* 519 barges waiting on Friday, down from more than 1,000
* River open to one-way, slow-speed commercial traffic
* No estimate on how soon river would fully reopen
By Karl Plume
Feb 1 (Reuters) - Mississippi River barge traffic remained congested on Friday near Vicksburg, Mississippi, where a barge accident and oil spill shuttered the major shipping artery on Sunday, but the backlog of boats was slowly declining.
The U.S. Coast Guard reopened the river on Wednesday to one-way traffic through a 16-mile safety zone from river mile marker 425 to 441, where the backlog of barges had ballooned to more than 1,000.
The backup thinned to 519 barges by midday Friday, including 11 northbound tows pushing a total of 138 barges and 26 southbound tows with 381 barges, said Coast Guard spokesman Jonathan Lally.
Ordered to remain at a slow speed and spaced 1-1/2 to 2 miles apart, 87 tow boats hauling 1,339 barges have transited the safety zone thus far, he said.
Cleanup crews have deployed 5,300 feet of boom to contain the spill and recovered more than 8,000 gallons of oil-water mixture since the barge collided with a railroad bridge and ruptured one of its 80,000-gallon tanks.
Crews have lightered 252,000 gallons of crude oil from the damaged barge, which had been carrying more than 668,000 gallons. Once the cargo is lightered, the barge will be removed to a maritime facility in Vicksburg.
The Coast Guard would not estimate how soon the river would fully reopen to shipping traffic.
"The response crews are working as quickly and as safely as they can. There are crews that are going to conduct shoreline cleanup as well as the skimming operations that they've done to ensure that the impact on the environment is minimal," Lally said.
Shippers rely on the Mississippi River and other inland waterways to haul billions of dollars worth of grain, coal, fertilizer and other commodities every year. Some 55 to 65 percent of all U.S. grain and soybean exports are transported down the Mississippi River to the Gulf of Mexico.
The shipping disruption did not seriously impact grain exports at the Gulf, although prices for soybean barges south of the closure briefly jumped early in the week before retreating.
The oil spill was the latest in a recent string of logistical headaches for shippers.
Low water along a busy shipping corridor between St. Louis and Cairo, Illinois, has threatened to disrupt traffic on the drought-drained river since December. Dredging operations and other work aimed at keeping barges flowing have also snarled traffic at times.
The river system's busiest lock was closed for a day last week after a barge collided with a lock gate. (Reporting by Karl Plume in Chicago; Editing by Bob Burgdorfer)