WATER QUALITY ASSESSMENT OF THOMPSON CREEK WATERSHED (TCW), SAN GABRIEL MOUNTAINS, CA

The area of study for this field investigation is the Thompson Creek Watershed (TCW) between the San Gabriel Mountains and the Angeles National Forest. There is a popular 5-mile hiking trail apart of the Claremont Wilderness Park where locals can escape the surrounding suburban area and witness wildlife such as deer, coyotes, and small fish to name a few. TCW discharges at the base of the Cobal Canyon into Thompson Creek Dam where the water is stored before percolating and recharging the Six Basin aquifer. Despite wildlife and communities relying on the quality of this water, little information is publicly available; thus, this project assesses the basic water quality of TCW.

A total of 24 water samples were collected throughout watershed from springs and accessible areas along the Thompson Creek channel during a 6-month period from February 2019 to July 2019. At the time of sampling, field measurements of pH and flow velocity were taken. Water samples were analyzed for total alkalinity and anions (F, Cl, Br, NO₃, and SO₄) at the Hydrogeology Laboratory at Cal Poly Pomona. Cations were analyzed at the Environmental Chemistry Laboratory at UC Riverside.

Results tested for seasonal changes and were compared to primary and secondary standards of the Environmental Protection Agency (EPA). Data collected from springs will be examined separately, as those results overwhelmingly showed high concentration levels; however, data collected along the stream that flows directly to the Thompson Creek Dam show promising results. Anions fluoride, chloride, nitrate, bromide, and sulfate displayed concentrations below the secondary EPA standards. Chloride, a proxy for salinity, ranged from 4.1 mg/L to 37.5 mg/L and sulfate concentrations ranged from 1.97 mg/L to 73.0 mg/L, both of which are well below the secondary standard of 250 mg/L. Cations studied-arsenic and iron-provided more insight on the overall quality. Arsenic concentrations were non-detectable, but iron concentrations were above the secondary standards of 0.3 mg/L ranging from non-detect to 0.82 mg/L. Overall, the data collected shows no abundance of harmful chemicals and suggests TCW is suitable for public consumption.