SPRING, CREEK, AND GROUNDWATER CHEMISTRY IN THE BIG CHICO CREEK ECOLOGICAL RESERVE AND THE BUTTE BASIN, CALIFORNIA

The Big Chico Creek Ecological Reserve (BCCER) is located in the canyon of Big Chico Creek in the foothills of the Cascade Range east of the Sacramento Valley of northern California. Three geologic formations are exposed and dip at less than ten degrees to the east. Marine sandstones of the 75–80Ma Chico Formation are overlain by the ca. 13Ma Lovejoy Basalt, which is overlain by 2–4Ma Tuscan Formation (3 units of lahar-deposited sandstones, breccias, and tuffs). To the east of the BCCER the Tuscan Formation is the principal water supply aquifer in the Butte Basin. Water chemistry data have been collected for numerous ephemeral and perennial springs emerging from all three formations in the BCCER and compared to creek and aquifer water chemistry to investigate recharge, flow pathways, mixing, and water-rock interactions. Field values for electrical conductivity, pH, and oxidation-reduction potential as well as lab analysis of major ions indicate a difference in water origins. This difference can be attributed to water-rock interactions occurring along subsurface flow paths. The measured pH range of the Tuscan Formation waters is 6–6.5 while waters from the Chico Formation are 7.5. The measured average electrical conductivity value for waters of the Tuscan Formation is 225 µS/cm with measured OR values (>100mV) indicating an oxidizing environment. Measured EC values for waters of the Chico Formation average 715 µS/cm with measured OR values (<0mV) indicating an oxidizing environment. Reported chemistry of Big Chico Creek varies seasonally reflecting relative proportions of spring discharge and meteoric inputs. Reported values for creek waters exhibit a pH range of 7.5–8 with little variation due to seasonal changes. The reported EC for the creek varies both spatially and temporally where increased salinity can be attributed to discharge and contact of the Chico Formation. Equilibrium modeling was used to constrain water chemistry evolution and indicates aqueous speciation appropriate to the mineralogy of the different formations.