Ames

What it means: 2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most widely used herbicides in the world, applied to lawns, turf, pastures, and crops to control broadleaf weeds. It remains in active use in the United States. 2,4-D is water-soluble and can enter drinking water through agricultural and residential runoff and leaching into groundwater.

Health effects: Chronic exposure to 2,4-D is associated with kidney, liver, and adrenal gland damage in animal studies. IARC classifies it as possibly carcinogenic to humans (Group 2B); EPA's classification is not likely to be carcinogenic under current guidelines. Individuals with kidney or liver conditions may face greater risk from long-term exposure.

Limit: 0.07 mg/L (70 ppb)

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)