Atmospheric modeling of the July 1991 metam sodium spill into California`s Upper Sacramento River

The California Office of Emergency Services asked the Department of Energy`s Atmosphere Release Advisory Capability (ARAC) at the Lawrence Livermore National Laboratory to determine the maximum credible air concentrations from a spill of metam sodium into California`s Upper Sacramento River. About 19,000 gallons of metam sodium herbicide were spilled into the river approximately 3 miles north of Dunsmuir, California, due to a tank-car derailment on the night of July 14, 1991. The herbicide moved in the river toward the northernmost finger of California`s largest reservoir, Lake Shasta, 45 miles to the south. As it flowed down the deep canyon, the water-soluble metam sodium decomposed into hydrogen sulfide and methylamine gases. Residents along the river were advised to evacuate the area, and a 50-mile stretch of Interstate 5 was temporarily closed. Response officials were also concerned that sunlight would readily evaporate the enlarged slick once it arrived into the still water of Lake Shasta on July 16. On July 15, ARAC used its three-dimensional emergency response modeling system to determine the highest instantaneous and 8-hour average air concentrations of toxic gas by- products over upper Lake Shasta. A quick response was possible using on-line topographic and geographic data bases in combinationmore » with forecasted southwestern surface winds. The worst-case calculation showed that the gases would be well below any health hazard.« less