Evidence policy

Sources and methodology

Initial guidance is based on primary public sources and applicable state or local agencies. Each published record includes its source URL, review date, scope, and limitations.

Private wells

US EPA private wells and CDC well-water testing guidance.

See the data-backed test library →

Septic systems

Our national planning baseline uses EPA guidance for system care and inspection, common service questions, and malfunction warning signs. State, local, permit, and system-specific requirements override it.

Open the transparent estimator →

Geography

US Census ZCTA guidance and USGS water resources.

Modeled well prevalence or ZCTA data helps prioritize research; it does not prove that a particular property has a private well or septic system.