Pesticides have percolated through the ground and into well water in at least 34 states - significantly more contamination than was previously reported by the federal government.About 90 percent of rural Americans get their drinking water from the ground. An estimated 13 million wells serve 117 million people nationwide.

The Environmental Protection Agency gathered test data from 45,000 wells and found that 5,500 of them were tainted with harmful levels of at least one pesticide.

The EPA had that information last February, but it issued a written report saying only 19 pesticides had been found in wells in 24 states. However, EPA officials verbally acknowledged that between 50 and 60 pesticides were found in 30 states.

Environmental groups demanded to see the data earlier this summer. The Washington headquarters of Public Interest Research Group (PIRG) assembled the data in a report that is scheduled to be released today. PIRG came up with 73 different pesticides found in wells in 34 states.

An EPA spokesman told us the agency report in February used lower figures because it counted only the cases that had been "confirmed" by the federal government. "We are not trying to hide that issue at all," the spokesman said.

The EPA has promised to release its own updated numbers in October based on continued work to double-check the information. A groundwater contamination survey based on the EPA's own test data will not be completed until 1990.

While the EPA's written data may be conservative, the report from PIRG may lean a little too far in the other direction.

Some of the test information is not holding up to scientific scrutiny by the EPA, according to agency scientist, Patrick Holden. Even taking that into consideration, Holden said the final figures, to be released Oct. 1, will exceed the 19 pesticides in 24 states cited in the EPA's February report.

Sen. Dave Durenberger, R-Minn., and Rep. Jim Oberstar, D-Minn., introduced tough proposals last spring to control groundwater pollution. But the chances are slim that the bills will pass before Congress recesses in October.

The data from PIRG does not mean that one out of every four wells in the country is contaminated with a pesticide.

The data comes from 101 studies by state officials, chemical companies and research institutions, and they were focusing on problem areas. Nevertheless, the data indicates groundwater contamination is more widespread than previously thought.

Among other things PIRG reported:

- Of the 73 pesticides, at least 25 can cause cancer, 18 can cause birth defects and 14 can cause genetic damage.

- California, with 31 different pesticides in tested wells, led other states.

- Aldicarb, a chemical that can cause vomiting, stomach pain and blurred vision, was found in concentrations as high as 200 parts per billion in Wisconsin and 575 parts per billion on Long Island, N.Y.