NON-STATIONARY HYDROSTRATIGRAPHIC MODEL OF CROSS-CUTTING ALLUVIAL FANS

The groundwater system of southern Sacramento County was formed by the glacially dominated American River fan and the non-glacial Cosumnes River fan. Recent hydrostratigraphic analysis with borehole log data shows that the American River gravelly channels not only migrated to the south, cutting into the Cosumnes sediments in the Holocene, but also did the same during the Quaternary, leaving deep, coarse grained channels embedded in what would otherwise be considered Cosumnes fan sediments. Three dimensional Markov chain models are created for each of the two systems using transition probability geostatistics. Each system is simulated separately and then combined to produce a regional three-dimensional representation of subsurface heterogeneity which preserves cross-cutting relationships between American River coarse grained sediments and the Cosumnes River fan. This geologic model is incorporated into a simple flow model to investigate the impact of these channels on regional groundwater flow and transport.