Congressional investigators, proposing a major policy change, are urging the Environmental Protection Agency to consider even low levels of pesticides in groundwater a potential health risk.

The new strategy is needed because "the knowledge of the possible toxic effects of pesticides is not complete, and new information could raise serious concern in the future," said a General Accounting Office report released Wednesday.It also said that once established in groundwater, pesticides could be extremely costly or even impossible to remove.

In another major proposal, the GAO said evidence of groundwater contamination should play a major role in triggering priority EPA reviews of specific pesticides.

Current EPA criteria for beginning the special reviews do not specifically address groundwater contamination, according to the report, presented to the House Energy and Commerce investigations subcommittee.

The differences between the GAO proposals and EPA policy are crucial to the approximately 100 million Americans who use groundwater for drinking purposes. Forty percent of Americans overall and 90 percent in rural areas drink groundwater from wells or their community water systems.

Victor J. Kimm, a top EPA pesticides official, confirmed in an interview Tuesday that the agency does not consider low levels of pesticides in groundwater to be a significant health risk - although the contamination must be carefully monitored.

He said the EPA is ready to act quickly if contamination rises to unsafe levels.