Within the next two weeks, the Fairfax County Water Authority will complete the purchase of a 270-foot deep rock quarry beside its Occoquan Reservior, providing the county with a place to dump sediment from its water-purification plants and a billion-gallon reservior that can be tapped during droughts.

The liquid wastes from the water-treatment plants -- primarily silt or mud -- will be pumped into the quarry, where it will settle to the bottom, leaving clear water, according to Water Authority spokesman James Warfield.

At present, 5,500 tons of sediment a year are discharged into the Occoquan River, just below the Occoquan Reservoir dam and just above the Prince William County town of Occoquan. The muddy discharge has annoyed Occoquan residents since the reservior was built in 1967. It also has violated federal laws against dumping into rivers since 1972, when those laws went into effect.

The cost of a 44-acre quarry is $300,000, and design and construction of pipes from the agency's two water-treatment plants is expected to cost about $2.2 million. The quarry purchase, planned for about eight years, is not expected to increase water rates. The authority announced three months ago that rates will remain the same for 1980 as in the past three years.

The huge quarry will solve sediment disposal problems for the next 125 years, Warfield said, and will provide a reserve water supply for the 600,000 Northern Virginians who get their water from the Water Authority. The slurry of water and silt to be pumped into the quarry will contain alum and lime, used to treat drinking water, but once they settle, the quarry water actually will be purer than the Occoquan Reservior water, Warfield said.

The quarry can hold approximately one billion gallons of water, about a two-week supply. The Occoquan Reservior holds almost 10 million gallons, and if plans to raise the dam height two feet are approved, it will have a capacity of 11 million gallons.