Nearly a million Californians drink water contaminated with arsenic, nitrate or other pollutants, even as drought strains the state’s water supplies.

California’s state auditor found that 371 water systems serving more than 920,000 people had unsafe levels of contaminants, with most failing systems in the Central Valley and the state’s southeastern desert, according to a report published Tuesday. While numbers improved from the previous year, when 418 systems had unsafe water, the audit found that for every year since 2017 at least 300 had tainted supplies. More than two-thirds of water systems now on the list serve disadvantaged communities, and hundreds more are considered to be at risk of failing.