The Interior Department yesterday proposed to resume its ability to lease federal coal in the West and drastically lowered its forecasts of the potential environmental impact because of the reduced demand for coal.

The department's Bureau of Land Management said in a draft environmental impact statement that coal-related employment in the West, under its "medium" demand forecast, would be only 51,330 in 1990, an 82 percent reduction from the medium forecast it made in 1979.

Other environmental forecasts were similarly reduced. Dust and soot thrown into the air annually would be down 54 percent to 333,200 tons, total acreage disturbed would fall 65 percent, annual water use for mining would drop 90 percent to 77,500 acre-feet (about 25 billion gallons), and mining accidents would number 3,318 instead of 4,284.

These forecasts were based on the assumption that 1990 production from western sources would total 302 million tons -- a reduction of 55 percent from the department's 1979 forecast -- and 1.08 billion tons in the nation, a reduction of 30 percent.

The forecasts also assumed that the department would in fact decide to resume leasing after the final environmental impact statement expected this summer.

Leasing was temporarily suspended by Congress in 1983, and not resumed by then-Secretary William P. Clark when the July 1, 1984, deadline passed. Clark later decided it could not resume until a new environmental statement was prepared.

Congress imposed the suspension following criticism that the government failed to get fair market value for leases it sold in the Powder River Valley of Wyoming and Montana in 1982.