This report documents the connection between coal ash and hexavalent chromium. It reviews the sources, toxicity, and known coal ash dump sites where chromium has been found in groundwater. The report identifies studies of numerous power plants where testing of coal ash leachate found extremely high levels of hexavalent chromium. The report also identifies 28 coal ash disposal sites in 17 states where groundwater was documented to exceed existing federal or state standards for chromium and to exceed by many orders of magnitude the proposed California drinking water goal for hexavalent chromium. These contaminated coal ash dump sites are likely the tip of the iceberg. The threat of drinking water contamination by hexavalent chromium is present in hundreds of communities near unlined coal ash disposal sites across the United States. While the EPA doesn’t need another reason to define coal ash as a hazardous waste when disposed, it certainly has one now.