The report by the State Water Resources Control Board was ordered by the state Legislature. It says from 2002 to 2010, 680 out of 3,000 community water systems in the state relied on one or more contaminated groundwater wells. Those contaminated wells served 21 million people.
Arsenic was the most detected naturally-occurring contaminant. Nitrate was the most detected human-caused contaminant. Chemical fertilizers and livestock manure are the main source of nitrate contamination in groundwater.
The report stresses that most of the communities treat the water, so what residents get is safe.
Information from the Associated Press was used in this report.