1,2,3-trichloropropane, or TCP — a chemical that the Environmental Working Group calls an ” extremely potent carcinogen” — has over time seeped into a number of public water systems, where it can “persist for centuries,” according to the nonprofit. A proposal from the California State Water Resources Control Board would set a maximum level of TCP in tap water at five parts per trillion — the lowest level that existing filtration systems can reliably detect, Stephanie O’Neill reported last week.

It “is a top priority for the state water board,” spokesman Andrew DiLuccia told California Healthline.

TCP — a chemical byproduct of a defunct nemacide manufactured by Shell and Dow — disproportionately impacts the San Joaquin Valley, where much of the country’s produce is grown. It has also seeped into urban areas, including in Los Angeles, San Bernardino and San Mateo counties, probably from industrial and hazardous waste sites, according to the board.