On December 22, a billion gallons of poisonous sludge – largely coal ash, a byproduct of coal burning – broke through an earthen dike at the Kingston Fossil Plant in Tennessee. This industrial accident destroyed area homes, killed wildlife, and brought to the forefront long-running health concerns over heavy metals in the ash.

Wildlife may be threatened for years to come by trace amounts of arsenic, cadmium, mercury, thallium, and other toxins in the coal ash. Eventually, any toxic effects in animals could work their way up the food chain to humans, officials say. The Tennessee Department of Environment and Conservation has issued an advisory against eating striped bass caught in rivers around the spill zone as well as a precautionary advisory for catfish and sauger for children, pregnant women, and nursing mothers.

Conservationists are particularly concerned over the fate of freshwater mussels, which live on river bottoms, where sediment and pollution accumulate. Losing mussels could result in greater pollution levels in area rivers, because a single mussel can filter several gallons of water a day, according to the U.S. Fish and Wildlife Service. Mussels are also a major food source for ducks, birds, and fish, which could suffer if the mussels are tainted by the ash spill.

Massive amounts of ash are sold for use in concrete, mulch, construction fill, and other purposes. Although the U.S. government considers the ash a health and environmental risk, it remains unregulated. The U.S. Environmental Protection Agency (EPA) has long held that coal ash poses no substantial risks to the environment. Currently, the EPA strongly supports the substance’s use in commercial products such as paints, kitchen countertops, concrete, and agricultural products.

A 1998 study by the Electric Power Research Institute, a nonprofit association allied with the power industry, found health risks from coal ash are minimal and pointed out that heavy metals make up a small proportion of coal ash and that these same substances occur naturally in rocks and sand. However, other experts cite evidence that the toxins in coal ash – also called coal combustion residues (CCR) – build up in bodies over time, sometimes with lethal effects. Specifically, coal-ash spills have caused behavioral and physical problems in many vertebrates and invertebrates as well as local extinction of some fish species.

A group of six Tennessee environmental groups recently sent a letter to President Obama requesting that he move to declare coal ash a hazardous waste and for greater regulation. In addition, the Southern Alliance for Clean Energy has filed a lawsuit seeking to force the Tennessee Valley Authority (TVA) – the federally owned utility that produced and stored the ash – to restore ecological health to the spill zone.