Price George's County police have uncovered what they say is a massive system of illegal payments by garbage collection firms to employes at the county-run dump near Upper Marlboro.

The police investigation let to a grand jury probe of the allegations and resulted yesterday in the firing of two county workers who were said to be involved in the kickback scheme.

County Executive Lawrence Hogan estimated that the illegal payments may have cost the county at least $100,000 a year in dumping fees. Hogan said he plans to revoke the dumping licenses of 12 garbage collection companies using the 300-acre Brown Station Road landfill and to cancel county contracts with two of those firms.

Hogan and the county police would not disclose the companies that allegedly were involved in the illegal payments. The two county employes fired for their alleged roles in the scheme were Samual R. Sbona, 32, of Durwood, and Henry E. Hankins, 44, of Mount Rainier. Both men were operators at the landfill's weigh station.

Garbage collection companies are charged for use of the dump by the weight of the garbage they deposit there. Every time a truck passes over the scales, the weight -- at $7 a ton for normal loads and $14 a ton for tires -- is added by computer to the firm's account with the county.

Police believe that some county employes allowed garbage trucks from some firms to use the dump without being weighed and thus were not recorded on the firm's account with the county.

These firms, according to police investigators, would then give the dump employes 40 percent of what they should have paid to the county, and pocket the rest.

Police also believe that the empty weight of garbage trucks used by these firms -- which is used to calculate the amount of garbage brought to the dump by each truck -- was illegally inflated by employes. As a result, when the trucks were weighed in with a full load of garbage, the firm would not pay as much as it should have.

Officials from the county public works department were weighing all trucks that came to the dump yesterday to see if their empty weight had been inflated.

Hankins could not be reached for comment yesterday. Sbona said, "I just can't believe the whole thing happened. I'm just waiting to get it straightened out." He would not discuss any of the allegations.

Each day, 350 to 400 trucks including private pickups, use the dump, according to county officials. The dump brings in about $2 million a year to the county.

The investigation of the weigh station operation began last May when an informant, who was not a county employe but works for one of the firms using the dump, told Hogan of the alleged kickbacks.

County police began undercover surveillance from a bar near the dump and posed as truck drivers. Their surveillance and a check of the Public Works Department billing accounts indicated that not all the trucks crossing the scales at the weigh station had been recorded into the accounts by the computer, which has a manual cutoff switch at the station.

Police and county officials said they have no idea how long the alleged kickback scheme has been going on.