CREWS mounted an air pump on a barge in Shasta Lake over the weekend to begin vaporization of a half-mile-long plume of deadly herbicide dumped into the reservoir from a derailed train.The plan to aerate the plume of pesticide was hatched by Southern Pacific railroad officials and agreed to by representatives of the 35 agencies involved in the cleanup as the most safe and efficient method to rid California's largest reservoir of the contamination, said Lenny Clavecilla, a spokesman for the state Fish and Game Department. Crews hope that blowing compressed air into the herbicide massed near the reservoir's northern end will speed its dispersal and dilution in the 555 billion gallons of water stored in the reservoir. "The plan is to aerate it to the surface and that process and agitation will contribute to the chemical breakdown," Mr. Clavecilla said. To prevent poison vapors from escaping into the atmosphere, hoses will spray a "dome of water" over the site to collect fumes and return them to the lake, where they will be further diluted by the water, Clavecilla said. The aeration operation, believed to be the largest such undertaking, was to begin with tests late Saturday afternoon. If all goes well, the operation will continue around the clock. It was estimated that it would take about two days to dissipate the spill. Between 11,000 and 12,000 gallons of metham sodium, a soil fumigant widely used in California, spilled from a Southern Pacific tank car that derailed last week in a narrow canyon of the river in the far northern part of California. The chemical killed more than 100,000 fish as it crept along a 50-mile stretch of the Sacramento River channel between the derailment site and Shasta Dam. It also appeared to have killed plants close to the bank before sinking to a depth of about 20 feet below the surface of the reservoir behind the dam. Fish and Game officials, seeking to reduce further wildlife mortality, were placing uncontaminated dead fish near roosting area of hawks, eagles, and vultures to lure them away from the tainted food. The contaminated fish would be removed from the area and disposed of when the area is deemed safe. Water engineers reported Saturday that the concentration of metham sodium had thinned from about 26 parts per million Friday to about 1/10 of one part per million Saturday. Officials called the level near that acceptable for long-term drinking water. About 19,500 gallons of the chemical was being transported in the tanker car at the time of the accident. A Southern Pacific spokesman said about 7,000 gallons of the herbicide was pumped out of the tanker after the accident before it could enter the river. The San Francisco Chronicle reported in its Saturday edition that the derailment might have been caused by uneven loading of the cars. Quoting a source close to the investigation, the Chronicle said a string of six gondola cars heavily loaded with scrap metal at the rear of the train may have placed excessive drag on the curve where the derailment occurred.