The health threat from a 20-year-old underground gas tank spill that spread from Prince George’s County into Northeast Washington is slowly dissipating, but the risk to Riggs Park residents of cancer from other contaminants is surprisingly high, a new study finds.

Solvents, including cancer-causing perchloroethylenes, or PCEs, most often associated with dry cleaners, were found at concentrations well into the dangerous range, according to the 2008 study performed by S.S. Papadopulos & Associates Inc.