DISCHARGE AND GEOCHEMICAL CHARACTERISTICS OF EVEY CANYON AND ICEHOUSE CANYON SPRINGS, SAN GABRIEL MOUNTAINS, DURING EXTENDED DROUGHT

Evey Canyon and Icehouse Canyon (IH Cyn) are major tributaries that feed the San Antonio Creek watershed in the San Gabriel Mountains. An improved understanding of flow rates and water quality is important for water resources management of downstream cities and stakeholders. Presented herein are results of flow gauging and geochemical analyses acquired during years 2 and 3 of an extended drought. Since May 2013, we gauged and sampled two types of springs: 1) direct discharge from the cracks and fractures of fault zones in the quartz diorite bedrock, and 2) drainage from permeable landslide deposits in contact with bedrock. Hydrographs of the trunk stream below Evey springs show two base-flow recession periods interrupted by a minor recharge event in December 2013. Evey Creek discharge varied from 71 to 22 gpm, with 2014 base flow values lower than the previous year. Total dissolved solids increased systematically (192 to 257 mg/l) downstream from the source springs, with sulfate values increasing from 25 to 38 mg/l. These trends suggest increased interaction with the surrounding rock and soil units. In IH Cyn, we sampled four springs for oxygen-hydrogen isotope and tritium analysis to assess the degree mixing of deeper bedrock-sourced groundwater with shallower waters related to landslide discharge. The ¹⁸O and deuterium analyses 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 source. IH Cyn data have more negative values on the meteoric line than Evey Canyon data, consistent with colder, higher elevations of IH Cyn watershed. Results of tritium analyses range from 1.9 to 3.0 tritium units (TU). Using a base value of 11.5 TU for local rain collected October 2013, we calculate “ages” between 32 and 24 years for the four springs. These results suggest that IH Cyn springs tap a deep, old groundwater source that originated as local precipitation.