EFFECTS OF GEOLOGICAL HETEROGENEITY on APPARENT REACTION RATES AND ISOTOPE FRACTIONATION

Field-based methods to estimate reaction parameters from contaminant concentrations and isotopic compositions are often based on assumptions of homogeneous hydraulic conductivity. For complex geological systems, differences between field-scale (apparent) estimated reaction rates and isotopic fractionations and local-scale (intrinsic) effects are poorly understood. For a heterogeneous alluvial fan aquifer near Merced, California, numerical models and field observations were used to study the effects of physical heterogeneity on reaction parameter estimates derived from monitoring wells with small screened intervals. Field measurements included major ions, age-tracers, stable isotopes, and dissolved gases. Parameters were estimated for the O₂ reduction rate, denitrification rate, O₂ threshold for denitrification inhibition, and stable N isotope fractionation during denitrification. For six geostatistical realizations of the aquifer, inverse modeling was used to establish reactive transport simulations that were consistent with field observations. In subsequent numerical experiments, the estimated model parameters served as “true” (intrinsic) values for comparison with “apparent” parameters estimated from simulated bulk sample concentrations. In these scenarios non-Gaussian dispersion decreased the magnitude of apparent reaction rates and isotope fractionations more than Gaussian mixing alone. The effect was more pronounced with increasing distance of travel from the water table to the monitoring well. The ratio of apparent to true rate constants and fractionation parameters was less than 0.1 in some scenarios with long travel times or rapid reactions. The decreased magnitude of apparent isotope fractionation caused by mixing can explain common differences between published laboratory and field estimates. Likewise, simulated effects of heterogeneity on apparent O₂ threshold values for denitrification are consistent with previous reports of higher values in aquifers than in the laboratory. These results highlight the extent to which hydrogeological complexity can influence the interpretation and prediction of reactive transport based on field data.