History of Clean Water Standards

“If you can’t measure it, you can’t manage it.”

Water protection is a complex topic that involves a variety of approaches including regulations, wastewater treatment, hazard mitigation, land use controls, and public education (just to name a few). In the U.S., the first federal regulations of drinking water can be traced back to 1914. But to assess whether water is clean, it is important to test. But what is water tested for? And how low can a pollutant be detected in water? 

Making the List

There are millions of different organic and inorganic substances (natural and synthetic). Not all of these are necessarily “contaminants.” Under the Safe Drinking Water Act of 1974 (with amendments in 1986 and 1996) a “contaminant” is any physical, chemical, biological, or radiological substance or matter in water. Water can contain a variety of substances but not all of these are necessarily a health risk. In fact, despite the millions of potential candidates, only a small portion of these chemicals are considered potential pollutants that we routinely test for in surface water and groundwater. These are based on chemicals that are either widely used, have a known health effect, commonly occur in the environment, or have a high chance that people will be exposed to them. For example, in 1962 there were a total of 28 substances under regulation. Today, the U.S. Environmental Protection Agency (EPA) has a list of 126 “Priority Pollutants” that are regulated and for which analytical test methods have been developed.

 In addition, the National Primary Drinking Water Regulations specify a number of contaminants with established maximum contaminant levels (MCL) to protect drinking water supplies. This list has grown over the years. In 1996, there were about 70 contaminants on the list and currently, there are over 90. Other lists include the CERCLA Priority List of Hazardous Substances (covering 275 chemicals) for Superfund sites as well as the U.S. Department of Transportation’s Marine Pollutant List (or Environmentally Hazardous Substances), which currently contains almost 600 different constituents. States, territories, and authorized tribes also have a role in the process of identifying potential pollutants.

In addition to these regulated contaminants, there is an ongoing recognition of emerging contaminants.  They are referred to as emerging because these chemicals typically have only recently been discovered to be a potential threat to human health. To gather scientifically valid data on the national occurrence of these non-regulated contaminants, the EPA requires public water systems to test every five years for an additional list of chemical contaminants through the Unregulated Contaminant Monitoring Rule (UCMR). The latest version, UCMR 5, was approved by the EPA in December 2021.

Determining a Number

There are many types of numerical standards established by federal and state governments in the U.S. that cover drinking water, groundwater, surface water, as well as bottled water. Some standards are designed to protect human health or aquatic health while others protect water bodies for purposes of recreation or scenic enjoyment. The process of deciding on these numbers can be complex with many steps and involves lots of science. In the case of Drinking Water Standards (i.e. MCLs), these are generally created by the EPA through the establishment of a Maximum Contaminant Level Goal (MCLG) which is defined as “the level at which no known or anticipated adverse effects on the health of persons occur and which allows an adequate margin of safety.”  The MCLG is based on epidemiologic or toxicological studies that identify non-cancer or cancer effects and typically does not consider the limits of laboratory detection or the current ability of treatment technologies. If the chemical contaminant is a carcinogen, then the EPA sets the MCLG to zero. If the chemical contaminant is a non-carcinogen (but may cause adverse, non-cancer health effects) then the MCLG is based on an EPA-derived “reference dose” which is an estimate of the amount of a chemical that a person can be safely exposed to on a daily basis.

For example, arsenic previously had an MCL set at 50 ug/L; however, during the 1990s, new studies and improved technologies allowed for the MCL to be lowered to be considered given that the MCLG had been set to zero. The EPA considered setting several different options for a new arsenic MCL including 3 ug/L, 10 ug/L, and 20 ug/L. After balancing the feasibility, the health benefits, and the costs, the arsenic MCL of 10 ug/L was established in January 2006.

Establishing MCLs for new or emerging contaminants and defining an enforceable water quality standard for them can be challenging. For example, per-and poly-fluoroalkyl substances (PFAS) are a group of thousands of chemicals of which several (at least) have potential adverse health effects at very low levels. As a result, there has been considerable effort and research to create an MCL that is protective of human health but is also achievable by modern analytical methods and technology.

In 2016, the EPA established a level for two types of PFAS substances called PFOA and PFOS. Specifically referred to by EPA as a Health Advisory Level (HAL), which are not legally binding, these numbers were set to provide guidance on safe levels of these chemicals to humans and at that time were set at 70 parts per trillion (ppt). Based on new information, these levels were dramatically lowered to 0.004 ppt for PFOA and 0.02 ppt for PFOS in 2022. Some have questioned this and just recently a federal court struck down a challenge by industry over the EPA’s HALs.

In March 2023, EPA proposed regulations for six specific PFAS chemicals. If finalized, after going through a public comment period, EPA would cap the drinking water standard of PFOA and PFOAS at four parts per trillion which is currently the lowest value that can be reliably measured. In the meantime, a number of states have set their own standards. Four other PFAS chemicals would be regulated as a mixture. Regardless, as shown in the case of PFASs, finding and setting enforceable limits is a challenge and will be an ongoing process for the foreseeable future.

Creating scientifically defensible standards that balance costs and benefits and that are achievable with current technology is demanding and evolving.


Creating a realistic standard that can be enforced requires:

  • Establishing a level that can be effectively reached by an analytical laboratory
  • Treatment with available technology
  • Proof that the cleanup standard is clearly supported by the toxicological data

The process of establishing the cleanliness of a test water sample depends on what you are looking for and how low the pollutant can be analyzed. Therefore, when establishing enforceable standards or cleanup levels, a balance must be struck between what is safe and what is feasible. This is a challenging but important process that aims to protect the environment from hazardous chemicals.