Factors Affecting Degradation of MSMA in Soil

The effects of herbicide concentration, soil water content, and temperature on the fate and degradation of the monosodium salt of methylarsonic acid (MSMA) were evaluated in four soils. Available arsenicals were extracted, purified, and separated using reversed-phase high-performance liquid chromatography (HPLC). Residual MSMA was determined with a graphite furnace atomic absorption spectrophotometer at 193.7 nm. Initial loss of MSMA was rapid, and degradation followed first-order kinetics. The rate of change of the rate constant was temperature dependent only at soil water contents less than field capacity. MSMA dissipation was significantly faster in finer textured soils with continuous flooding and under controlled laboratory conditions (<180 days) than under field (ca. 350 days) conditions. Also, in finer textured soils, degradation of MSMA resulted in significant increases in arsenate and cacodylic acid (dimethyl arsinic acid) over native levels under flooded conditions. However, MSMA treatments contributed only a small fraction to total soil arsenic which dissipated to original concentrations by 120 days. The results indicate that under present recommended use patterns it is highly unlikely that MSMA will accumulate in the environment or carry over from one growing season to the next.