Aqueous geochemistry of waters and hydrogeology of alluvial deposits, Pinnacles National Park, California

First posted May 17, 2022 For additional information, contact: Director, WMA- Laboratory & Analytical Services DivisionU.S. Geological SurveyUSGS Headquarters12201 Sunrise Valley DriveReston, VA 20192 A cooperative study between the National Park Service (NPS) and U.S. Survey (USGS) characterized groundwater quality hydrogeology in parts of Pinnacles Park. The water-quality investigation assessed geochemistry springs, wells, surface water, precipitation analyzed rock formations that affect water chemistry through water-rock interaction. used geophysical level data to characterize groundwater-flow processes alluvial deposits Bear Chalone Creek watershed.Analysis aqueous geochemical parameters samples from perennial water-supply waters was conducted for collected after dry season (autumnal) wet (vernal) assess changes due levels or flow resulting precipitation. bulk during also analyzed. Bedrock were help constrain sources, paths, weathering. investigations show a correspondence source spring-water can be attributed mineralogy rock. narrow range strontium isotopes samples, sourced geochemically mineralogically disparate rocks, indicates bedrock is relatively old has reached quasi-steady state with respect weathering susceptible minerals.Groundwater-level monitoring indicated table shallow—from 0 10 meters (m) below land surface. In southern alluvium, rose declined by several over each annual cycle this study. northern Valley, modestly two seasons but third season. groundwater/surface-water interaction occurs along reach Sandy Creek. Groundwater discharges upstream part reach, becomes partly consumed evapotranspiration, infiltrates farther downstream. runoff-generated surface-water intermittent stream reaches major component recharge. After onset significant streamflow, creek rapidly recharges until rise nearly level. generally remain high throughout season, then gradually decline dry.