One of the last obstacles to production of ethyl alcohol at a Northern Virginia distillery for use in the manufacture of gasohol evaporated yesterday.

Fairfax Supervisor Martha Pennino, who has led the fight against the A. Smith Bowman Distillery's plan on environmental grounds, said yesterday she was dropping her opposition because a study found no evidence that the production would add significantly to pollution.

Residents of a Reston subdivision that adjoins the distillery had claimed its emissions contributed to the neighborhood's problem with black, sooty mold. Residents said the fungus, which grows on their roofs, trees and fences, is a nuisance and a possible health hazard.

However, a county health department study found no evidence to blame the distillery for the mold, which the department said may have been caused by other environmental factors. It also found no evidence that the mold is harmful to humans.

"We've concluded that the new use of the plant will not increase the evaporation of ethanol into the atmosphere significantly, and therefore will not add significantly to the growth of fungus even if the distillery is contributing to the problem," said Fairfax County planning director Theodore J. Wessel.

With Pennino's opposition removed, Wessel said, it is likely that Bowman's request for special permission to produce ethanol will be approved by the Fairfax Planning Commission and Board of Supervisors within the next week.

According to Bowman treasurer Tim Brown, approval of the request will allow the distillery to begin production in March of 600,000 to 1 million gallons of ethanol yearly at a price of approximately $1.70 a gallon.

The ethanol would then be sold to gasoline distributors who would mix it in a 1-10 ratio with gasoline to make gasohol. It is expected that the gasohol will be sold to Northern Virginia drivers at a cost comparable to unleaded gasoline.

Gasohol is being pushed by some energy conservationists as a means of stretching the gasoline supply. If available in large quantities, the grain-based ethanol could increase gasoline supplies by 10 percent, its supporters say.

The county health department plans to continue studying the Reston mold problem for at least a year, Pennino said.