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National Contaminant Occurrence Database (NCOD)

EPA developed the NCOD to satisfy the statutory requirements set by Congress in the 1996 amendments to the Safe Drinking Water Act (SDWA) to maintain a national drinking water contaminant occurrence database using samples data for both regulated and unregulated contaminants in public water systems.

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

Unregulated contaminant occurrence data are from monitoring in public water systems for contaminants not having health-based standards set under the SDWA at the time of the monitoring. The data assist the Administrator in determining whether or not to regulate those contaminants.

  • Historical State Data (Rounds 1 & 2)
    The Round 1 dataset contains public water system monitoring results for 62 (then) unregulated contaminants, generally collected between 1988 and 1992, from 40 states and primacy entities. The Round 2 dataset contains public water system monitoring sample data for 48 (then) unregulated contaminants, generally collected between 1993 and 1997, from 35 states and primacy entities.
  • Unregulated Contaminant Monitoring Rule 1 Data
    The Unregulated Contaminant Monitoring Regulation supporting the first cycle (UCMR1) of monitoring, conducted under EPA oversight, through the revised UCMR program, was published in the Federal Register on September 17, 1999. The UCMR1 required monitoring for 26 contaminants. Monitoring occurred January 2001-December 2005.
  • Unregulated Contaminant Monitoring Rule 2 Data The Unregulated Contaminant Monitoring Regulation supporting the second cycle (UCMR2) of monitoring, conducted under EPA oversight, was published in the Federal Register on January 4, 2007. The UCMR2 requires monitoring for 25 contaminants using five analytical methods. Monitoring is scheduled January 2008-December 2010.

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Six-Year Review of National Drinking Water Regulations

The Safe Drinking Water Act (SDWA) requires EPA to review each National Primary Drinking Water Regulation (NPDWR) at least once every six years and revise them, 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 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/primacy agencies. This data represents 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/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, Indian Tribes, volunteer groups, academics, and 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, or 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 has 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, and groundwater and surface water quality measures.

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