Perchlorate has impacted surface streams and groundwater at a California Department of Defense contractor site. The site is a former solid rocket propellant manufacturing facility that operated from 1959 through 2004. The site occupies 5,200 acres, with operations primarily situated in alluvium deposited within broad stream valleys. The confluence of several small streams occurs at the downgradient property boundary at a creek which then flows into a drinking water reservoir for a nearby city.

Several distinct plumes of perchlorate are present in groundwater at the site. Perchlorate, an inorganic chemical compound consisting of chlorine bonded to four oxygen atoms (ClO₄^-), can interfere with iodide uptake by the thyroid gland and may result in decreased thyroid hormone production. Concentrations of perchlorate have been measured in groundwater at the site exceeding 800,000 micrograms per liter (µg/l). Perchlorate is migrating to surface water by hydraulic connection with groundwater and from runoff over contaminated surface soils during storm events. The primary health concern is the potential for a nearby drinking water reservoir to be impacted by perchlorate migrating through surface streams. However, perchlorate has never been detected in the reservoir, and off-site surface water samples rarely exceed 3 µg/l.

Most of the on-site streams are intermittent, with ninety percent of the annual runoff occurring November to April. Monthly surface water monitoring conducted since 1998 at designated creek stations has frequently detected the presence of low concentrations of perchlorate during winter months. A recent change in sampling methodology, in which individual flow-proportioned samples were collected during storm events, resulted in the detection of much higher perchlorate concentrations, including the highest recorded off-site surface water concentration (61 µg/l). However, the storm sampling demonstrated that perchlorate loading in streams is generally of short duration, and overall, it appears that very little perchlorate mass is delivered to the reservoir.