California sets the first drinking water standard for 1,2,3-TCP after the ingredient in crop fumigants is found in hundreds of water wells.

Starting in the 1940s, the synthetic chemical 1,2,3-TCP or 1,2,3-trichloropropane, was an ingredient in certain soil fumigants that were widely used on farms in California and other states. 1,2,3-TCP is a highly toxic chemical classified as a human carcinogen.

This month, the California State Water Resources Control Board set the standard for 1,2,3-TCP at an incredibly low concentration of 5 parts per trillion (ppt). The drinking water standard, also known as a maximum contaminant level (MCL), defines the allowable concentration of a contaminant in tap water. For comparison, the drinking water standards for better known carcinogenic chemicals are much higher: the MCL for TCE is 5 parts per billion (1,000 times higher than the new TCP standard); the California MCL for benzene is 1 part per billion (200 times higher than the new TCP standard).

Ironically, 1,2,3-TCP was not an active ingredient in the agricultural products. It was instead a contaminant that occurred in their manufacture. 1,2,3-TCP was banned from use in soil fumigants in the 1990s. However, because the chemical does not bind to soil or break down easily in the environment, it has leached into groundwater in many agricultural communities and contaminated dozens of private and municipal drinking water wells.

Based on 2015 data, the California Division of Drinking Water has estimated that 103 water systems serving approximately 920,000 Californians have detected 1,2,3-TCP above 5 ppt in at least one drinking water source.