A 10-inch sewer line in Fairfax County that ruptured nine days ago was still broken yesterday, forcing diversion of about 1.5 million gallons a day of untreated sewage into the Potomac River.

Richard Gozikowski, director of waste water treatment for the county, said an unusually high-water table, sand and water like "quicksand" and cold weather have delayed repairs on the pipe which runs between Belle Haven pumping station and the Westgate sewage treatment plant south of Alexandria.

The county has had to hire District Associates Inc., a local construction firm, to make repairs on the pipe which lies about 14 feet underground, Gozikowski said. He said repairs may be completed today.

Until Wednesday, repair crews had been working around the clock. "I'll hate to see the bill on this one," Gozikowski said. He said fixing the Belle Haven line will be the most expensive sewer repair for the county since June 1972 when Hurricane Agnes knocked out a major trunk line at the lower Potomac sewage treatment plant.

Workmen at the broken line said yesterday repairs have been difficult because "boiling muck" - waster and sand - around the pipe does not give it enough support, causing it to sag and break. They have had to replace about 42 feet of pipe.