Perchlorate in Drinking Water

Last Update: January 6, 2010

Perchlorate is a regulated drinking water contaminant in California, with a maximum contaminant level (MCL) of 6 micrograms per liter (µg/L).  The MCL became effective October 2007.   For information provided to public water systems by the CDPH Drinking Water Program about the implementation of the MCL and the scheduling of monitoring, see links at the bottom of this page.

Perchlorate and its salts are used in solid propellant for rockets, missiles, and fireworks, and elsewhere (e.g., production of matches, flares, pyrotechnics, ordnance, and explosives).  Their use can lead to releases of perchlorate into the environment.  Perchlorate's interference with iodide uptake by the thyroid gland can decrease production of thyroid hormones, which are needed for prenatal and postnatal growth and development, as well as for normal metabolism and mental function in the adult.   

Monitoring, first in 1997 by the Drinking Water Program and then by public water systems, showed perchlorate to be a widespread drinking water contaminant, occurring in several hundred wells, mostly in southern California (see early findings).  Perchlorate was also found in the Colorado River, an important source of water for drinking and irrigation, where its presence resulted from contamination from ammonium perchlorate manufacturing facilities in Nevada. 

For this presentation, we've used monitoring results since January 1, 2006, which are available for about 10,600 sources --"sources" include pending, inactive, and destroyed or abandoned sources; monitoring wells; agricultural wells; and sources with peak detections below the DLR.  From these, we extracted the perchlorate findings in active and standby sources that had reported detections at or greater than 4 µg/L and greater than 6 µg/L, concentrations that correspond to perchlorate's detection limit for purposes of reporting (DLR) and the perchlorate MCL, respectively.  These are presented in Table 1.  Where raw and treated samples or other results indicate more than one sampling point for the same source, they are counted as coming from a single source. All detections, however, are included here (Excel, 1.9MB) .

These findings are helpful in identifying areas in which perchlorate has affected sources of drinking water (principally wells), but they should not be interpreted as representative of water being served by public water systems. Consumers interested in finding out more about the quality of their drinking water should refer to their water systems' annual consumer confidence reports (CCRs).  Many CCRs for California water systems are available on the US EPA's website.

Data in Table 1 should be considered draft, since they will change with subsequent updates.