INVESTIGATING VARIABILITY OF TRANSMISSIVITY VALUES IN THE CORNING SUB-BASIN, NORTHERN SACRAMENTO VALLEY, CALIFORNIA

The Corning sub-basin is located in the northern part of California's Sacramento Valley, primarily within Tehama County, and is the primary water supply for the Tehama West Watershed. The aquifer system consists of late Tertiary to Quaternary deposits, including Holocene alluvium, Pleistocene terrace deposits (Modesto and Riverbank Formations), and Pliocene Tehama and Tuscan Formations, with the Tehama Formation serving as the primary groundwater source. Variability in transmissivity values is expected across these formations, but how they are distributed is unknown. Current groundwater models oversimplify this variability, limiting the effectiveness for sustainable water management, particularly as the region faces increasing water demand.

The objective of this research is to examine how aquifer transmissivity varies across different areas of the Corning sub-basin and to compare these observed values with those used in current groundwater models. Additionally, this study seeks to determine whether the variability is related to depth or lateral facies changes.

The methods for this research involved analyzing all available well completion reports (WCRs) in the Corning sub-basin (n=3819). WCRs containing key information, such as lithology, screen interval depths, reliable well locations, and pump test data (including drawdown and specific capacity), were selected for further analysis (n=87). Transmissivity values were then calculated using the empirical relationship between specific capacity and transmissivity from Razack and Huntley (1991). These values were then compared to current groundwater models by multiplying the modeled hydraulic conductivity (K) at the well site with the modeled aquifer thickness (b). The calculated transmissivity values were also plotted throughout the sub-basin to determine the controls on aquifer heterogeneity. Preliminary results show considerable difference between modeled and calculated values. In addition, the variability is likely influenced by variations in depositional environments within a fluvial/alluvial system as well as the depth of the screened intervals. The findings of this research can help calibrate existing groundwater models that currently use simplistic distributions of transmissivity values.