Constellation Energy and the owner of a sand and gravel mine in Gambrills have known since 1999 that fly ash dumped at the site could threaten nearby groundwater with cancer-causing chemicals, according to documents obtained by the Maryland Gazette.

Yet, the two companies did not tell the public or try to take steps to mitigate the risk until 2003, public state records and reports show.

And the Maryland Department of the Environment has been aware of the problem since the companies first identified it, but has continuously allowed fly ash to be dumped at the mine and approved plans to deposit the substance at more sites in...