Naturally occurring arsenic is a metallic mineral that is found in the Earth's crust. It is commonly found in groundwater. The United States Geologic Survey estimates that almost half of groundwater in the U.S. contains naturally occurring arsenic greater than 1 part per billion (ppb), with about 10% exceeding 10 ppb.
In January 2006, the United States Environmental Protection Agency reduced the federal Maximum Contaminant Level (MCL) for arsenic in drinking water from 50 ppb to 10 ppb. In November 2008, California followed suit, revising its State MCL to 10 ppb. More information from the California Division of Drinking Water about arsenic in drinking water can be found here.
The District's drinking water sources include 15 active and two standby groundwater wells. Only two of these wells have arsenic above California's Detection Limit for Purposes of Reporting (DLR), which is set at 2.0 ppb, and none of the District's wells contain arsenic at or above the MCL. In fact, all arsenic levels are one-half or less than the MCL. Even though the arsenic levels are well below the drinking water standard, in 2003 the District began investigation to treat (reduce) the detectable levels of arsenic. Treatment began in 2006 at one plant and at a second plant in 2007.
The District operates two treatment plants that reduce the amount of arsenic that is delivered to customers. The treatment plant technology is based on oxidation with sodium hypochlorite (chlorine bleach) and ferric chloride to enhance the ability of arsenic to be filtered out through the filter media (anthracite and sand).
The District's arsenic treatment plants are successful in achieving roughly 50% removal of the arsenic content of the raw groundwater, so that drinking water entering the distribution system has arsenic levels at or below the DLR.