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.
SixYear 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 revise them, if appropriate. SDWA specifies that any revision must maintain or increase public health protection. 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
Six Year Review contaminant occurrence data
Six Year Review detailed data in text files
Data included in the Six Year Review PivotTable
This PivotTable contains information on Six Year Review sample data. Any or all of the following facts for a contaminant
- # analyses performed
- # PWSs with analyses
- # detects
- # PWSs with detects (analytical detections above the reporting limit)
- minimum detect
- maximum detect
- mean detect
- minimum non-zero MRL
- maximum MRL
- mean non-zero MRL
-can be categorized by any combination of the following water system attributes:
- EPA Region
- State
- Water system type
- Size category *
-or any combination of the following sample attributes:
- Sample collection year *
- Source water type where the sample was taken *
* To avoid double-counting, the Pivot Tables will not sum the number of PWSs with and without detects across this dimension/attribute. Details on these attributes are provided in the Pivot Tables
Data fields included in the Six-Year Review database table
- PWSID (Public Water System Identification Code)
- EPA region
- State
- Water system type
- Size category (based on people served by the system)
- 1 = Very Small <=500
- 2 = Small 501-3,300
- 3 = Medium 3,301-10,000
- 4 = Large 10,001-100,000
- 5 = Very large >100,000
- Population served
- Source_type
- Contaminant Code
- Contaminant Name
- Detect?
- NonDetect?
- Result (for detects)
- MRL (for non-zero non-detects)
- Sample collection date
Details on these attributes are provided in the Pivot Tables
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.
