6.1 Safe Drinking Water Act (SDWA) Regulations & MCLs

The quality of drinking water distributed to the public is strictly governed by a comprehensive framework of federal and state regulations. Understanding the historical context, statutory requirements, and specific rules under the Safe Drinking Water Act (SDWA) is essential for any water distribution operator. Regulatory compliance is not merely about avoiding fines; it is the fundamental mechanism through which public health is protected from waterborne pathogens, toxic chemicals, and harmful byproducts.

The Safe Drinking Water Act (SDWA): Historical Context and Framework

Passed by Congress in 1974 and significantly amended in 1986 and 1996, the Safe Drinking Water Act (SDWA) serves as the primary federal law ensuring the quality of Americans' drinking water. Under the SDWA, the Environmental Protection Agency (EPA) is authorized to set national health-based standards for drinking water to protect against both naturally occurring and man-made contaminants that may be found in the water supply. The SDWA applies to every public water system (PWS) in the United States, defined as a system providing water for human consumption to at least 15 service connections or regularly serving at least 25 individuals.

The EPA operates on a system of primacy, meaning that while the EPA sets the national floor for regulations, individual states, tribes, or territories can be granted the authority to implement and enforce the SDWA within their jurisdictions. To obtain and maintain primacy, a state must adopt regulations that are at least as stringent as the federal standards, though many states choose to enact even stricter limitations.

Primary vs. Secondary Drinking Water Standards

Regulatory limits under the SDWA are divided into two distinct categories: Primary Standards and Secondary Standards. Understanding the difference between these is critical for operators, as they carry different legal weights and address different types of water quality issues.

National Primary Drinking Water Regulations (NPDWRs)

Primary standards are legally enforceable standards that apply to public water systems. They are designed exclusively to protect public health by limiting the levels of specific contaminants that can adversely affect human biology. The development of a primary standard involves multiple steps:

1. Maximum Contaminant Level Goal (MCLG): First, the EPA determines a non-enforceable health goal. The MCLG is the level of a contaminant in drinking water below which there is no known or expected risk to health, allowing for a margin of safety. For known carcinogens (cancer-causing agents) and microbial pathogens, the MCLG is always set to zero.
2. Maximum Contaminant Level (MCL): Once the MCLG is established, the EPA sets an enforceable standard called the MCL. The MCL is set as close to the MCLG as feasible, taking into consideration the best available treatment technology and the economic costs of achieving that level. If it is not economically or technologically feasible to measure a contaminant, the EPA may dictate a specific Treatment Technique (TT) instead of an MCL.

National Secondary Drinking Water Regulations (NSDWRs)

Secondary standards are non-enforceable guidelines regulating contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water. These parameters are quantified as Secondary Maximum Contaminant Levels (SMCLs). While the EPA recommends adherence to SMCLs, they do not mandate compliance. However, high levels of secondary contaminants can cause customer complaints, prompting state regulatory agencies to enforce them on a localized basis. Examples include iron (SMCL of 0.3 mg/L) and manganese (SMCL of 0.05 mg/L), which cause red or black water respectively.

The Revised Total Coliform Rule (RTCR)

Bacteriological safety is arguably the most critical aspect of water distribution. Originally promulgated in 1989, the Total Coliform Rule (TCR) was significantly updated in 2013 to become the Revised Total Coliform Rule (RTCR).

Under the old TCR, a system was in violation of the MCL if more than 5% of its monthly samples were positive for total coliforms. The RTCR eliminated the MCL for total coliforms (though it maintained the MCL for E. coli) and shifted the regulatory framework to a "find and fix" approach. Total coliforms are used as indicator organisms—their presence suggests that a pathway exists for environmental contamination to enter the distribution system, meaning dangerous pathogens could also be present.

When a system exceeds a specified frequency of total coliform-positive samples, it triggers an assessment to identify sanitary defects.

• Level 1 Assessment: A basic evaluation of the system, typically conducted by the water system operator, focusing on source water, treatment processes, distribution system conditions, and sampling practices.
• Level 2 Assessment: A more comprehensive and detailed evaluation triggered by an E. coli MCL violation, or multiple Level 1 triggers. This must be conducted by an approved party, often the state regulatory agency or a third-party expert.

Any sanitary defects identified during these assessments must be corrected by the system within a specified timeframe, ensuring that vulnerabilities are permanently addressed rather than merely documented.

The Lead and Copper Rule (LCR) and Improvements (LCRI)

Unlike most contaminants which are found in the source water, lead and copper primarily enter drinking water through the corrosion of plumbing materials in the distribution system and inside customer homes. Therefore, the LCR establishes an Action Level (AL) rather than an MCL, measured at the customer's tap.

Historically, the Action Level for lead has been 15 parts per billion (ppb) and 1.3 parts per million (ppm) for copper, based on the 90th percentile of tap water samples. If the 90th percentile exceeds the Action Level, the system must take steps to control corrosion, such as adjusting pH or adding orthophosphate inhibitors.

However, the regulatory landscape for lead is rapidly evolving. The EPA has introduced the Lead and Copper Rule Improvements (LCRI), representing the most significant overhaul of the rule in decades. Key mandates of the LCRI include:

• 100% Replacement of Lead Service Lines (LSLs): Systems are generally required to replace all lead service lines within 10 years, shifting the burden entirely toward the elimination of lead infrastructure.
• Lower Action Level: The lead Action Level is reduced from 15 ppb to 10 ppb, triggering public education and corrosion control optimization sooner.
• Improved Sampling: Tap sampling protocols have been updated to ensure that the highest levels of lead are captured (such as requiring a 1-liter sample after stagnation or using a 5th-liter sample in homes with LSLs).

Disinfectants and Disinfection Byproducts Rules (DBPR)

While adding disinfectants like chlorine is essential to kill pathogens, these chemicals can react with naturally occurring organic matter (NOM) in the water to form harmful disinfection byproducts (DBPs). The Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules (DBPR) balance the risks of microbial pathogens against the risks of long-term exposure to DBPs, which are suspected carcinogens.

The DBPR regulates two main groups of byproducts:

• Total Trihalomethanes (TTHMs): The enforceable MCL is 0.080 mg/L (or 80 ppb).
• Haloacetic Acids (HAA5): The enforceable MCL is 0.060 mg/L (or 60 ppb).

The Stage 2 DBPR specifically requires systems to evaluate their distribution networks and identify locations with the highest potential for DBP formation—usually areas with high water age, such as dead ends or large storage tanks. Compliance is based on the Locational Running Annual Average (LRAA), meaning a system cannot average a high-DBP location with a low-DBP location to achieve compliance; every individual sampling site must meet the MCL.

Additionally, the rules establish Maximum Residual Disinfectant Levels (MRDLs) to ensure that systems do not over-chlorinate. The MRDL for chlorine and chloramines is 4.0 mg/L (as Cl2). Like MCLs, MRDLs are enforceable health-based standards.