Data Sources & Methodology
Where does Sewage Monitor's data come from, what does it include, and — just as importantly — what does it miss? We believe in radical transparency about our sources and their limitations.
Primary Data Source: EPA ECHO
All SSO event data on Sewage Monitor is sourced from the EPA's ECHO (Enforcement and Compliance History Online) system — specifically the ICIS-NPDES SSO/Bypass Eventtable available through ECHO's bulk data download interface.
What is EPA ECHO?
ECHO is the EPA's public-facing compliance and enforcement database. It aggregates information from the Integrated Compliance Information System for NPDES (ICIS-NPDES), which is the federal repository for permit compliance data submitted by states and regulated facilities. ECHO covers more than 900,000 regulated facilities across air, water, and hazardous waste programs, but the piece relevant to Sewage Monitor is the wastewater discharge reporting data — specifically, the SSO event reports that permitted utilities submit when they have a spill.
ECHO is designed primarily for regulators, environmental attorneys, and researchers. Its interfaces are not built for casual public use. Sewage Monitor processes the raw data to make it accessible to everyone.
What the ICIS-NPDES SSO Table Contains
Each row in the ECHO SSO dataset represents a single reported overflow event. The fields include:
| Field | Description |
|---|---|
| NPDES Permit ID | The unique federal permit number for the reporting utility |
| Event Start Date | When the overflow began |
| Event End Date | When the overflow was controlled (if known) |
| Volume (gallons) | Self-reported estimate of total discharge volume |
| Cause Category | Primary cause: blockage, I/I, pump failure, etc. |
| Reached Water Body | Yes/no indicator whether discharge reached surface water |
| Water Body Name | Name of the receiving water body, if applicable |
| Latitude / Longitude | Location of the overflow (not always available) |
| NPDES Facility Name | Name of the permitted utility or treatment plant |
| State | Two-letter state abbreviation |
How to Access the Data Yourself
The ECHO bulk download is free and requires no account. Navigate to echo.epa.gov/tools/data-downloads/sewer-overflow-download-summary and download the SSO Event CSV file. The file is updated periodically by EPA as states submit new reports. You can load it into Excel, R, Python/pandas, or any data analysis tool for your own research.
Critical Data Limitations
Understanding what this data cannot tell you is just as important as understanding what it can. These are not minor caveats — they are fundamental constraints that affect how the data should be interpreted.
⚠ Phase 2 eRule: Not all states are fully reporting yet
The EPA's SSO Electronic Reporting Rule (eRule) requires utilities to submit SSO reports electronically through NetDMR (Net Discharge Monitoring Reports). The Phase 2 requirements, which took effect for large systems in late 2022 and are still rolling out for smaller systems, mean that historical data coverage is very uneven. States that were early adopters of electronic reporting have years of data in ECHO; states that are just coming into compliance may have sparse records before 2023–2024. A state with few events in ECHO may simply not have been reporting, not necessarily have had fewer overflows.
⚠ Volume estimates are imprecise — often very imprecise
Reported gallon volumes are utility self-estimates made at the time of the event. For a small, contained spill, a utility employee can make a reasonable visual estimate. For a large overflow that spreads across a parking lot or into a drainage ditch during a storm, the estimate may be a rough order-of-magnitude guess. Some utilities report very round numbers (10,000 gallons, 50,000 gallons) that suggest estimation rather than measurement. Treat volume figures as rough indicators of scale, not precise measurements.
⚠ Significant reporting lag — events appear weeks later
After an SSO occurs, the utility files a report with its state environmental agency. The state then enters or transmits that data to the ICIS-NPDES system, which feeds ECHO. This chain of custody introduces lag that can range from days to weeks. Sewage Monitor refreshes from ECHO daily, but recent events may not appear for 2–4 weeks after they occur. Do not interpret the absence of recent events as proof that recent overflows have not occurred.
⚠ Many SSOs are never detected or reported at all
Sewer systems are largely underground and out of sight. Small overflows that don't make it to a waterway may go unnoticed. Utilities in resource-constrained municipalities may not have the inspection capacity to identify and document every event. Academic research consistently suggests the true number of SSO events is substantially higher than what appears in regulatory databases. Zero events in a particular county does not mean zero overflows — it may mean zero reported overflows.
⚠ California uses CIWQS, not ECHO
California operates its own water quality compliance system called CIWQS (California Integrated Water Quality System), administered by the State Water Resources Control Board. California wastewater utilities primarily report SSO data to CIWQS rather than to federal ECHO. As a result, California is significantly underrepresented in Sewage Monitor's current data. We are working to incorporate CIWQS data directly. For California-specific SSO data, visit the SWRCB's CIWQS portal.
⚠ State-by-state coverage varies significantly
Beyond California, reporting completeness varies by state based on when electronic reporting was adopted, how well-funded the state environmental agency is, and how rigorously permit conditions are enforced. States with active EPA enforcement presence and well-funded DEQ offices tend to have more complete records. See our States page for our assessment of coverage quality by state.
⚠ Coordinate data is often missing or inaccurate
Many SSO reports lack valid geographic coordinates, or have coordinates that locate the utility's headquarters rather than the actual overflow site. Events without valid coordinates appear in our database and state filters but cannot be displayed on the map. The actual number of events in any geographic area is higher than what the map shows.
Secondary Data Sources (Planned)
EPA ECHO SSO data is our foundation, but a complete picture of sewer overflow impacts requires additional context. The following sources are either already incorporated in limited ways or are planned for future integration:
Surfrider Foundation Blue Water Task Force
Volunteer-based bacteria monitoring program that samples beach water quality across the US and posts results publicly. Provides ground-truth validation of whether bacteria counts spike following nearby overflow events. We plan to correlate BWTF monitoring stations with nearby SSO events.
USGS National Water Information System (NWIS) Streamflow Data
USGS maintains thousands of real-time streamflow gauges across the US. Downstream flow data during and after SSO events helps contextualize the dilution capacity of receiving waterways — a 10,000 gallon spill into a drought-affected creek is very different from the same spill into a major river in flood stage.
NOAA Precipitation Data (NCEI)
Rainfall is the primary driver of inflow/infiltration SSOs. Correlating overflow events with NOAA precipitation records allows us to distinguish wet-weather overflow clusters from dry-weather structural failures — an important distinction for understanding cause patterns.
EPA Sewershed Boundaries Dataset
EPA has developed sewershed boundary data that maps the drainage areas contributing to particular wastewater treatment facilities. Layering SSO points against sewershed boundaries would improve geographic attribution and allow better analysis of which communities bear the overflow burden.
California CIWQS (State Water Resources Control Board)
California's own compliance database, which contains the most comprehensive SSO records for the nation's most populous state. Pending API access and data normalization, we plan to incorporate CIWQS data to fill the California gap in ECHO coverage.
How Data Is Refreshed
Sewage Monitor downloads a fresh copy of the EPA ECHO SSO bulk data file every day via an automated process. The download runs early in the morning UTC and processes the CSV into our database, updating existing records and adding new ones.
The process works as follows:
- Cron job triggers daily download of the ECHO SSO event CSV from the EPA ECHO bulk download endpoint
- CSV is parsed and each row is upserted into the Sewage Monitor database using the NPDES permit ID + event start date as a composite key
- Events with invalid or missing coordinates are stored but flagged as map-ineligible
- The map and list views reflect the updated database within minutes of the daily refresh completing
Because EPA ECHO itself has reporting lag (as described above), daily refreshes do not mean real-time data. We are displaying the most current data available from the federal system, which may still lag actual events by days to weeks.
How to Cite This Data
If you use data from Sewage Monitor in research, journalism, or advocacy work, please cite both the original EPA source and Sewage Monitor:
For the primary data, you may also cite directly: GAO. (2023). Sewer Overflows: EPA Should Work with States to Improve Data Quality. GAO-23-105479. gao.gov/products/gao-23-105479
Key External Resources
Main entry point for EPA compliance data
Direct download of the raw SSO event CSV
Government Accountability Office report on data quality gaps
Nationwide assessment of wastewater infrastructure investment needs
American Society of Civil Engineers wastewater infrastructure grade
California's own SSO compliance reporting system
Data Corrections & Tips
If you notice a data error, a missing event you know occurred, or if you work for a utility or state agency with data corrections to share, please reach out. We are committed to improving data quality and accuracy over time.
Contact: data@sewagemonitor.com
Note: Sewage Monitor can only display what EPA ECHO contains. For corrections to the underlying federal data, the appropriate path is through your state environmental agency's NPDES reporting process.