ABSTRACT
On July 14, 1991, an accidental spill of the pesticide metam sodium (‘‘metam”) occu1Ted by derailment of a tank car of the Southern Pacific Railway at the Cantara Loop near Dunsmuir, CA. The tank car rupture resulted in the discharge of up to 27,000 kg of the pesticide into the Upper Sacramento River. The contaminant was 72subsequently transported from the spill site downstream to Shasta Lake, a distance of approximately 77.5 km. During transit, metam decomposed by hydrolysis and photolysis into methyl isothiocyanate (MITC) that was diminished in aqueous con centration by volatilization to the atmosphere along the river.
The spill event was simulated mathematically using the two-dimensional hydrodynamic model RMA-2, modified to represent the run-riffle-pool character of the steep gradient stream in the mountainous terrain, coupled with the com panion finite element water quality model RMA-4Q, modified to represent the unique physical and chemical processes governing the fate and transport of metam and MITC.
The models were calibrated against field observations of the passage of MITC at downstream locations during several days following the accident. Sensitivity testing of alternative scena1ios considering various combinations of hydrolysis, photolysis, and volatilization rates revealed the relative importance of these processes in determining the fates of metam and MITC in the stream and in the overlying atmosphere.
