National Contaminant Occurrence Database (NCOD)

EPA developed the National Contaminant Occurrence Database (NCOD) to satisfy statutory requirements in the 1996 Amendments to the Safe Drinking Water Act (SDWA). SDWA requires EPA to assemble and maintain a national drinking water contaminant occurrence database using information for both regulated and unregulated contaminants in public water systems (PWSs).

This site describes water sample analytical data that EPA is currently using and has used in the past for analysis, rulemaking, and rule evaluation. The data have been checked for data quality and analyzed for national representativeness.

NCOD data include the following:

• Unregulated contaminant occurrence data
• Six Year Review of national drinking water regulations
• Ambient and/or source water data

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Unregulated contaminant occurrence data

EPA uses the Unregulated Contaminant Monitoring Rule (UCMR) to collect data for contaminants that are suspected to be present in drinking water and do not have regulatory standards set under the Safe Drinking Water Act (SDWA). The monitoring provides EPA and other interested parties with nationally representative data on the occurrence of contaminants in drinking water, the number of people potentially being exposed, and an estimate of the levels of that exposure. These data can support future regulatory determinations and other actions to protect public health.

• Unregulated Contaminant Monitoring Rule 5 Data
  The fifth Unregulated Contaminant Monitoring Rule (UCMR 5) was published on December 27, 2021. UCMR 5 requires monitoring for 29 per- and polyfluoroalkyl substances (PFAS) and lithium. Sample collection under UCMR 5 will occur from January 2023 through December 2025. Occurrence data will be updated on a quarterly basis until completion of data reporting in 2026. Data are added and possibly removed or updated over the course of this reporting cycle following further review by analytical laboratories, PWSs, states, and EPA.
• Unregulated Contaminant Monitoring Rule 4 Data
  The fourth Unregulated Contaminant Monitoring Rule (UCMR 4) was published in the Federal Register on December 20, 2016. UCMR 4 required monitoring for 10 cyanotoxins and 20 additional contaminants. Sample collection under UCMR 4 occurred from January 2018 to December 2020.
• Unregulated Contaminant Monitoring Rule 3 Data
  The third Unregulated Contaminant Monitoring Rule (UCMR 3) was published in the Federal Register on May 2, 2012. UCMR 3 required monitoring for 30 contaminants: 28 chemicals and two viruses. Sample collection under UCMR 3 occurred from January 2013 to December 2015.
• Unregulated Contaminant Monitoring Rule 2 Data
  The second Unregulated Contaminant Monitoring Rule (UCMR 2) was published in the Federal Register on January 4, 2007. UCMR 2 required monitoring for 25 contaminants. Sample collection under UCMR 2 occurred between January 2008 and December 2010.
• Unregulated Contaminant Monitoring Rule 1 Data
  The first Unregulated Contaminant Monitoring Rule (UCMR 1), under the redesigned UCMR program, was published in the Federal Register on September 17, 1999. UCMR 1 required monitoring for 26 contaminants. Sample collection under UCMR 1 occurred between January 2001 and December 2005.
• Historical State Data (Rounds 1 & 2)
  The Round 1 state dataset contains PWS monitoring results for 62 (then) unregulated contaminants, generally collected between 1988 and 1992, from 40 states and primacy entities. The Round 2 state dataset contains PWS monitoring results for 48 then-unregulated contaminants, generally collected between 1993 and 1997, from 35 states and primacy entities.

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Six-Year Review of national drinking water regulations

The SDWA requires EPA to review each National Primary Drinking Water Regulation (NPDWR) at least once every six years and make revisions if appropriate. The purpose of the review, called the Six-Year Review, is to identify those NPDWRs for which current health effects assessments, changes in technology, and/or other factors provide a health or technical basis to support a regulatory revision that will maintain or strengthen public health protection.

To support the national contaminant occurrence and exposure assessments performed under the Six-Year Review process, EPA analyzes SDWA compliance monitoring data from public water supplies for regulated drinking water contaminants. This analysis allows EPA to characterize the national occurrence of contaminants to help the Agency determine if there may be a meaningful opportunity to improve public health protection.

• Six-Year Review 3 occurrence data 
  In December 2016, the Agency published the review results for the Agency’s third Six-Year Review (called Six-Year Review 3). EPA analyzed occurrence data for the Six-Year Review 3 for 76 regulated contaminants, using data provided by 54 states and primacy agencies. The data represent compliance monitoring samples collected from January 2006 through December 2011.
• Six-Year Review 2 occurrence data
  In March 2010, the Agency announced the review results for the Agency’s second Six-Year Review (called Six-Year Review 2). EPA analyzed occurrence data for the Six-Year Review 2 for 69 regulated contaminants, using data provided by 47 states and primacy agencies. The data represent compliance monitoring sample results collected between January 1998 and December 2005.
• Six-Year Review 1 occurrence data
  In July, 2003 the Agency announced the review results for the Agency’s first Six-Year Review (called the Six-Year Review 1). EPA completed its first detailed contaminant occurrence analyses in 2003 for 69 regulated contaminants, using data provided by a national cross-section of 16 states. Most of the sample data were collected between 1993 and 1997.

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Ambient and/or source water data

EPA maintains two data management systems containing water quality information for the nation's ambient waters, the Legacy Data Center and STORET. Both systems contain raw biological, chemical, and physical data on surface and ground water collected by:

• Federal, state, and local agencies
• Tribes
• Volunteer groups
• Academics
• Others

All 50 states, territories, and jurisdictions of the U.S., along with portions of Canada and Mexico, are represented in these data systems.

• Legacy Data Center
  The Legacy Data Center (LDC) contains historical water quality data dating back to the early part of the 20th century and collected up to the end of 1998.
• STORET
  The STORET database contains data collected beginning in 1999, along with older data that have been properly documented and moved from the LDC.

U.S. Geological Survey (USGS)
The USGS disseminates water data it has collected to the public through a system called the National Water Information System (NWIS). Many types of data are stored in the NWIS network, including:

• Site information
• Time-series (flow, stage, precipitation, chemical)
• Peak flow
• Ground water and surface water quality measures

National Water Information System (NWIS)