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Private Well 101: Drinking Water Standards

Yilin Zhuang, Andrea Albertin, and Arthur G. Hornsby


Drinking water comes from a variety of sources, including public water systems, private wells, or bottled water. While public water systems are monitored under the federal Safe Drinking Water Act, private wells are not regulated (US Environmental Protection Agency 2004). Private well users are responsible for the management and protection of their wells and water quality.

This EDIS publication is for Florida homeowners who are interested in learning more about drinking water standards. It also serves as a reference for well owners to understand their drinking water quality.

Primary Drinking Water Standards

Drinking water supplied by municipal water systems is monitored for many contaminants. As authorized by the 1974 Safe Drinking Water Act and its amendments, the US Environmental Protection Agency (EPA) has established limits on the concentration of certain drinking water contaminants allowed in public water supplies. These limits are set to protect your health and ensure that your water is of good quality. The Florida Department of Environmental Protection (FDEP) has accepted the National Standards as published herein.

The EPA standards for drinking water fall into two categories: Primary Standards and Secondary Standards. Primary Standards are based on health considerations and are enforced by the EPA. They protect you from three classes of toxic pollutants:

  • Pathogens: Disease-causing organisms such as bacteria, fungi, or viruses.
  • Radioactive elements: Substances that emit radiation, such as radium, uranium, and plutonium. Radiation can cause cancer in people and other living things.
  • Toxic chemicals: Substances that can injure or kill people.

Additionally, the Primary Standards establish a limit, called the Maximum Contaminant Level (MCL), on the highest allowable concentration of a contaminant in drinking water supplied by municipal water systems. The MCL is usually expressed in milligrams per liter (mg/L). Table 1-1 to Table 1-6 contain the current primary drinking water standards.

How Primary Standards Are Set

EPA regulators develop Primary Standards for drinking water contaminants based on three criteria:

  • The contaminant causes adverse health effects.
  • It is detectable in drinking water.
  • It is known to occur in drinking water.

When establishing Primary Standards for a drinking water contaminant, the government first looks at all the toxicological data on that contaminant, usually obtained from acute and chronic animal studies. Occasionally human clinical or epidemiological data are also available. Experts use this information to estimate the concentration of a drinking water contaminant that may be toxic and the concentrations, if any, that may cause no adverse effects. Because the levels of contaminants found in drinking water are rarely high enough to cause acute health effects, health officials are most concerned about chronic health effects such as cancer, birth defects, miscarriages, nervous system disorders, and organ damages. These health effects may occur after prolonged exposure to small amounts of a substance.

If the EPA decides not to regulate a contaminant based on the above three criteria, they may decide to develop a health advisory. A health advisory is a nonenforceable federal limit. It serves as technical guidance for federal, state, and local officials.

Regulators treat cancer-causing substances (carcinogens) differently from contaminants that cause other health effects.

Noncancerous Chemicals

For chemicals that cause adverse health effects other than cancer, Acceptable Daily Intake (ADI) levels are determined. ADI is the daily dose of a substance that a person can ingest over a lifetime without harming their health. The ADI level also includes a conservative safety margin. Regulators use the ADI to establish a Maximum Contaminant Level Goal (MCLG). The MCLG is the concentration of a contaminant that experts believe a person can drink safely over their lifetime. It is based entirely on health considerations and, as a health goal, is set at a level where no adverse health effects should occur. The MCLG is not enforced by the EPA. It is used to set enforceable drinking water standards, the Maximum Contaminant Level (MCL). The MCL is the Primary Standard measurement enforced by the EPA. It is set as close as possible to the MCLG. When setting the MCL, EPA regulators consider the feasibility and the combined cost of analyzing water for a contaminant and for treating water to remove the contaminant in addition to public health considerations. Therefore, the MCL is often less stringent than the MCLG.

Cancerous Chemicals

When establishing primary standards for chemicals that are believed to cause cancer, no concentration is considered safe. Therefore, the lifetime goal, the MCLG, is set at zero. However, a zero level is not always feasible to achieve. For example, laboratories may not be able to detect carcinogens found at low levels, or the cost of maintaining the levels of carcinogens below the MCL may not be feasible. In addition, when carcinogens are found at very low concentrations, the risk of cancer becomes so small that it is considered negligible. Therefore, regulators must decide what level of risk is acceptable. It may be one excess cancer in 10,000 persons or one excess cancer in 1 million persons exposed over a lifetime of 70 years. The concentration of chemical estimated to cause this "acceptable level" of risk is called the Risk Estimate. It is then used to set the MCL.

Table 1-1 to Table 1-6 list the current primary drinking water standards for different categories of contaminants: microorganisms, disinfection byproducts, disinfectants, inorganic chemicals, organic chemicals, and radionuclides.

Secondary Drinking Water Standards

Secondary Standards regulate contaminants that cause offensive taste, odor, color, corrosivity, foaming, and staining in drinking water. The concentration limit is called the Secondary Maximum Contaminant Level (SMCL). Secondary Standards are not enforced because they are not considered to pose a risk to human health at the recommended SMCL. They are guidelines for water treatment plant operators and state governments attempting to provide communities with the best quality water possible. Table 2 lists the current secondary drinking water standards.

Drinking Water Standards Are Not Absolute

Drinking water standards do reflect sound scientific judgment and are based on the best and most current knowledge available. They also include margins of safety to reduce adverse health effects and protect human health. The Safe Drinking Water Act also requires EPA to review each national primary drinking water regulation at least once every six years and revise them, if appropriate. As part of the "Six-Year Review," EPA evaluates any newly available data, information, and technologies to determine if any regulatory revisions are needed. Revisions must maintain or strengthen public health protection.

However, it is also important to understand that Primary Standards for drinking water contaminants do not guarantee that water with a contaminant level below the standard is risk-free, nor do they mean that water with a higher level is unsafe. It is mainly because setting drinking water standards is an imperfect process:

  • Regulatory decisions are often complicated by economic, political, and social considerations.
  • Data relating human health effects to chemicals in drinking water are limited, and scientists have difficulty predicting the effects of drinking small amounts of chemicals for many years.
  • The standards do not take into account the possible presence of other chemicals, which may increase or decrease the toxicity of the contaminant.

Current Drinking Water Standards

As mentioned earlier, the EPA has set MCLs for microorganisms, disinfection byproducts, disinfectants, inorganic chemicals, organic chemicals, and radionuclides. The EPA periodically issues standards for additional organic and inorganic chemicals, microbes, and viruses. Many more organic chemicals known to be present in drinking water are not currently regulated by either state or federal standards.

Working through state governments, the EPA monitors community drinking water. When a standard is exceeded, the EPA requires that contaminant level to be reduced to the MCL. The corrective treatment is left to the individual water system, usually a private utility.

State Responsibilities

Ultimately, regulatory officials in your state set and enforce drinking water standards for EPA-regulated contaminants and for other contaminants. However, states are not permitted to set standards that are less stringent than the MCLs set by the EPA.

Drinking water standards represent conservative judgements of scientists and regulators and are based on all available information on the health effects of drinking water contaminants. Although current drinking water standards do not guarantee that the glass of water you draw from your tap will be absolutely safe and pure, they do reflect sound scientific judgment and are based on all the knowledge that is available.

Private Well Owner Responsibility

As a private well owner, you are responsible for the quality of your own drinking water. Private wells are usually not required to test their drinking water to meet primary drinking water standards. However, you can use the public drinking water standards as guidelines when evaluating the quality of your drinking water.

For more information about drinking water standards, please see US EPA, “Drinking Water Requirements for States and Public Water Systems”:

Publication History

Stewart, Judith C., Ann T. Lemley, Sharon I. Hogan, and Richard A. Weismiller. 1989. “Health Effects of Drinking Water Contaminants.” Cornell University and University of Maryland.

Stewart, Judith C., Ann T. Lemley, Sharon I. Hogan, Richard A. Weismiller, and Arthur G. Hornsby. 2001. “Drinking Water Standards.” University of Florida Institute Food and Agricultural Sciences.


US Environmental Protection Agency. 2004. “Understanding the Safe Drinking Water Act.”

US Environmental Protection Agency. January 26, 2022. “National Primary Drinking Water Regulations.”

US Environmental Protection Agency. February 17, 2022. “Secondary Drinking Water Standards: Guidance for Nuisance Chemicals.”

Table 1-1. US EPA Primary Drinking Water Standards—Microorganisms.

Table 1-2. US EPA Primary Drinking Water Standards—Disinfection Byproducts.


Table 1-3. US EPA Primary Drinking Water Standards—Disinfectants.


Table 1-4. US EPA Primary Drinking Water Standards—Inorganic Chemicals.

Table 1-5.US EPA Primary Drinking Water Standards—Organic Chemicals.


Table 1-6. US EPA Primary Drinking Water Standards—Radionuclides.


Table 2. US EPA Secondary Drinking Water Standards.


Peer Reviewed

Publication #SL159

Date: 6/29/2022


Fact Sheet

About this Publication

This document is SL159, one of a series of the Department of Soil, Water, and Ecosystem Sciences, UF/IFAS Extension. Original publication date February 2001. Revised June 2022. Visit the EDIS website at for the currently supported version of this publication.

About the Authors

Yilin Zhuang, regional specialized agent II, UF/IFAS Extension Central District; Andrea Albertin, regional specialized agent and Extension agent II, UF/IFAS Extension Northwest District; and Arthur G. Hornsby, professor emeritus (deceased), UF/IFAS Department of Soil, Water, and Ecosystem Sciences; UF/IFAS Extension, Gainesville, FL 32611.


  • Yilin Zhuang