Truck fleets and repair shops that try to project a clean image on the road may be sitting on top of a dirty, costly and dangerous problem where they live.

The problem is steam cleaning and its effect on groundwater pollution.A presentation on the topic at the recent meeting of The Maintenance Council of the American Trucking Associations in Orlando, Fla., came as a shock to many fleet supervisors who have been cleaning their trucks in the same spots for years.

According to Ron Fink, president of RGF Environmental Systems in Fort Lauderdale, the trouble is that the greasy runoff from steam cleaning seeps into the ground and in time reaches the groundwater with serious consequences for both the environment and the company's liability.

Mr. Fink cited a recent instance in which a garage in Miami was fined $37 million for groundwater pollution blamed on cleaning operations. For thousands of truck fleets and waste haulers, daily steam or high pressure hot water cleaning has become routine.

Over time, constant cleaning in one area can produce a build-up in the ground of not only petroleum residues but toxic heavy metals such as mercury, lead and beryllium, Mr. Fink said.

In as little as four years of such operations, seepage can reach a depth of 12 feet where it then contaminates groundwater. In less than 10 years, the discharge descends to 20 feet, the level of the potable water supply, according to Mr. Fink.

Mr. Fink's company makes a self-contained system that recycles steam cleaning discharge to avoid continuing contamination, so it has good reason to publicize such figures.

But an official at the Office of Groundwater Protection of the Environmental Protection Agency in Washington confirmed the seriousness of the problem.

The agency is working on a program to make states aware of the many thousands of smaller sources of groundwater pollution like truck garages and service stations, which use steam cleaners.

It is a pretty serious problem, said an EPA official. People who conduct these kinds of operations haven't been real fastidious about tracking their waste stream.

What we're dealing with here is large numbers of very small contributors, the official said.

While there are no estimates of how many steam cleaners are in operation across the country, Kevin Collette, vice president and marketing director of RGF, estimates that there are between 10,000 and 15,000 steam cleaners in just his immediate area in Florida.

The RGF officials say that recycling is the only practical and acceptable way of dealing with the steam cleaning problem. EPA officials agree that the alternatives all involve some degree of risk.

Disposal into a central municipal sewer system is at least likely to subject the discharge to primary and secondary treatment, the EPA official said, although this is not true in all cases. Disposal into storm drains may often result in direct discharge into rivers and streams.

And while recycling may solve the problem of continuing discharge into the groundwater, it does nothing to treat the contamination built up from years of previous steam cleaning.

Mr. Collette said he feels that environmental authorities would look favorably on any operation that shows it has made an effort to deal with its problem by installing a recycling system.

But the EPA official indicated that any forgiveness would likely depend on the type and seriousness of previous pollution.

They'll certainly look kindly on him from that day forward, the official said.

When asked how an agency would deal with an operator who had recently installed a recycling system to prevent further polluting discharge, the official responded, They'd kind of give him a pat on the back and a slap on the wrist at the same time.