Water Quality
home >  maps & data >  water quality
Types of Drinking Water Contaminants

Drinking water contaminants may be classified as inorganic compounds, organic compounds, disinfection byproducts, radionuclides, and infectious agents. Tracking California provides data for certain contaminants of concern, listed below. Each contaminant is monitored and regulated against standards such as a maximum contaminant level (MCL) or a required treatment technique (TT) established by the US Environmental Protections Agency (EPA). For a complete list of primary drinking water contaminants and their regulatory limits and health effects, see the EPA's National Primary Drinking Water Regulations. All information on public water system violations and estimates of impacted populations provided below are from the California Division of Drinking Water's (DDW) 2016 Annual Compliance Report.

Inorganic Compounds: Arsenic, Nitrate, and Lead

Inorganic compounds are chemicals of mineral materials which do not have a carbon base. According to the DDW, an estimated 138,830 people in California were potentially impacted by drinking water that violated water quality standards for inorganic compounds in 2016. Arsenic, nitrate, and lead are examples of inorganic compounds.

In some locations, arsenic may be naturally occurring in soil, rocks, and minerals and can enter source water as these natural deposits erode over time. It is also found in electronic production waste and in some pesticides used primarily in orchards, and runoff from these sites may cause arsenic to enter surface water sources. Consuming high levels of arsenic over a long period of time may cause skin damage and damage to the circulatory system and may increase risk of cancer. In 2016, 127 public water systems had at least one violation where nitrate levels were above the MCL.

Nitrate enters water sources from runoff from fertilizer use, leaks from septic tanks and sewage, and erosion of natural deposits. Infants that consume water containing more than 10 micrograms per liter of nitrate may experience serious health impacts. Among infants, nitrate can deplete the body of oxygen, causing symptoms such as a blueish coloring of the skin, shortness of breath, and even death if left untreated. In 2016, 108 public water systems had at least one violation where nitrate levels were above the MCL.

Unlike arsenic and nitrate, lead usually enters water systems in the distribution system after the treatment process due to corroding lead pipes. During childhood, exposure to any level of lead can cause delayed physical or mental development, attention deficits, and learning disabilities. In 2016, 18 public water systems had at least one violation of the Lead and Copper Rule. More information and data on lead exposure can be found on Tracking California's Lead page.

Organic Compounds: Atrazine

Organic compounds are chemicals that include carbon compounds. Atrazine, a type of organic compound, is found in herbicides frequently used in agricultural production. In areas where atrazine-containing herbicides are used, atrazine may enter groundwater sources through soil absorption, or surface water sources from runoff. Consuming high levels of atrazine over a long period of time may increase risk for cardiovascular or reproductive health problems. In 2016, 3 public water systems had at least one violation indicating levels of organic compounds above the MCL; an estimated 320 people in California were potentially impacted by drinking water that violated water quality standards.

Disinfection Byproducts: Trihalomethanes and Haloacetic Acids

Disinfection byproducts (DBP) are formed during the treatment process when disinfectants such as chlorine are added to water to kill harmful bacteria and viruses. Trihalomethanes and haloacetic acids are two types of DBPs that form when chlorine interacts with organic matter in water. While scientific evidence on the health impacts of long-term exposure to DBPs is inconclusive, there is some evidence that exposure to water with high levels of trihalomethanes over a long period of time may increase risk for liver, kidney, central nervous system, or reproductive system problems, and may increase risk of certain types of cancer. There is slightly stronger evidence that long-term exposure to water with high levels of haloacetic acids may increase risk of cancer. In 2016, 38 public water systems had levels of trihalomethanes above the MCL and 22 public water systems had at least one violation indicating levels of haloacetic acids above the MCL; an estimated 221,309 people in California were potentially impacted by drinking water with levels of DBPs above the MCL.

Radionuclides: Uranium and Radium

Radionuclides are radioactive compounds that release nuclear radiation as they decay over time. Radionuclides such as uranium and radium occur naturally and can enter source water from the erosion of natural deposits. Exposure to high levels of uranium can have a toxic effect on the kidneys and can increase the risk of cancer. Drinking water with high levels of radium may also increase risk of cancer. In 2016, 28 public water systems had at least one violation indicating levels of uranium above the MCL; an estimated 11,418 people in California were potentially impacted by drinking water with levels of radionuclides that exceeded the MCL.

Infectious agents: Bacteria, Viruses, and Parasites

Tracking California does not collect data on infectious agents such as bacteria, viruses, or parasites because public water systems are not required to test their water for specific organisms or viruses. Instead, water systems must test for indicator organisms such as total coliform, which, if present in drinking water, indicates a water supply has been contaminated and may contain a variety of harmful bacteria. Consuming water that contains bacteria, viruses, or parasites most often causes gastrointestinal illness; symptoms may include fever, diarrhea, and vomiting. In 2016, 433 public water systems had at least one violation indicating levels of total coliform above the MCL; an estimated 425,554 people in California were potentially impacted by drinking water with levels of total coliform that exceeded the MCL.

Despite these violations and potential exposures to disease-causing bacteria, California has not had any known drinking water-related disease outbreaks in the last 9 years according to the Centers for Disease Control and Prevention (CDC). This could be because public water systems are effectively monitoring their water quality and immediately notifying their customers when the level of total coliform exceeds the MCL to prevent anyone from drinking the contaminated water. However, these statistics are based only on disease outbreaks and do not indicate that no one in California has experienced adverse health impacts from exposure to drinking water contaminants.