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What it means: Nitrate (NO₃) is the most common chemical contaminant in US groundwater, entering from fertilizers, septic systems, and animal waste. It is odorless and tasteless — the only way to know if it's present is to test.

Health effects: Methemoglobinemia in infants under 6 months: gut bacteria convert nitrate to nitrite, which blocks oxygen transport in blood. Pregnant women should also take precautions. Chronic adult exposure at elevated levels may increase cancer risk.

Limit: 10 mg/L as nitrogen (NO₃-N)

What it means: The 2013 RTCR (effective 2016) replaced the original TCR. It triggers automatic E. coli testing whenever total coliform is found. A single E. coli-positive sample is an acute MCL violation. It also requires systems exceeding 5% total coliform-positive samples to conduct a formal assessment and correct any deficiencies, helping prevent contamination before it escalates.

Health effects: Total coliform bacteria indicate that the disinfection barrier has been breached or fecal contamination is entering the system. E. coli specifically indicates fecal contamination and the possible presence of disease-causing pathogens (norovirus, Salmonella, Shigella, Campylobacter). Illness can be severe in elderly, young children, and immunocompromised individuals.

Limit: Zero E. coli; ≤5% of monthly samples total coliform-positive