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Perchlorate in Drinking Water

Last Update: February 26, 2014

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. 

For information on the history of perchlorate in California drinking water, click here.

Health Concerns

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.   

Its effects on the thyroid gland are the basis of the 6-µg/L public health goal (PHG) established in 2004 by Cal/EPA's Office of Environmental Health Hazard Assessment (OEHHA). (PHGs contribute to the development of MCLs, as described here.) 

In December 2012 OEHHA released a revised draft technical support document for a 1-µg/L PHG for perchlorate in December 2012; a previous draft, also for a 1- µg/L, was released in January 2011.

Analytical Laboratory Information:  Alternative Perchlorate Preservation Recommendation

From CDPH Drinking Water and Radiation Laboratory (DWRL), May 2012:

Due to a concern that perchlorate may be subject to biodegradation, the newer US Environmental Protection Agency (US EPA) methods (e.g., US EPA Methods 331, 332, 314.1, and 314.2) all require field filtration into a sterile bottle and shipping samples refrigerated.   However, USEPA Method 314, a much older method, does not require these steps.  Perchlorate microbiological reduction requires anaerobic conditions, a carbon source and an active perchlorate-degrading microbial population.  Therefore, field filtration is not necessary as long as samples are aerobic and this can be readily accomplished in the field by half filling the sample containers, agitating them, and chilling them on ice.  For example, a half-filled 100 mL container has 15 mg oxygen in the headspace while the water sample can dissolve only 0.35 mg oxygen (35oC) or 0.45 mg oxygen (25oC) or 0.75 mg oxygen (0oC).  Agitating the sample dissolves air and the headspace is a large reservoir for additional oxygen.

If these precautions are taken, there is no need to filter samples in the field.  Samples should be chilled after collection and during transport to the receiving laboratory at temperatures preferably below 6 oC.  Once in the laboratory, they should be stored refrigerated until analysis.   Data produced from samples collected under these conditions and analyzed by any of the aforementioned US EPA perchlorate methods would be considered acceptable for drinking water compliance in California. If samples contain particulates, it is advisable to filter them prior to analysis to protect the analytical instrumentation.

Monitoring Results

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 history).  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 2010-2013 monitoring results from the water quality monitoring database (available here) and we extracted the perchlorate findings in active and standby sources that had two or more reported detections at or greater than the 4-µg/L detection limit for purposes of reporting (DLR) and greater than the 6-µg/L MCL.  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 greater than the DLR, however, are included here (Excel, 1.1MB) Opens new browser window..

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. 


Table 1. Active and Standby Sources with Perchlorate Detections (2010 - 2013)

   Peak detection at or 
above 4 µg/L
Peak detection 
above 6 µg/L
County (ID) No. of Sources No. of Systems No. of Sources No. of Systems Peak Conc.
Contra Costa (7) 1 1 1 1 7.9
Fresno (10) 1 1 - - 4.5
Kern (15) 2 2 1 1 14
Los Angeles (19) 98 31 68 21 108
Monterey (27) 1 1 - - 4.8
Orange (30) 11 7 3 2 9
Riverside (33) 49 8 38 8 68
Sacramento (34) 2 1 2 1 13
San Bernardino (36) 57 19 37 16 94
San Diego (37) 10 2 8 2 9.9
Santa Barbara (42) 1 1 - - 4.6
Santa Clara (47) 5 4 3 3 10
Sutter (51) 3 3 1 1 10
Tulare (54) 6 3 5 3 20
Ventura (56) 1 1 - - 5.2


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Early Perchlorate Findings in Drinking Water
Last modified on: 2/26/2014 5:53 PM