INVESTIGATION OF ANOMALOUS SULFATE CONCENTRATIONS WITHIN THE SAN ANTONIO CANYON WATERSHED OF THE SAN GABRIEL MOUNTAINS, CALIFORNIA

Although water supply is regularly tested in North Fork Barrett Canyon, this study is imperative with regards to the residents who tap directly from the local streams and springs. The area is also used heavily for recreation, where both visitors and animals drink from the local water. In the past, sulfate in nearby Cascade Canyon has been as high 1,706 ppm (Yaralian, 2017). Current results show sulfate is as high as 836 ppm. Sulfate concentrations appear to decrease downgradient. The objective of this research is to analyze various streams and springs, as well as local rocks, for the source of these anomalous concentrations of sulfate.

The San Antonio Watershed lies in the eastern San Gabriel Mountains and is bounded to the north by Mt. Baldy. The canyons of interest include the main San Antonio Canyon, along with some of its tributaries, Evey Canyon, Barrett Canyon, and Cascade Canyon. Water samples were collected bi-weekly from November 2018 to August 2019, in addition to spot samples and site monitoring, although some sites had flowing water only during the wet season. The samples were analyzed for alkalinity, pH, oxidation-reduction potential for selected samples, and concentrations of anions (IC, at CPP) and cations (ICP-OES, at UCR). Hydrochemical data from past research by alumni and faculty is also included in this study, as well as precipitation records from 2017 to 2019 that cover a very dry water year followed by an anomalously wet water year. This cycle may cause seasonal fluctuations in sulfate concentration.

Sulfate in water systems has three main sources; the decomposition of organic matter via microbes, precipitation from environmentally impacted areas, and local geology. The latter source is hypothesized to have a far more significant role, due to the abundance of metamorphosed marine sediments, igneous intrusions, mylonites, and basement granitoids. Spring Hill landslide is of particular interest as it may contribute groundwater to the Barrett and Cascade watersheds, and sulfur-bearing horizons are locally exposed. The most ubiquitous sulfide mineral present in the area is pyrite, which typically releases high amounts of sulfate and iron as it weathers. The local rocks were sampled and analyzed (XRF, at WSU) to supplement the water data by narrowing down the sulfate source to a specific area and rock unit.