Cleaning up an oil spill is simplified by two facts of nature: oil is lighter than water, and oil and water do not mix.

Unfortunately, some other facts of nature are complicating cleanup near Pittsburgh, where a storage tank at Ashland Oil Co. collapsed Saturday and dumped 1 million gallons of diesel fuel, which is similar to fuel oil, into the Monongahela River.

Oil spills on a river will flow with the water, even racing slightly ahead of it as the oil spreads over the water.

"The biggest factor in containing a river spill is time. You want to get your booms and skimmers in front of the oil before its spreads too far," said Donald Aulenbach, an environmental engineer and oil-spill specialist at Rensselaer Polytechnic Institute in Troy, N.Y.

"The longer you let that oil flow, the more it's going to emulsify {break up and disperse in the water} and get away from you."

Preliminary indications are that at least six hours elapsed between the collapse of the tank and placement of the first boom several miles downstream.

About 13 hours elapsed before a second boom, 20 miles downstream in Pittsburgh, was in place to catch oil that had escaped the first boom.

Possible delay in placing the booms was only one of several questions raised by the accident. Among the areas investigators are expected to probe as the cleanup effort continues are:Was the 4 million-gallon tank, which was 40 years old and recently moved from another site, properly inspected for damage in moving and reassembly? Ashland officials said today at a Pittsburgh news conference that it was erected without a county permit and normal testing procedures, but they said they were uncertain whether that was a factor in the accident.

Was the earthen dike surrounding the tank, which is 10 percent larger than the tank and intended to contain spills, adequately constructed? The tank ruptured so suddenly while being filled for the first time that the force of gushing oil was enough to slosh over the dike and wash away part of it. The dike held about 2.5 million gallons of oil, but an additional 1 million gallons escaped into the river. Were booms and trained crews available as quickly as they should have been to apply established spill-containment procedures before too much oil flowed too far?

Although questions remain about the incident, oil spills happen often enough that their behavior is fairly well understood.

Although oil and water do not mix as alcohol and water do, the churning action of rivers can emulsify oil, breaking it into tiny droplets that disperse in the water. Oil spills are easily emulsified by choppy water, by going over a waterfall or by being pumped through locks or dams, all of which are present on the Monongahela.

Depending on the water's turbulence, emulsified oil can ride several inches or several feet below the surface. Although the larger droplets tend to rise, continued turbulence can keep them dispersed deep in the water. Smaller droplets may stick to particles of sediment or other matter in the river and eventually sink because of the particles' weight.

The phenomenon is similar to shaking up an oil-and-vinegar salad dressing. Turbulence causes the oil to emulsify in the vinegar. If particles of herbs and other ingredients are present, the emulsion lasts longer.

Cold weather is on the side of the cleanup effort. The colder the diesel fuel, the thicker it gets and, therefore, the more resistant to breaking up.

Oil-spill experts say the most effective way to control a spill is to place booms -- long floating barriers that reach about a foot above the water and two feet below the surface -- to trap the floating oil. Booms, often made of rubber or similar materials, are stretched across a river, usually in the form of a giant "U."

Skimmers, which work like vacuum cleaners, then pump up the oil and transfer it to a barge or some other storage facility.

Some booms are continuous lengths of barrier, but others are made of segments 20 to 30 feet long and leakage at the joints is inevitable. Often a second or third boom is placed farther downstream to catch oil that escapes the first.

At least five booms have been stretched over the Monongahela at various points along a 50-mile stretch downstream of the collapsed tank.

"In practice, you can't usually get all the oil out," Aulenbach said. "Eventually, however, it gets diluted the farther it flows. Granted, dilution is not the solution to pollution, but it does occur and that helps you."

Environmental scientists also note that oil is biodegradable. Although oil spills can be a threat to some kinds of wildlife, and high concentrations can be toxic to many microorganisms, once the oil is dispersed sufficiently, certain kinds of bacteria feed on it, gradually converting it to harmless substances.