Montgomery County Executive Charles W. Gilchrist has ordered that the proposed Rock Run sewage treatment plant discharge its effluent below all the intake facilities used daily to draw water from the Potomac River for treatment as drinking water.

The discharge pointed selected by Gilchrist is in Little Falls Branch about 200 feet above its confluence with the Potomac and just inside the county's boundary with the District.

While the discharge point is 2,000 feet above a recently completed emergency water intake, located just inside the District and designed for use during periods of drought when the river's flow is low, Gilchrist said there would be no danger of the effluent mixing in the water that would be pumped by the intake.

On the infrequent occasions when the emergency intake is used, Rock Run effluent would be shunted to the Blue Plains regional sewage treatment plant in the city instead of being discharged in Little Falls Branch.

Some environmentalists believe that the effluent, even if it is very clean, would impose a health hazard if mixed with water that was tapped for drinking.

Gilchrist's decision -- the least controversial of a number of options -- endorses the recommendations of a citizens' advisory committee and a technical task force which have been considering alternatives for the past nine months.

Two alternatives involved discharge points closer to the plant site in Potomac.Either would have been cheaper than extending a pipeline to the Little Falls Branch point but they are above water intakes used by the Washington Suburban Sanitary Commission and the District. Officials of the city, Maryland and the federal Environmental Protection Agency (EPA) had warned they would oppose selection of either site.

The planned predecessor to Rock Run, the giant regional plant that would have been built at Dickerson, was doomed by the EPA in part because its discharge point would have been above water intakes.

Gilchrist could have chosen a discharge point below even the emergency water intake, which is located just west of Chain Bridge. It would have been just over the county line, inside the District. But Gilchrist said, "The added expense and difficulty involved were not worth the gain in environmental considerations"

Rock Run, which would cost $70 million to $120 million to build, has been planned by Nmontgomery as its answer to sewage treatment needs beginning about mid-decade. The plant would be located on WSSC-purchased land at the old Avenel Farm in Potomac.

If the planning process continues without major delays, the plant could be ready in 1985. Gilchrist's decision this week, because it selected one of the least controversial of the discharge points, is likely to get approval from Maryland's new Environmental Health Administration, which must give the crucial discharge permit.

Convinced that the EPA would not help fund the project, Montgomery County and the WSSC decided to build the project with local sewer revenues. The cost would be borne in part by the $1,560 fee for new connections and an 11 percent increase in the yearly charges paid by WSSC sewer customers in Montgomery and Prince George's counties.

In his letter to the WSSC announcing his selection, Gilchrist also urged the agency to give special consideration to using alternatives to chlorine as a disinfectant for Rock Run's effluent. Chlorine has been suspected as being possibly toxic to some species of fish, particularly shad.

"There's no extensive data that clearly shows that chlorine is a direct cause (of fish kills)," he said, "but there is enough evidence to concern me. This is something we haven't faced up to."

Two alternatives, used widely in Europe, involve use of ozone or ultraviolet rays as a substitute for chlorine in disinfecting effluent.

Gilchrist, in his message to the WSSC, also said he favors using the same level of sewage treatment at Rock Run. This is scheduled to be implemented at Blue Plains when that facility goes to advanced processes over the next year. This level of treatment was also favored by the citizens'advisory panel, but one member of the 11-person committee endorsed more stringent, and more expensive, treatment.

Gilchrist said a higher level of treatment would hold no environmental advantages as long as Blue Plains, using a lower level, discharged 15 times as much effuent as the proposed Rock Run plant.