Following describes available data for several constituents. Additional data for total dissolved solids, nitrate, iron and manganese, which are generally considered not a concern in the SGA area, can be found in the Basin Management Report.
Arsenic: A naturally occurring element in the earth’s crust. The MCL for arsenic is 10 micrograms per liter (µg/L). Of the 236 distinct arsenic samples from the period, 67 were at or below the analytical detection level of 2 µg/L. Of the remaining wells with values above the detection level, the average was 3.6 µg/L, with one well exceeding the MCL.
Radon: A naturally occurring radioactive gas believed to cause lung cancer in humans. Although radon from drinking water sources contributes only a small percentage of overall exposure to radon from all sources, EPA in 1999 issued a proposed rule for a maximum concentration of 300 picoCuries per liter (pCi/L). Relative to the proposed rule, radon could be a potential future concern for local public water suppliers in the North Area Basin. Of 101 samples from public supply wells collected between 1994 and 2002, the average concentration of radon exceeded 395 pCi/L. Fifty-nine of the wells (58 percent) exceeded 300 pCi/L, with 16 of the wells exceeding 600 pCi/L.
Hexavalent Chromium (CrVI): A heavy metal that is commonly found in low concentrations in drinking water. It can occur naturally, but has also been sourced historically from industrial operations. A State of California MCL was established at 10 µg/L effective July 1, 2014. The occurrence of CrVI is widespread in the SGA area. Of 215 distinct wells sampled between 2001 and 2014, 15 were below the detection level of 1 µg/L, 116 wells were between 1 µg/L and 5 µg/L, 65 had concentrations from 5 µg/L up to 10ug/L, and 19 had concentrations greater than 10ug/L. The average concentration of well above the detection level is 5.2 µg/L.
Tetrachloroethylene (PCE): A volatile organic compound (VOC) used as a component of solvents, hydraulic fluids, paint thinners, and dry cleaning agents. PCE currently has an MCL of 5 µg/L, but could be lowered in the future. Of 142 wells sampled from the period, 118 wells were below the detection level of 0.5 µg/L. Of the remaining wells with detections, six had concentrations exceeding the MCL.