Drinking water quality standards explain the rules that determine the quality of drinking water. Water can contain harmful substances, but there are no worldwide standards that all countries agree on. Even when standards exist, the allowed amounts of these substances can differ by up to ten times between different sets of standards. Many countries create their own rules for drinking water. In Europe, this includes the European Drinking Water Directive, and in the United States, the United States Environmental Protection Agency (EPA) sets standards based on the Safe Drinking Water Act. China has its own standard called GB3838-2002 (Type II), which was created by the Ministry of Environmental Protection in 2002. For countries without laws or rules about these standards, the World Health Organization (WHO) provides guidelines on what standards should be met.

Most drinking water standards are written as guidelines or goals, not strict rules. Few have legal backing or are enforced. Two exceptions are the European Drinking Water Directive and the Safe Drinking Water Act in the United States, which require countries to follow specific standards legally. In Europe, member states must create local laws to follow the directive. The European Commission can impose penalties on countries that don't comply.

Range of standards

Drinking water standards include lists of specific numbers, and also describe where samples should be taken, how samples should be collected, how often samples should be taken, the methods used to test samples, and the qualifications needed for laboratories (AQC). Some standards also require calculations to check if a level meets the standard, such as finding an average. Other standards provide detailed rules for handling test results, including how to deal with changes over time and seasons, how to add up related measurements, and how to handle results that seem unusual.

For example, when comparing drinking water quality in Kenya and Ethiopia to published guidelines (thresholds), scientists looked at several standards: the Kenyan drinking water standard, the Ethiopian standard, the WHO health guideline, the WHO aesthetic guideline, and the EAS (East African Standards) for natural drinking water. Scientists also included non-regulatory health-based screening levels (HBSLs) for cobalt, lithium, silver, strontium, and thallium from the United States Geological Survey and the EPA in their analysis.

Parametric values

A parametric value in this context is often a measurement of how much of a substance is present, such as 30 milligrams per liter of iron. It can also be a count, like 500 E. coli bacteria per liter, or a statistical value, such as the average amount of copper being 2 milligrams per liter. Many countries set parametric values for substances that might affect health, as well as for other substances that are unlikely to affect health. These include measurements of color, turbidity, pH, and organoleptic (aesthetic) factors such as taste and odor.

It is possible and acceptable to describe the same parameter in different ways, which might seem to suggest different standards. For example, nitrite can be measured as nitrite ions or expressed as nitrogen. A standard of "nitrite as nitrogen" set at 1.4 milligrams per liter equals a nitrite ion concentration of 4.6 milligrams per liter. This appears to be a difference of nearly three times.

Standards by country

Countries that use guideline values as their standards include Canada, which has guidelines for a limited number of water quality parameters, New Zealand, where laws require water providers to try their best to meet standards, and Australia.

In Australia, drinking water quality standards are created by the Australian Government National Health and Medical Research Council (NHMRC) through the Australian Drinking Water Guidelines. These guidelines set limits for different types of contaminants, such as pathogens, aesthetic issues, organic and inorganic chemicals, and radioactive materials. They also provide instructions on managing water quality during treatment and distribution.

China has its own drinking water standard called GB3838-2002 (Type II), which was established by the Ministry of Environmental Protection in 2002.

The European Union’s Drinking Water Directive includes the following parametric standards. These standards must be enforced by laws in all EU countries. The values listed here are simplified, but the original directive often includes additional notes or explanations for some of these values.

• Acrylamide: 0.10 μg/L
• Antimony: 5.0 μg/L
• Arsenic: 10 μg/L
• Benzene: 1.0 μg/L
• Benzo(a)pyrene: 0.010 μg/L
• Boron: 1.0 mg/L
• Bromate: 10 μg/L
• Cadmium: 5.0 μg/L
• Chromium: 50 μg/L
• Copper: 2.0 mg/L
• Cyanide: 50 μg/L
• 1,2-dichloroethane: 3.0 μg/L
• Epichlorohydrin: 0.10 μg/L
• Fluoride: 1.5 mg/L
• Lead: 10 μg/L
• Mercury: 1.0 μg/L
• Nickel: 20 μg/L
• Nitrate: 50 mg/L
• Nitrite: 0.50 mg/L
• Pesticides: 0.10 μg/L
• Total Pesticides: 0.50 μg/L
• Polycyclic aromatic hydrocarbons: 0.10 μg/L (sum of specific compounds)
• Selenium: 10 μg/L
• Tetrachloroethene and trichloroethene: 10 μg/L (sum of specific compounds)
• Trihalomethanes: 100 μg/L (sum of specific compounds)
• Vinyl chloride: 0.50 μg/L

In the United States, the federal law that controls drinking water quality is the Safe Drinking Water Act (SDWA). This law is carried out by the Environmental Protection Agency (EPA), mainly through state or territorial health agencies.

The EPA has set standards for more than 90 contaminants, grouped into six categories: microorganisms, disinfectants, disinfection byproducts, inorganic chemicals, organic chemicals, and radionuclides. States and territories must follow rules that are at least as strict as the EPA’s to keep primary responsibility for managing drinking water. Many states also use their own standards, which may be stricter or include more parameters. Many countries use EPA standards as a reference for scientific and public health guidance.

World Health Organization guidelines

The World Health Organization (WHO) Drinking-water Quality Guidelines suggest the following maximum levels for certain naturally occurring substances that can harm health:

• Arsenic 10 μg/L
• Barium 10 μg/L
• Boron 2400 μg/L
• Chromium 50 μg/L
• Fluoride 1500 μg/L
• Selenium 40 μg/L
• Uranium 30 μg/L

• Benzene 10 μg/L
• Carbon tetrachloride 4 μg/L
• 1,2-Dichlorobenzene 1000 μg/L
• 1,4-Dichlorobenzene 300 μg/L
• 1,1-Dichloroethane 30 μg/L
• 1,2-Dichloroethene 50 μg/L
• Dichloromethane 20 μg/L
• Di(2-ethylhexyl)phthalate 8 μg/L
• 1,4-Dioxane 50 μg/L
• Edetic acid 600 μg/L
• Ethylbenzene 300 μg/L
• Hexachlorobutadiene 0.6 μg/L
• Nitrilotriacetic acid 200 μg/L
• Pentachlorophenol 9 μg/L
• Styrene 20 μg/L
• Tetrachloroethene 40 μg/L
• Toluene 700 μg/L
• Trichloroethene 20 μg/L
• Xylene 500 μg/L

Comparison of parametric values

The table below shows how certain details about levels set by the World Health Organization (WHO), the European Union, the Environmental Protection Agency (EPA), and China's Ministry of Environmental Protection compare.