(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.
Fifty-two percent of those sites are contaminated with cancer-causing arsenic, and 60% are polluted with lithium, which is linked to neurological damage, according to the report.
Last year, a federal appeals court ordered the EPA to re-evaluate the Obama-era rule, saying it did not go far enough. For example, it suggested that the EPA should go further to require that coal ash ponds be lined, because government data showed that "a significant portion" of the 500-plus ponds covered under the rule "are likely to contaminate groundwater."
The Trump administration finalized new regulations in July, but weakened rather than tightened some regulations, said Lisa Evans, an attorney with Earthjustice. The new rules allowed states or the EPA to waive monitoring requirements in some situations.
EPA spokesman John Konkus said Monday that the agency is reviewing the report "and cannot comment on its contents yet." He said the monitoring requirement "helps provide transparency and a mechanism to oversee a facility's compliance with the regulations."
The report's authors said they're "understating" the contamination, because data is available only on coal ash sites actively in use. Ponds and landfills that hold coal ash but are not receiving new deliveries are not included.
"They're not regulated, they're not monitored, and we don't know much about them," Russ said of the unregulated pits.