HYDROGEOLOGY AND GEOCHEMISTRY OF BEDROCK VS. LANDSLIDE-SOURCED SPRINGS IN THE EASTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

We present preliminary results of geological mapping, flow gauging and geochemical analyses from springs in the Palmer Canyon, Evey Canyon and Icehouse Canyon tributaries of the San Antonio Creek watershed. The May through July, 2013 observation period captured very low base flow conditions after two successive years of abnormally low precipitation recharge. Two types of springs were mapped, gauged and sampled in each sub-drainage: 1) direct discharge from fault zones or fracture networks in quartz diorite bedrock, and 2) drainage from boulder-cobble clast landslides along basal contacts with impermeable crystalline rock. An important structural feature is the N45E-striking Evey Canyon fault zone that transects the upper parts of Evey and Palmer watersheds and dips about 60 degrees NW, up-gradient. We recorded a dramatic downstream-gain in discharge from 0 to 38 gallons per minute (gpm) along a 300 meter segment of Evey Canyon that crosses this fault zone. Farther downstream, additional gain to 57 gpm records contribution from the Evey landslide deposit. Springs in E-striking Icehouse Canyon discharge from Cedar landslide that overlaps the N80W/55NE Icehouse Canyon fault. This 100+ meter wide zone of damaged quartz-diorite gneiss dips beneath a southerly inclined water table in the landslide and may create a groundwater dam and/or permeable fracture network to facilitate transmission of deep groundwater and mixing with shallower landslide-derived discharge. Preliminary alkalinity and conductivity measurements suggest low total dissolved solids in the Evey and Palmer springs (194-216 mg/l), with values slightly elevated to 247mg/l downstream from the Evey landslide. pH values range from 7.42 to 8.55 in these samples, consistent with most natural waters. Still pending are tritium ages and hydrogen-oxygen isotope analyses from several springs that should aid in quantifying relative components of deep vs. shallow groundwater and associated flow paths. Measurements of major ions are also in progress and may provide insight into mixing relationships. Our future work plan is to resample these springs during and after the next significant precipitation recharge events that are expected to include El Nino type storms.