Arsenic: How Low Should We Go?

The U.S. Environmental Protection Agency (USEPA) is in the throes of issuing a proposed rule for arsenic in drinking water: A proposed maximum contaminant level (MCL) of 5 µg/L is expected. Public comment will be requested as to whether the final MCL should remain at that level, be lower (3 ug/L) or be higher (10 µg/L). Some within the agency question whether the proposed MCL of 5 µg/L is justified, given the uncertainties of health effects at levels seen in the U.S. and the cost of treatment.

The Office of Management and Budget (OMB) is currently reviewing the proposed rule package. Industry lobbyists claim the proposed MCL is too low, and is not based on sound science. Environmental advocates argue that the proposed MCL is too high, and that arsenic poses significant health risks at very low levels. Contention over the adequacy of the existing arsenic MCL has been raging for over a decade, but recent health studies have tipped the balance.

SDWA Required EPA to Revise MCL for Arsenic
The US Public Health Service first set a drinking water standard for arsenic at 50 µg/L in 1942. Following passage of the 1974 Safe Drinking Water Act (SDWA), USEPA subsequently adopted this standard as an MCL, noting that no illness was known to occur below this level. The 1986 SDWA required USEPA to revise the MCL for arsenic; this effort was delayed based on the need to better understand health effects and to assess removal technologies.

No MCL goal (MCLG) exists for arsenic, but a proposed MCLG of zero is expected. Both the USEPA and the International Agency for Research on Cancer classify arsenic as being carcinogenic to humans. The harmful effects of arsenic are legendary—and well documented in both human and animal studies—though uncertainty remains regarding the human health effects at low concentrations.

Studies Indicate Link Between Arsenic Exposure and Forms of Cancer
Current arsenic risk assessments draw heavily from studies of a large population in Taiwan exposed to arsenic in well water. In the 1960s, a population of 40,421 was divided into three groups based on arsenic content of their well water (high, >0.60 mg/L; medium, 0.30—0.59 mg/L; and low, <0.29 mg/L). A clear dose-response relationship was found between exposure to arsenic and the frequency of dermal lesions, a peripheral vascular disorder known as "blackfoot disease," and skin cancer. The strengths of this study include good medical records of disease incidence, and the large population examined. However, the individual exposures to arsenic is not known. Some villages have more than one well, with differing arsenic concentrations. In addition, methodological weaknesses complicate interpretation of the results.

Since the 1970s, a number of epidemiological studies have been conducted in Taiwan and other parts of the world. Several studies have found exposure to arsenic in drinking water to be associated with internal cancers, in both males and females including bladder, kidney, skin, and lung, and cancers of the prostate and liver in males, but data are limited and insufficient for quantitative assessment of internal cancer risks.

Uncertainties remain regarding the risks associated with ingestion of arsenic in drinking water, especially at low concentrations. In 1995, USEPA estimated the risk of one case of skin cancer per 10,000 people at 2 µg/L. The national cost of meeting an arsenic MCL of 2 µg/L was estimated at $2.1 billion annually. Ion exchange, reverse osmosis, and lime softening were the treatment technologies considered. Household costs were estimated at $40 per household per year for large systems and $1,500 per household per year for small systems. USEPA sought deferral of the arsenic rule to provide time for additional studies that would narrow the uncertainty associated with the health effects, given the high cost impact.

SDWA Amendments Required Arsenic Regulation
The 1996 SDWA amendments set a schedule for regulation of arsenic, required USEPA to prepare a research plan, and mandated a study of health effects by the National Research Council (NRC). A proposed rule was required by Jan. 1, 2000, and a final rule by Jan. 1, 2001.

USEPA has been actively conducting studies and engaging stakeholders in discussion of arsenic issues (http://www.epa.gov/ogwdw/ars/arsenic.html). In March 1999, the NRC released a report estimating the risk for male bladder cancer at the current MCL at 1 to 1.5 per 1,000. Studies indicate that arsenic may cause 2-5 times more lung cancer deaths. As a result most of the NRC committee members predicted that a combined risk for all cancers at the current MCL could be on the order of 1 in 100. NRC did not specifically evaluate lung cancer risk or combined risk of cancers in the report, and recommended that USEPA do those analyses. NRC concluded that although additional research is needed, the current arsenic MCL does not achieve USEPA's goal for public health protection and requires downward revision as promptly as possible.

In 1992, the California Office of Environmental Health Hazard Assessment proposed a public health goal for arsenic at 0.002 µg/L based on extrapolation of the Taiwan data, without regard to economic consequences. A final public health goal was deferred until USEPA completed work on a revised MCL. In 1996, the World Health Organization sought to balance health uncertainties at low concentrations with economic considerations by setting a provisional guideline for arsenic at 10 µg/L.

USEPA's research plan identified long-term research needs, which will not be complete before 2001 when the new MCL is due. Even so, the scientific consensus is that the current arsenic MCL must be lowered—but agreement ends there—with strong feelings on all sides. One thing we do know: "How low should we go?" will be decided as a risk management decision, not simply by available science.

OMB is due to complete their review in mid-May 2000. A formal proposal is expected from USEPA in June 2000 and following will be a 90-day public comment period. A proposed MCL of 5 µg/L will have a high cost impact. But is a higher MCL defensible given customer attitudes towards arsenic? Could arsenic become an election issue? If so, exciting times are before us.

About the Author:
Frederick W. Pontius, P.E., is president of Pontius Water Consultants, Inc., Lakewood, Colorado, specializing in drinking water regulatory affairs, compliance, water quality and treatment. Fred has 20+ years in the water and wastewater industries. He is a frequent conference and seminar speaker, was a former staffer with the American Water Works Association from 1982 to 1999, and is a contributing editor to the Journal of the American Water Works Association (since 1991). He may be reached at fredp@pontiuswater.com.

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