760-766 Gw S-o 04

In hydrogeological investigations, the scale at which model parameters (e.g., hydraulic conductivity) are measured is usually quite different from the scale at which heads and contaminant concentrations are measured. The scale of grid blocks used in numerical simulations is usually different from the aforementioned two scales. However, measured concentrations and heads are used for calibration and validation of models developed at a much larger scale. For a relatively homogeneous aquifer, the concentration distribution over a numerical block may be so smooth that the deviation of point concentrations may be reasonably small. For heterogeneous aquifers, the concentration distribution over a numerical block may be far from smooth and the discrepancy between measured and simulated point concentrations may be significant. Davis (1986) found that field concentration data could not be adequately matched by coarse-grid (two to six times wider than the actual alluvial channels) numerical simulations that neglect the small-scale hydraulic conductivity and use large values of longitudinal dispersivities. Fitts (1996) found that significant concentration uncertainties were introduced by the use of macrodispersivity to represent mixing caused by small-scale velocity field variation. He used log(ca/cm), where ca is actual concentration and cm is modeled concentration at the same point, to represent the deviation of predicted concentrations from measured concentrations and found that the distribution of log(ca/cm) had a standard deviation of 0.7 for the field tracer experiments conducted in slightly heterogeneous aquifers at Cape Cod, Massachusetts, and the Borden site in Ontario, Canada. Peng et al. (2000) found that the use of a large dispersivity to compensate for ignorance of the true hydraulic conductivity distribution led to a loss of accuracy in predicting breakthrough curves. High-resolution numerical simulation results (Tompson et al. 1996; Naff et al. 1998a, 1998b) show that the spatial variation of aquifer properties at small scales may significantly affect the transport of solutes, especially reactive chemicals. Numerical simulations of Weissmann et al. (2002) show a high degree of variability in tracer residence time distribution among 10 different realizations of a heterogeneous aquifer. Abstract The influence on solute transport of the small-scale spatial variation of aquifer hydraulic conductivity (K) was analyzed by comparing results from fine-grid (2 m by 2 m) simulations of a synthetic heterogeneous aquifer to those from coarse-grid (8 m by 4 m) simulations of an equivalent homogeneous aquifer. Realizations of the K field of the heterogeneous aquifer were generated, using the Monte Carlo approach, from a lognormal distribution with mean log K of 2 (K in m/d) and three levels of log K variance of 0.1, 0.5, and 1.0. Numerical simulation results show that the average standard deviation of point concentrations increased from 1.21 to 5.78 when the value of log K variance was increased from 0.1 to 1.0. The average discrepancy between modeled concentrations (obtained from a coarse-grid deterministic numerical simulation) and the actual mean point concentrations (obtained from fine-grid Monte Carlo numerical simulations) increased from 0.91 to 4.23 with the increase in log K variance. The results from this study illustrate the uncertainty in predictions from contaminant transport models due to their inability to simulate the effects of heterogeneities at scales smaller than the model grid.