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State Water Resources Control Board <br />Division of Water Quality <br />GAMA Program <br />GROUNDWATER INFORMATION SHEET <br />Arsenic <br />The purpose of this groundwater information sheet is to provide general information <br />regarding a specific constituent of concern (COC). The information provided herein <br />relates to wells (groundwater sources) used for public drinking water, not water served <br />at the tap. <br />GENERAL INFORMATION <br />Constituent of Concern <br />Arsenic As <br />Aliases <br />"Arsenic Black", "Colloidal arsenic", "Gray arsenic" <br />(numerous additional aliases exist for the compound <br />forms of arsenic) <br />Chemical Formula <br />As <br />Main forms of arsenic in natural waters are arsenate <br />[As(V)] and arsenite [As(III)] <br />CAS No. <br />7440-38-2 <br />Storet No. <br />01002 <br />Summary <br />Arsenic is a regulated trace element with an established <br />State Maximum Contaminant Level (MCL) in drinking <br />water of 10 µg/L. Arsenic is a semi -metal element that <br />occurs naturally in the environment, but can also be <br />released to the environment by human activities. Based <br />on SWRCB data from 2007 to 2017, 931 active and <br />standby public water supply wells (of 9,381 sampled) had <br />at least one detection of arsenic above the MCL. Most <br />wells with detections above the MCL occurred in Kern <br />(155), Los Angeles (68) and San Bernardino (44) <br />counties. The maximum measured concentration was <br />650 pg/L, in a Sonoma county well. <br />Revised October 2017 <br />