Underground water supplies, the source of drinking water for half the American population, face a growing problem of chemical contamination that has spread to every state and is expected to grow worse, according to a study by the Office of Technology Assessment released yesterday.

The study said that although only a small percentage of ground water supplies are impaired, the threat is enormous.

Meanwhile, the Environmental Protection Agency said in a draft report that its system for monitoring ground-water contamination from hazardous waste dumps is not working very well.

The draft EPA report, requested by the Office of Management and Budget, said the agency "has only a rough idea of the number of facilities subject to monitoring requirements" under the 1976 Resources Conservation and Recovery Act.

"EPA's policy of turning the RCRA program over to the states as soon as possible has resulted in authorizing many states that were ill-prepared to implement the program," the draft report said.

It also said that several states have a "very lax" enforcement policy on preventing seepage of hazardous wastes into ground water and that the EPA, "for several administrative reasons, has declined to enforce or pursue cases involving the major performance standards."

The 1976 law outlines how waste sites are to be operated but leaves most enforcement to he states.