SPATIAL HYDROCHEMICAL TRENDS IN ARROYO SECO, A MAJOR TRIBUTARY OF THE LOS ANGELES RIVER

The Arroyo Seco stream channel transitions from its headwaters in the San Gabriel Mountains, through urban areas in Los Angeles, to the confluence with the LA River. In the San Gabriel Mountains, flows in Arroyo Seco originate from springs emitting groundwater from low-solubility igneous rocks, producing water with Total Dissolved Solids (TDS) of about 250 mg/L. The headwater channel has average values of nitrate and ammonia of 1.5 mg/L NO3-N and 0.04 mg/L NH3-N, respectively. Selenium is only 1.2 μg/L total Se in headwaters. Most of the flows in the headwaters are collected in groundwater recharge basins above Devil's Gate Reservoir. The first significant flows in the urban section of Arroyo Seco occurs from tributary inflows above and below Devil's Gate Reservoir, and from dry weather urban runoff. Continuing along the urban stretch of Arroyo Seco, TDS rises from about 650 mg/L to about 1200 mg/L near the junction with the LA River. Nitrate increases from about 0.6 to 7.0 mg/L NO3-N along the urban reach, while ammonia increases minimally from about 0.02 to 0.07 mg/L NH3-N. Selenium increases from <0.4 ug/L to 3.0 μg/L total Se in the same section of stream. These changes are associated with groundwater inflows through voids in the concrete lining of Arroyo Seco. To confirm this interpretation, 25 groundwater samples were collected from springs and channel voids where groundwater flows into Arroyo Seco. Groundwater varies from 620 to 1,600 mg/L TDS and 7.6 to 11.2 mg/L NO3-N, with ammonia usually close to or below detection limits. Selenium ranges from 2 to 70 μg/L total Se in groundwater. The hydrochemical signature of influent groundwater is highly correlated with geologic units in the Arroyo Seco Watershed, transitioning from low solubility igneous rocks in the headwaters, to marine sandstones, siltstones, and subsidiary mudstones of the Topanga Formation, to claystones, siltstones, and subsidiary sandstones of the Monterey Formation. The Monterey Formation is known for sequestered selenium that has been shown to be released into groundwater due to nitrate-facilitated oxidation. Isotopic data is being analyzed to confirm and determine the primary sources of surface water, including both native and imported sources. Other isotopes and hydrochemical tracers are being used to determine sources of nitrate, sulfur, and selenium in both surface water and groundwater in Arroyo Seco.