Sources of arsenic in water include natural sources, particularly rocks and soils, and also releases from its use as a wood preservative, in semi-conductors and paints, and from mining and agricultural operations. Elevated levels of arsenic are found more frequently in ground water than in surface water. Because small communities typically rely on wells for drinking water, while larger cities often use surface-water sources, arsenic tends to occur in higher concentrations more frequently in water used by small communities. In the United States, the average arsenic level measured in ground-water samples is less than or equal to 1 part per billion (ppb, or micrograms per liter [μg/L]); however, higher levels are not uncommon. Compared with the rest of the United States, Western states have more water systems with levels exceeding 10 ppb; levels in some locations in the West exceed 50 ppb. Parts of the Midwest and New England also have some water systems with arsenic levels exceeding 10 ppb, but most systems meet the new standard. When issuing the rule, EPA estimated that roughly 4,000 (5.5%) of regulated water systems, serving a total of 13 million people, were likely to exceed the 10 ppb standard. The previous drinking water standard for arsenic, 50 ppb, was set by the U.S. Public Health Service in 1942. EPA adopted that level and issued an interim drinking water regulation for arsenic in 1975. This standard was based on estimated total dietary intake and non-cancer health effects. In 1986, Congress amended the Safe Drinking Water Act (SDWA), converted all interim standards to National Primary Drinking Water Regulations, and included arsenic on a list of 83 contaminants for which EPA was required to issue new standards by 1989. EPA's extensive review of arsenic risk assessment issues caused the agency to miss the 1989 deadline. As a result of a citizen suit, EPA entered into a consent decree with a new deadline for the rule of November 1995. EPA continued work on risk assessment, water treatment, analytical methods, implementation, and occurrence issues, but in 1995 decided to delay the rule in order to better characterize health effects and assess cost-effective removal technologies for small utilities. In the 1996 SDWA Amendments ( P.L. 104-182 ), Congress directed EPA to propose a new drinking water standard for arsenic by January 1, 2000, and to promulgate a final standard by January 1, 2001. Congress also directed EPA to develop a research plan for arsenic to support the rulemaking effort and to reduce the uncertainty in assessing health risks associated with low-level exposures to arsenic. EPA was required to conduct the study in consultation with the National Academy of Sciences. In 1996, EPA requested the National Research Council (NRC) to review the available arsenic toxicity data base and to evaluate the scientific validity of EPA's risk assessments for arsenic. The NRC issued its report in 1999 and recommended that the standard be reduced, but it did not recommend a particular level. The NRC affirmed that the available data provided ample evidence for EPA's classification of inorganic arsenic as a human carcinogen, but that EPA's dose-response assessment, which was based on a Taiwan study, deserved greater scrutiny. The NRC explained that the data in the study lacked the level of detail needed for use in dose-response assessment. The Council also reported that research suggested that arsenic intake in food is higher in Taiwan than in the United States, further complicating efforts to use the data for arsenic risk assessment. Based on findings from three countries where individuals were exposed to very high levels of arsenic (several hundreds of parts per billion or more), the NRC concluded that the data were sufficient to add lung and bladder cancers to the types of cancers caused by ingestion of inorganic arsenic; however, the NRC noted that few data addressed the risk of ingested arsenic at lower concentrations, which would be more representative of levels found in the United States. The NRC concluded that key studies for improving the scientific validity of risk assessment were needed, and recommended specific studies to EPA. In June 2000, EPA published its proposal to revise the arsenic standard from 50 ppb to 5 ppb and requested comment on options of 3 ppb, 10 ppb, and 20 ppb. EPA stated that the proposal relied primarily on the NRC analysis and some recently published research, and that it would further assess arsenic's cancer risks before issuing the final rule. As proposed, the standard would have applied only to community water systems. Non-transient, non-community water systems (e.g., schools with their own wells) would have been required only to monitor and then report if arsenic levels exceeded the standard. In the final rule, published on January 22, 2001 (66 FR 6976), EPA set the standard at 10 ppb and applied it to non-transient, non-community water systems, as well as community water systems. The agency gave the water utilities five years to comply (the maximum amount of time allowed under SDWA). EPA estimated that 3,000 (5.5%) of the 54,000 community water systems, and 1,100 (5.5%) of the 20,000 non-transient, non-community water systems, would need to take measures to meet the standard. In developing standards under the Safe Drinking Water Act, EPA is required to set a maximum contaminant level goal (MCLG) at a level at which no known or anticipated adverse health effects occur and that allows an adequate margin of safety. (EPA sets the MCLG at zero for carcinogens [as it did for arsenic], unless a level exists below which no adverse health effects occur.) EPA must then set an enforceable standard, the MCL, as close to the MCLG as is "feasible" using the best technology, treatment, or other means available (taking costs into consideration). EPA's determination of whether a standard is feasible typically has been based on costs to large water systems (serving more than 50,000 people). Less than 2% of community water systems (roughly 750 of 54,000 systems) are this large, but they serve roughly 56% of all people served by community systems. Congress has long recognized that the technical and cost considerations associated with technologies selected for large cities often are not applicable to small systems. In the 1996 amendments, Congress expanded SDWA variance and exemption provisions to address small system compliance concerns. The small system variance provisions require that for each rule establishing an MCL, EPA must list technologies that comply with the MCL and are affordable for three size categories of small systems. If EPA does not list affordable compliance technologies for small systems, then it must list variance technologies. A variance technology need not meet the MCL, but must be protective of public health. If EPA lists a variance technology, a state then may grant a variance to a small system, allowing the system to use a variance technology to comply with a regulation. EPA has not identified variance technologies for arsenic or any other standards because, based on its current affordability criteria, EPA has determined that affordable compliance technologies are available for all standards. Thus, small system variances are not available. Congress took issue with EPA's assessment that small system variance technologies were not merited for the arsenic standard, and in 2002, directed EPA to review the criteria it uses to determine whether a compliance treatment technology is affordable for small systems. In March 2006, EPA proposed three options for revising its affordability criteria (71 FR 10671). Under the current affordability criteria, EPA considers a treatment technology affordable unless the average compliance cost exceeds 2.5% of the area's median household income. Based on this measure, EPA determined that affordable technologies are available for all SDWA standards. The proposed options under consideration are well below the current level: 0.25%, 0.50%, and 0.75% of an area's median household income. EPA also stated that it expects to address in the revised criteria the issue of how to ensure that a variance technology would be protective of public health. According to EPA, the final criteria would apply only to the new Stage 2 Disinfectants/Disinfection Byproducts Rule and future rules, and not to the arsenic rule. Exemptions potentially offer a source of compliance flexibility for small systems. States may grant temporary exemptions from a standard if, for certain reasons (including cost), a system cannot comply on time. The arsenic rule gives systems five years to comply with the new standard; an exemption allows another three years for qualified systems. Systems serving 3,300 or fewer persons may receive up to three additional two-year extensions, for a total exemption duration of nine years (a total of 14 years to achieve compliance). In the final rule, EPA noted that exemptions will be an important tool to help states address the number of systems needing financial assistance to comply with this rule and other SDWA rules (66 FR 6988). However, to grant an exemption, the law requires a state to hold a public hearing and make a finding that the extension will not result in an "unreasonable risk to health." Because the exemption process is complex, states have seldom granted them. State officials have noted that "unreasonable risk to health" has never been defined, and that states must make a separate finding for each system. Many states have granted few or no exemptions for the arsenic rule. When proposing a rule under SDWA, EPA must publish a determination as to whether or not the benefits of the standard justify the costs. If EPA determines that costs are not justified, then it may set the standard at the level that maximizes health risk reduction benefits at a cost that is justified by the benefits. EPA determined that the "feasible" arsenic level (for large systems) was 3 ppb, but that the benefits of that level did not justify the costs. Thus, EPA proposed a standard of 5 ppb.