FATE AND TRANSPORT OF METAM SPILL IN SACRAMENTO RIVER

A mass balance model was developed and applied to the Sacramento River in northern California during the July 1991 Sacramento River metam-so dium spill. The transport and reactions of metam-sodium, a soil fumiga nt, and the volatile and toxic methyl isothiocyanate (MITC) were simul ated during the two-and-a-half days of movement alone a 68-km stretch of river. Results from modeling were compared with field data for MITC , which is the only product measured downriver after the spill. Agreem ent between the simulated and measured values of MITC concentrations w ere found at Doney Creek (65.9 km downstream). Results illuminated the complexities and unique characteristics associated with the multiple kinetic processes of the chemical plume in the river. In particular, t he photolysis of metam-sodium followed zero-order kinetics for high co ncentrations and first-order kinetics for low concentrations, a unique phenomenon consistent with the finding reported in a laboratory study . Concentrations of metam-sodium for transition from zero- to first-or der, obtained by calibration and model sensitivity analyses, were in t he same range as those in the reported laboratory results.