(CNN) Waste ash from hundreds of coal-fired power plants has contaminated groundwater in 39 states with toxic substances like arsenic, lithium and mercury, according to a report by two environmental groups that was based on data the plants reported to the US Environmental Protection Agency.

The report, released Monday by the Environmental Integrity Project and Earthjustice, highlights more than a dozen instances in which those substances have reached drinking water supplies. The full extent of the effect on drinking water supplies is not known because private sources of drinking water are not tested, the report said.

"Virtually all coal plants are poisoning our water," said author Abel Russ, an attorney with the Environmental Integrity Project.

The ponds and landfills used to store coal ash are frequently unlined, allowing toxins to leach into groundwater.

The report is based on groundwater monitoring data from more than 4,600 wells. It compared measured levels to drinking water or other standards. Contamination was found in groundwater near 242 of the 265 plants that recently reported data required by the 2015 rule

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