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-sodium spill. The transport and reactions of metam-sodium, a soil fumigant, and the volatile and toxic methyl isothiocyanate (MITC) were simulated during the two-and-a-half days of movement along 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. Agreement between the simulated and measured values of MITC concentrations were 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, the photolysis of metam-sodium followed zero-order kinetics for high concentrations 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 zeroto first-order, obtained by calibration and model sensitivity analyses, were in the same range as those in the reported laboratory results.