutions require local as well as global mass conservation.In practice, the flow Problem is often solved numerically, and velocities are interpolated to arbitrary particle locations.Advective particle tracking models can be made mass conservative by using a divergence-free velocity interpolation scheme [Schafer-Perini and Wilson, 199i]. However, additional criteria are necessary to formulate a mass conservative random-walk model. For example, discontinuities in the velocity or effective porosity may yield a dispersion tensor that is discontinuous in space. Local mass conservation conditions for the RWPM require that the dispersion tensor be continuous in spa...

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