WATER QUALITY OF THE SAN ANTONIO WATERSHED OF THE SAN GABRIEL MOUNTAINS, CALIFORNIA

The San Gabriel foothills are used extensively for recreation and residential sprawl. Recent analysis of spring discharge proximate to an abandoned mine has indicated the presence of arsenic at 29 ug/L, well above the Environmental Protection Agency (EPA) primary drinking water standard (personal communication with Dr. Stephen Osborn), motivating this study. Yet, there is insufficient publicly available data assessing the presence of arsenic and other water quality parameters throughout San Antonio Watershed (SAW) and surrounding areas. The purpose of this research is to assess arsenic and basic water quality of this area.

The geology consists of igneous, metamorphic, and sedimentary rocks, primarily Proterozoic and Mesozoic granitics. Gold deposits in the San Gabriels primarily occur within the mylonite gneiss of the Vincent Thrust Fault, due north of the study area, from the hanging walls in the rocks of the Pre-Cambrian gneisses, Triassic, Jurassic and Late Cretatceous granitoid, and mafic components of the Pelona Schist. Arsenic has been associated with gold deposits and could potentially be a source along with other metals in the water. There are multiple retired gold mines in this region with extensive intrusions and metamorphic rocks, suggesting a possibility to impact waters.

More than 100 Water samples were collected at Evey Canyon, San Antonio Canyon, Cascade Canyon, and Barrett Canyon throughout the course of ten months in order to observe any seasonal effects on water quality. Anions (F, Cl, Br, NO3, and SO4) were analyzed by Ion Chromatography in the Hydrogeology laboratory at Cal Poly Pomona. Alkalinity titration was conducted within 24 hours of sampling. Cation analyses were conducted at the Environmental Chemistry Laboratory at UC Riverside. Along with heavy metal concentration analysis of the samples, alkalinity, pH, and sulfide concentrations are also analyzed, as well as compositional analysis of the surrounding rocks. Preliminary results of cations known to the EPA as contaminants of drinking water show no traces of arsenic, cadmium, copper, manganese or lead in water samples collected. Trace concentrations of barium range from 0.00-0.57 mg/L and, zinc up to 0.015 mg/L and iron up to 0.160 mg/L, all under EPA primary drinking water standards.