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What it means: The 1991 LCR uses action levels rather than MCLs: if the 90th percentile of high-risk tap samples exceeds 15 ppb lead or 1.3 mg/L copper, the system must take escalating corrective actions. Violations typically indicate failure to maintain adequate corrosion control — the treatment that keeps lead from leaching out of pipes, solder, and fixtures.

Health effects: Lead: no safe level for children. Causes irreversible neurological damage, impaired cognitive development, behavioral problems, and reduced IQ in children under 6. In adults: hypertension, cardiovascular disease, kidney damage. Copper: acute gastrointestinal distress at high levels; liver/kidney damage at very high chronic levels.

Limit: Action levels: lead 15 ppb, copper 1.3 mg/L — 90th percentile of tap samples at high-risk sites

What it means: When a system violates a drinking water standard, it must notify customers promptly: within 24 hours for acute violations (e.g., E. coli), 30 days for non-acute MCL violations, and 1 year for monitoring failures. A violation means the notification was late or missing — not that the underlying water issue is necessarily severe.

Health effects: No direct health risk from the notification violation itself, but delayed notification means customers may have consumed contaminated water without the chance to take protective action (boil water orders, bottled water use).

Limit: No MCL — compliance requires meeting specific notification deadlines