WQRF Contaminant Occurrence Map


Data Looks at 57 Different Contaminants

The WQRF Contaminant Occurrence Map displays data in the United States for regulated drinking water contaminants that have an enforceable level (MCL or Action Level) above the health-based goal level (MCLG) and also aesthetic contaminants that can cause taste, odor and color or staining issues.

  • The Contaminant Occurrence data collection effort gathered water quality data over the last 10 years for 57 different drinking water contaminants in the United States.
  • The data comes from 46 state regulatory agencies, the US EPA’s 4th Unregulated Contaminant Monitoring Rule (UCMR4) and the EPA’s Safe Drinking Water Information System (SDWIS).
  • The mapping tool provides a visual illustration of this occurrence data provided by the states and through US EPA UCMR4.
  • The mapping tool provides a visual illustration of this occurrence data provided by the states and through US EPA UCMR4.
  • ​This map shows statistical summaries of all data, not regulatory compliance.


Regulated Contaminants
​with MCLG lower than MCL

​Full Report
Included in this report:
lead, arsenic, disinfection byproducts (individual species and totals), inorganics, organics (SOC’s and VOC’s), microbials and radionuclides

Aesthetic Contaminants

Executive Summary
Full Report

Included in this report:
chlorine residual, chloramine residual, hardness, iron, manganese and pH


Key Terms

Maximum Contaminant Level Goal (MCLG) 

The level of a contaminant in drinking water below which there is known or expected risk to health. MCLGs allow for a margin of safety and are non-enforceable public health goals.

Maximum Contaminant Level (MCL) 

The highest level of a contaminant that is allowed in drinking water. MCLs are set as close to MCLGs as feasible using the best available treatment technology and taking cost into consideration. MCLs are enforceable standards.

Maximum Residual Disinfectant Level Goal (MRDLG) 

The level of a drinking water disinfectant below which there is no known or expected risk to health. MRDLGs do not reflect the benefits of the use of disinfectants to control microbial contaminants.

Community Water System (CWS)

 This supplies water to the same population year-round. It serves at least 25 people at their primary residences or at least 15 residences that are primary residences (for example, municipalities, mobile home park, sub-divisions).

Maximum Contaminant Level (MCL) Transient Non-community Water Systems (TNCWS)

– These provide water to 25 or more people for at least 60 days/year, but not to the same people and not on a regular basis (for example, gas stations, campgrounds).

Non-transient Non-community Water Systems (NTNCWS) 

These regularly supply water to at least 25 of the same people at least six months per year, but not year-round (for example, schools, factories, office buildings, and hospitals which have their own water systems).

Treatment Technique (ITT)

 A required process intended to reduce the level of a contaminant in drinking water.

  • Lead and copper are regulated by a treatment technique that requires systems to control the corrosiveness of their water. If more than 10% of tap water samples exceed the action level, water systems must take additional steps. For copper, the action level is 1.3 mg/L, and for lead is 0.015 mg/L.

Maximum Residual Disinfectant Level (MRDL) 

The highest level of a disinfectant allowed in drinking water. There is convincing evidence that addition of a disinfectant is necessary for control of microbial contaminants.

Non-detect (ND) Data

This means the concentration was lower than the analytical instrument’s detection limit and are treated as a result of zero.