REDEFINING THE TUSCAN AQUIFER SYSTEM (TAS) IN THE NORTHERN SACRAMENTO VALLEY, CALIFORNIA

The Tuscan Aquifer System (TAS) is a north-south trending, widespread hydrologic system in the northern Sacramento Valley of California. Understanding the stratigraphy of an aquifer system is vital to the management of groundwater resources for municipalities and agricultural users. Previous interpretations of the TAS are based on large-scale outcrop interpretations that do not apply to the subsurface. For this study, we use over 460 boreholes, petrographic data from well cuttings from 8 wells, and two vintages of Airborne Electromagnetic (AEM) data profiles. In general, the main aquifer framework includes the time-equivalent Pliocene-aged Tuscan and Tehama formations that interfinger in the middle of the basin overlain by Quaternary formations. From oldest to youngest, we propose the following stratigraphic designations to represent the TAS: Lower Tuscan (LT), Upper Tuscan/Tehama 2 (UTT2), Upper Tuscan/Tehama 1 (UTT1), and Quaternary (Q). The LT contains mostly amalgamated coarse-grained fluvial (in the west) and alluvial facies (in the east) that thicken to the west. The overlying UTT2 layer has fine-grained dominated fluvial overbank and floodplain facies with isolated coarse-grained dominated channels. UTT1 is mostly a coarse-grained dominated fluvial system. Q contains mostly coarse-grained fluvial sediments with a persistent fine-grained dominated layer. The UTT1 and UTT2 units thin to the east under the city of Chico due to an erosional surface that places the overlying Q units above the LT. South of Chico, the UTT2 combines with the LT into a single unit (UTT2/LT). Finally, there is an interpreted incised valley that formed during the time of Tuscan deposition that filled with coarse-grained dominated fluvial material in the vicinity of Butte Creek. Aquifer stratigraphy that is based on subsurface data rather than regional outcrop interpretations allows groundwater models to distribute aquifer and aquitard textures more accurately leading to a better understanding of how pumping affects the TAS.