More than 90 percent of U.S. coal-fired power plants that are required to monitor groundwater near their coal ash dumps show unsafe levels of toxic metals, according to a study released on Monday by environmental groups, which cited the potential harm to drinking water. Data made public by power companies showed that of the 265 plants subject to the monitoring requirement, 241 - or 91 percent - showed unsafe levels of one or more coal ash components in nearby groundwater compared to EPA standards, according to the analysis by the groups.The report also found that 52 percent of those plants had unsafe levels of cancer-causing arsenic in nearby groundwater while 60 percent showed unsafe levels of lithium, which can cause neurological damage.