Toward new models of subsurface heterogeneity: An alluvial fan sequence stratigraphic framework with transition probability geostatistics

Modeling groundwater flow and contaminant transport in highly heterogeneous alluvial aquifer systems requires combining conceptual geological tools with numerical modeling techniques. Our approach incorporates sequence stratigraphic conceptual models with transition probability geostatistics to develop characterizations of stream-dominated alluvial fan aquifer systems. An advantage of this method is that it allows modeling of heterogeneity at several scales. We present an application of this approach on the Kings River alluvial fan, located in Fresno County, California.;Sequence stratigraphy provides a framework for understanding stratigraphic packaging within the alluvial fan, and it allows for prediction of the complex hydrofacies distributions. Laterally continuous, low-permeability paleosols mark unconformities that bound the largest-scale stratigraphic features, or sequences, within the Kings River alluvial fan. The paleosols were modeled separately from the other facies since they were developed after sequence deposition and are correlated to the large-scale sequence boundary rather than internal hydrofacies. Within each sequence, intermediate-scale hydrofacies distributions are modeled using a three-dimensional Markov chain followed by sequential indicator simulation. Hydrofacies do not correlate across sequence boundaries since the sequences are separated by unconformities; thus, each sequence was modeled individually. The realizations of paleosol distributions and internal sequence hydrofacies are combined into a single realization of the aquifer heterogeneity. This multiscale approach, where large-scale paleosols are modeled around the intermediate-scale internal hydrofacies, produces a geologically-realistic characterization of the aquifer heterogeneity.;The significance of this characterization procedure was tested using three scenarios of heterogeneity with different paleosol arrangements. Results of this modeling indicate that the paleosols tend to reduce vertical plume dispersion, where contaminants move across paleosols only at specific locations where vertical head gradients are large. In contrast, greater contaminant dispersion is predicted throughout the aquifer if paleosols are not included in the characterization.;Additionally, results show that groundwater consists of a mix of different ages. Therefore, use of environmental tracers to provide groundwater ages may be misleading. Instead, the environmental tracer concentrations should be used to calibrate models to produce age distributions that correspond to measured environmental tracer concentrations.