Despite the Environmental Protection Agency's public statements that proposed budget cuts will not harm its programs, the agency had a different story in a report it sent to the Office of Management and Budget last September.

In the report, released at a congressional hearing yesterday, EPA said that to reduce its fiscal 1983 operating budget by 29 percent below the fiscal 1981 level, it would have to cut several pollution assessment and control programs. This contrasts sharply with EPA's public reassurances that budget reductions would be achieved through increased efficiency, delegation of more authority to states and the completion of several projects.

The report was released at a Senate appropriations subcommittee hearing yesterday by Sen. Patrick J. Leahy (D-Vt.), who accused EPA of being "less than candid" about the impact of its proposed budget cuts. A study by Leahy's staff concluded that "the large reductions in the FY 83 budget represent real reductions in environmental protection . . . . "

EPA Administrator Anne M. Gorsuch conceded that there will be actual program cutbacks, but said "fiscal responsibility and environmental objectives are not mutually exclusive." She added that the Reagan administration "has a very strong commitment" to protecting the environment.

Deputy Administrator John W. Hernandez explained that EPA's intention was to "shave those things that don't produce good and quick results. That's efficiency."

The hearing focused on EPA's fiscal 1983 budget request to OMB last fall, which Leahy obtained through a source. His staff compared the document to EPA's budget report to Congress and found 70 instances in which EPA told Congress it could cut its budget because of increased efficiency, or the transfer or completion of programs, while at the same time telling OMB that budget cuts would result in a "reduction in effort" or total elimination of the programs.

Among the discrepanicies highlighted were:

* A proposal to reduce by 39 percent research and development on air pollution problems related to oxidants, most notably ozone, nitrogen oxide and carbon monoxide. EPA asked OMB for $12.3 million for fiscal 1983, compared with $18.47 million for this year. The request actually sent to Congress was pared even more--to $11.45 million.

In the congressional justification, EPA said it can achieve these cuts by "more efficiently" scheduling the work, while EPA told OMB the cut "represents a reduction in effort" in all of the program elements except one.

* A proposal to reduce from $9.27 million to $7.9 million the money for research on hazardous air pollutants. EPA told Congress that it could cut the funding level because one of the programs would be transferred, and, because of regulatory reform efforts, there would be less of a need to develop models and produce data on the way air pollutants are transported.

The OMB submission did list the transfer, but not the remaining reasons. Rather, it said the budget could be cut because of plans to eliminate research on whether volatile organic compounds affect reproduction, birth defects and the neurological and pulmonary system, and to eliminate various risk assessments.

* A request for $11.04 million for fiscal 1983, a 42 percent cut, for water quality research. EPA told Congress the decrease "reflects the completion of major efforts" in developing standards for major pollutants and "reduced investments in long-term projects that have no immediate impact on current program priorities." But EPA told OMB the budget cuts will be accomplished by slowing down or eliminating several programs to monitor toxic substances and assess health effects.