DISCHARGE AND OXYGEN-HYDROGEN ISOTOPIC CHARACTERISTICS OF FAULT ZONE-SOURCED SPRINGS IN EVEY AND PALMER CANYONS, EASTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

We present results of geological mapping, flow gauging and oxygen-hydrogen isotopic analyses from bedrock-sourced springs that feed the Evey Canyon and Palmer Canyon tributaries of the San Antonio Creek watershed. The June 2013 through July 2014 observation period captured very low base flow conditions during years 2 and 3 of an extended drought. An important structural feature is the N45E-striking Evey Canyon fault zone (ECFZ) that transects Evey and Palmer watersheds and dips about 60 degrees NW, up-gradient. We recorded a dramatic downstream-gain in discharge from 0 to 38 gpm along a 180 m segment of Evey Canyon crossed by the ECFZ. Similar gains occur in Palmer Canyon where direct discharge occurs from pervasively fractured quartz diorite. Both canyons were dry upstream from these sites, indicating that the ECFZ is an important groundwater source. To test the isotopic character of this groundwater and assess potential flow paths we ran ¹⁸O, deuterium and tritium analyses on three spring samples. ¹⁸O-deuterium data plot on the local meteoric water line, indicating that the original meteoric source water has not been affected by fractionation or mixing with an anomalous groundwater component. Evey-Palmer Canyon values are less negative than data from higher elevation Icehouse Canyon, consistent with a warmer source for the original precipitation. Tritium analyses from the three springs range from 1.5 to 3.7 tritium units (TU). These results yield calculated groundwater “ages” between 20 and 37 years, using a base value of 11.5 TU for local rain collected October 2013. The isotopic data suggest that groundwater discharge from the Evey fault zone during this very low base-flow period is likely dominated by decades-old groundwater originally derived from local precipitation. We postulate a lengthy flow path in which deep-seated groundwater in hanging wall fractures is channeled into the ECFZ and discharged in topographically low areas of Evey and Palmer Canyons.