Paper No. 14
Presentation Time: 12:00 PM
GEOLOGIC, STRUCTURAL, AND GEOCHEMICAL INVESTIGATION OF ROBUST SPRING DISCHARGE FROM A MAJOR LANDSLIDE IN THE EASTERN SAN GABRIEL MOUNTAINS, CALIFORNIA
Wingate spring system discharges from a major landslide in Kerkhoff Canyon, a tributary of San Antonio watershed located in the Eastern San Gabriel Mountains. Flow measurements taken during summer 2011 and spring- summer 2013 yield hydrographs following abnormally wet and dry recharge periods, respectively. Discharge from main Wingate spring varied from 334 to 281 gallons per minute during 2011 and 194 to 114 gpm during 2013. This spring demonstrates remarkably robust, sustained flow as recorded by low baseflow recession constants (0.0021 – 0.0017 days-1), compared to landslide-fed springs in nearby Icehouse Canyon (0.0068 days-1) and Upper San Antonio Canyon (0.0154 days-1). To investigate groundwater flow paths and reasons for the robust flow we mapped the local bedrock and surface deposits, and collected multiple spring water samples for geochemical analysis. Mapping reveals that landslide and alluvial deposits upgradient from Wingate Spring overlap a major strike-slip fault (San Antonio Fault) that displays a 100+ meter-wide damaged zone within quartz diorite gneiss. This transversely oriented structure may create a bedrock funnel to focus landslide discharge in addition to probable conduit for a deep groundwater source. Optimally oriented fracture networks may also provide pathways to discharge groundwater during drought periods. Preliminary geochemical analyses of alkalinity, pH, anions (Fluoride, Chloride, Nitrate, and Sulfate) from several springs on Wingate Ranch show intriguing results. Alkalinity ranges from 126 mg/L to 188 mg/L and pH from 7.25 to 7.89 which are consistent with natural waters of mountain front recharge. Initial anion data suggests that a spring discharging from alluvium has elevated nitrate (possibly a shallower source) compared to landslide-fed Wingate Springs. Still pending are results from tritium age dating and oxygen-hydrogen isotopes. The combined geochemical data should aid in the interpretation of near surface vs. deeper bedrock groundwater sources and flow paths within the study area. Wingate springs provide a unique laboratory for studying hydrogeologic controls of an important sustainable groundwater supply in recently uplifted crystalline mountain terrain.