from aaca.org's FAQ on coal ash:
Is coal ash hazardous?
Coal, like soil, rocks and other natural materials found in the earth’s crust contain trace amounts of heavy metal elements. The burning of coal results in some of these elements being oxidized in the coal ash that is produced. Typically, these heavy metals include arsenic, boron, cadmium, chromium, copper, lead, selenium and zinc.
Beginning in 1988 and continuing through 2000, the U.S. Environmental Protection Agency (EPA) studied extensively the risk that coal ash presents to the environment. First, in a report to Congress dated February 1, 1988 the EPA concluded that the ash resulting from the combustion of fossils fuels was not hazardous and did not need to be regulated as a hazardous waste under the Resource Conservation and Recovery Act (RCRA). In a Report to Congress dated March 1999, the EPA again confirmed that coal ash did not require regulation as a hazardous waste and encouraged the beneficial use of coal combustion byproducts. On May 22, 2000 , the Federal Register published the EPA’s final determination “ Notice of Regulatory Determination on Wastes from the Combustion of Fossil Fuels.” This Federal Register is available on ACAA‘s website under “What are CCPs?”
As stated earlier, coal ash contains varying concentrations of these heavy metals. Despite the large volumes of ash produced, the total quantity of heavy metals is relatively small, and an even smaller amount of these elements can be released to the environment. Annually, utilities report the total quantity of metals and other materials released in to the environment through Toxic Release Inventory (TRI) reporting requirements. For example, the largest quantities of heavy metals are in the form of manganese, barium and vanadium. Much smaller amounts of copper, lead and other metals are reported under TRI. The impact of these heavy metals is slight. Studies conducted by the University of North Dakota indicate that for most heavy metals, even if released directly into groundwater, the concentrations are so low that they would not adversely affect drinking water quality. A U.S. Geological Survey (USGS) fact sheet states that a “Standardized test of the leach ability of toxic trace elements such as arsenic, selenium, lead and mercury from fly ash shows that the amounts dissolved are sufficiently low to justify regulatory classification of fly ash as non-hazardous solid waste.” Other regulations and standards regarding the use and disposition of coal ash are in place and vary by application from state to state.
However, it is important to note that despite these relatively low concentrations, if improperly managed, any waste can have a negative impact on the environment. The United States and Canadian utility industry have implemented many methods of control and monitoring to ensure that when coal ash is disposed, there will be no adverse affect to human health or to the environment. Environmental stewardship is an important part of the utility industry.