| Clothianidin, a synthetic neonicotinoid, is registered for pre-flood (to field soil) and post-flood (to field water) application to protect California rice fields against the rice seed midge, Criotopus sylvertis , and the rice water weevil, Lissoroptrus oryzophilus. Our objective was to characterize the individual mechanisms governing transport and degradation under conditions representative of California rice fields (20-36°C; UV irradiation< 1,350 muW cm-2 at 310 nm; pH 7-9; flooded soil redox potential Ehs between -123 and -217 mV).;Clothianidin (pKa 11.0) is a neutral species within environmentally relevant pH ranges, has a low vapor pressure (3.8 x 10-11 Pa at 20°C), and is capable of strong absorbance of environmentally-relevant long (UV-A; 320 to 400 nm) and short (UV-B; 290 to 320 nm) UV irradiation. Based on these properties, we hypothesized that dissipation will be controlled by photolysis and microbial degradation under flooded conditions.;Clothianidin was confirmed to be non-volatile (from water) via the gas-purge method, as no loss from the aqueous phase was observed at 22 and 37°C; an upper limit KH value was calculated at 2.9x10-11 Pa m3 mol-1 (20°C). Soil-water partitioning was determined by the batch equilibrium method using four soils collected from rice fields in the Sacramento Valley, and sorption affinity (Kd ), sorbent capacity, desorption and organic carbon-normalized distribution (Koc) were determined. Values for pH, cation exchange capacity, and organic matter content ranged between 4.5 to 6.6, 5.9 to 37.9, and 1.25 to 1.97%, respectively. Log Koc values (22 and 37°C) ranged between 2.6 to 2.7, while sorption capacity was low at 22°C and further decreased at 37°C. Hysteresis was observed in soils at both temperatures, suggesting that bound residues do not readily desorb.;Photodegradation of clothianidin was characterized in deionized, Sacramento River, and rice field water samples. Pseudo first-order rate constants and DT50 values in rice field water (mean k = 0.0158 min-1; mean DT50 = 18.0 dequiv) were significantly slower than deionized (k = 0.0167 min -1; DT50 = 14.7 dequiv) and river water ( k = 0.0146 min-1; DT50 = 16.6 dequiv ) samples. Quantum yield &phis;C values demonstrate approximately 1% and 0.5% of the light energy absorbed results in photochemical transformation in pure and field water, respectively. Concentrations of the photodegradation product TZMU in aqueous photolysis samples were determined using LC-MS/MS analysis and accounted for ≤17% in deionized water and ≤8% in natural water.;Microbial degradation of clothianidin was evaluated under flooded (anaerobic) and non-flooded (aerobic) microcosms. Clothianidin was recalcitrant to microbial degradation under aerobic conditions. First-order transformation rates and DT50 values of clothianidin in representative flooded soil at 35 +/- 2°C (k = -7.16 x 10-2 +/- 3.08 x 10-3 d-1, DT50 =9.7 d) were significantly faster than 25 +/- 2°C microcosms °C ( k = -2.45 x 10-2 +/- 1.59 x 10-3 d-1, DT50 = 28.3 d). |
