Survey of Low Level Arsenic Occurrence in Surface and Groundwater in California

EXECUTIVE SUMMARY

The Association of California Water Agencies (ACWA) has just completed the Survey of Low Level Arsenic Occurrence in Surface and Groundwater in California. This survey documents the occurrence of arsenic, a naturally occurring element, in selected California drinking water sources at concentrations lower than have been documented previously. This survey also tabulates other source specific information that will be used to further characterize these water sources. This survey is the first part of a two-part study by ACWA to determine the impact on California water consumers of a revised drinking water regulation for arsenic.

Arsenic is one of 83 specific contaminants for which the U.S. Environmental Protection Agency (USEPA) is required to set a maximum contaminant level goal (MCLG) and a National Primary Drinking Water Regulation (NPDWR). The USEPA is in the process of revising the NPDWR for arsenic. Consideration is being given to lowering the current arsenic maximum contamination level (MCL) of 0.050 mg/L (50 parts per billion) to one that is somewhere between 0.002 and 0.020 mg/L (2 - 20 parts per billion). Under court order, the USEPA is required to propose a revised arsenic regulation by November 30, 1995. The final rule is scheduled for promulgation by November 30, 1997. Depending on the level of the new standard, ACWA recognizes that a revised regulation could have a significant impact on the state’s water suppliers and, in turn, the 31 million Californians they serve.

As the largest statewide coalition of public water agencies in the country, ACWA's 417 public agency members collectively are responsible for more than 90% of the water delivered in California. Approximately half of these agencies deliver domestic water to their customers. Their primary interest is to deliver safe drinking water to the people of California. In fulfilling its role, ACWA identifies issues of concern to the water community and the public it serves, seeks the best available scientific information on those issues and then communicates that information to its member agencies, the policy makers, regulators and the public.

Because of the potential impact a revised drinking water regulation for arsenic could have on the water consumers of California, both in cost and in limitation of water resources, ACWA made this survey a top priority, investing significant resources and time to its development. This occurrence survey provides an assessment of the amount of water delivered domestically in California that would be affected by a lowered arsenic MCL. Together with an in-progress cost of compliance evaluation, this survey will help ACWA member agencies and their customers better understand the impact that a revised regulation will have on California water resources, and the associated capital and operational costs. Additionally, information produced by this survey is submitted, where appropriate, to the USEPA for inclusion in the occurrence and exposure database that the USEPA is generating for the revision of the arsenic drinking water regulation. ACWA believes that use of the data produced from this survey will lead to a drinking water regulation that is based on the best scientific data currently available.

Approach

In 1993, ACWA requested its members to voluntarily participate in a survey to assess the occurrence of arsenic in California ground and surface water. From March 1993 to February 1994, more than 1500 samples were collected and analyzed using analytical techniques that would determine arsenic at very low concentrations. In order to better characterize these water sources, survey participants also provided agency and source specific information that would help define quantity, quality and geographic parameters of the sources of water being analyzed.

Results

Over 180 water agencies, utilities and cities from 27 counties throughout California participated in this study. While large, medium and small water systems were represented in the survey, a predominant number of these systems fell into the medium, large and very large size categories. A larger number of the participating agencies were located in the southern part of the state. Primary results of the survey show that:

The median value for arsenic concentrations in the more than 1500 water samples analyzed was 0.002 mg/L (2 parts per billion). This is extremely low, 25 times less than the current standard of 0.05 mg/L (50 parts per billion). Only one water source in more than 1500 water samples analyzed was determined to have an arsenic level above the current standard.

Both ground and surface water supplies could be affected by a revised regulation. Over 50% of the surface water that was analyzed contained detectable concentrations of arsenic. Over 65% of the ground water tested had detectable concentrations of arsenic.

These levels of arsenic occurrence have not previously been reported. From historic records, only 15% of the ground water wells analyzed in this survey had previously reported detecting arsenic. Historically, only 22% of the surface water sources analyzed in this survey had previously detected arsenic.

The increase in the number of water sources reported as having a level of detectable arsenic is due primarily to the improved analytical techniques utilized in this survey. The reporting detection limit for the analytical method used for this survey was 0.001 mg/L (1 ppb).

A number of the water quality parameters reported in this survey varied greatly, particularly when comparing ground and surface water. Location and source of the water are strong considerations in this variation. Because of this variation, treatment techniques to remove arsenic will have to be flexible enough to accommodate the wide range of water types found in California.

When comparing surface water samples collected before and after treatment, it was found that a small reduction in the concentration of arsenic was observed in the treated samples. That is to say, even though the water treatment currently being applied by these agencies may not be optimized for arsenic removal, some reduction in arsenic concentrations was seen in the treated samples.

When looking for arsenic at these low concentration levels, arsenic was found throughout California -- in north, south and central California counties. Ground water wells in such diverse locations as Sacramento, Mono and Los Angeles counties had detectable concentrations of arsenic. Major surface water sources such as the Sacramento River, the Colorado River, the State Water Project, and the Los Angeles Aqueduct all had detectable concentrations of arsenic.

Conclusions

Arsenic, at low concentration levels, occurs widely throughout California in both surface and ground waters. A revised regulation for arsenic in drinking water will affect California water suppliers and their customers. The degree of impact will depend on how low the MCL is set. In order to assess that impact, ACWA has initiated the second phase of this study, the cost of compliance evaluation. This evaluation will take the occurrence data generated by this survey and use it to assess the cost of treatment of these water sources. The cost of compliance study is scheduled for completion in July of 1995.