California Water Systems Ranked by Exposure Burden — 2026

These 50 California water utilities have the highest population-weighted violation burden over the past five years. Scoring combines EPA health-based and treatment-technique violations on a linear scale (10× / 6×), adds a log-scaled contribution for monitoring/reporting lapses, and applies a 20× weight for currently-unresolved health violations. The total is multiplied by each system's population served.

How to read this ranking

Each row links to a full utility profile with violation history, lead testing results, and service-area ZIPs. The demographic context columns are from independent data sources (ACS, not EJScreen) and are provided for readers who want to examine equity patterns alongside the operational data.

See the full methodology for calculation details, data vintages, and known limitations.

Frequently asked questions

What is "exposure burden"?

A single score combining how many people a water system serves with how severe its federal drinking-water violations have been over the last five years. The formula is population_served × severity_score, where severity_score linearly sums contamination-related events — 10× per health-based violation, 6× per treatment-technique violation, 20× per currently-unresolved health violation — and adds a logarithmic contribution from procedural violations (2× × log10(1 + monitoring_count), 1× × log10(1 + other_count)). The log scaling prevents large utilities with many sampling sites from dominating the list purely through paperwork accumulation. Units are arbitrary — only relative ranks are meaningful. Scaled by 1,000 for display readability.

Why weight violations by severity?

A raw 5-year violation count would put systems with many late monitoring reports above systems with actual contamination events — because a large utility with 100 sampling sites failing 5% of them accumulates more MR violations than a tiny utility with real tap-water contamination. The severity weights come from the EPA Safe Drinking Water Act enforcement hierarchy: maximum contaminant level violations and treatment technique failures are Priority 1 (actual public-health risk), while monitoring/reporting violations are Priority 2 (procedural). We publish the weights so readers can recompute the ranking under different assumptions.

Why multiply by population?

Two systems with the same violation record affect very different numbers of people when one serves 500,000 residents and the other serves 3,000. A ranking of "worst systems" that ignores this conflates violation-per-system with violation-per-person-year. Population-weighting is standard in environmental-epidemiology work (e.g., "person-years at exposure") and prevents a list dominated by small rural utilities with easy-to-accumulate monitoring gaps.

Is this the same as the Most Unresolved Violations list?

No. Unresolved violations count only currently-open health-based violations as of the latest EPA snapshot — a point-in-time view of where the Safe Drinking Water Act is being violated right now. Exposure burden is a 5-year accumulated view that weights all violations by severity and population. A system with one serious contamination event affecting a large city can outrank a system with many small unresolved monitoring gaps; the two lists surface different facts.

Where does the data come from?

Violation categorization and counts come from EPA's Safe Drinking Water Information System (SDWIS) via the ECHO enforcement database, refreshed monthly. Population served comes from EPA Community Water System Service Area Boundaries v3 cross-walked to Census 2020 population via Microsoft building-footprint weights. Demographics come from Census ACS 2019-2023.