Guymon

What it means: Arsenic is a naturally occurring metalloid found in bedrock and soil. It dissolves into groundwater from natural deposits and can also come from mining, smelting and past pesticide use. EPA lowered the MCL from 50 ppb to 10 ppb in 2001.

Health effects: Long-term arsenic exposure increases risk of bladder, lung, and skin cancers. It is also associated with cardiovascular disease, type 2 diabetes, and developmental effects in children. Arsenic is classified as a Group A human carcinogen by the EPA.

Limit: 0.010 mg/L (10 ppb), set in 2001 and enforceable since 2006; the previous MCL was 0.050 mg/L (50 ppb)

What it means: This code tracks the combined total nitrogen from both nitrate and nitrite. While each has its own MCL (nitrate: 10 mg/L, nitrite: 1 mg/L), the combined sum is also limited to 10 mg/L as nitrogen. The combined measurement applies when both forms are present, particularly in partially treated or mixed sources where nitrification may be incomplete.

Health effects: The combined limit protects against methemoglobinemia — the same risk as nitrate alone and nitrite alone. In infants under 6 months, gut bacteria convert these nitrogen compounds to forms that block oxygen transport in blood ('blue baby syndrome'). Pregnant women and individuals with certain enzyme deficiencies are also at elevated risk.

Limit: 10 mg/L as nitrogen (NO₂-N + NO₃-N combined)