Fenamiphos transport, transformation, and degradation in a highly weathered soil

Fenamiphos, a nematicide used on corn and sorghum, quickly oxidizes into two metabolites which have similar activities and toxicities, yet are more mobile and persistent than the parent compound. Given the soil and climatic conditions of the southeastern U.S., fenamiphos and its metabolites could be transported from the application site and contaminate off-site water bodies. A three-year study was conducted to evaluate (1) degradation and transport of the fenamiphos parent (F(p)) and its metabolites (sulfoxide, F(x), and sulfone, F(o)) from a 0.34 ha field site, and (2) the utility of the GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model in describing system response and simulating pesticide transport. Each year, fenamiphos was applied at 6.7 kg ha-1 a.i., broadcast and incorporated into the upper 100 mm soil layer before planting each crop. Concentrations of fenamiphos and its metabolites were determined from soil samples taken within the root zone at 50 mm intervals to a depth of 300 mm and from subsurface tile outflow at selected times throughout each sweet corn (Zea mays L.) and hybrid pearl millet (Pennisetum glaucum (L.) R. Br.) growing season. The GLEAMS model was used to simulate runoff, lateral subsurface flow (LSF), and F(p), F(x), and F(o) losses from the Cowarts loamy sand. An average of 6 and 21% of the total rainfall + irrigation was measured as runoff and LSF, respectively. GLEAMS model simulations were correlated with measured runoff (R2 = 0.81) and LSF (R2 = 0.89). Field half-lives (t1/2) were determined by comparing observed concentrations in soil by depth and time to those simulated with the GLEAMS model. Average t1/2 values from measured field data were 5, 28, and 14 days for F(p), F(x), and F(o), respectively. For the three-year study, about 6.2% of the total amount of applied fenamiphos (F(tot) = F(p) + F(x) + F(o)) was measured in LSF, while less than 0.1% of the applied fenamiphos was measured in surface runoff. F(x) was the dominant compound measured and simulated in the root zone and LSF, with 70 to 99% of measured F(tot) being F(x). Calibration of the GLEAMS model provided fit of the field data that indicated (1) F(p) dissipated rapidly while the two metabolites (F(x) and F(o)) formed (average F(p) t1/2 = 5.5 d); (2) t1/2 values for all compounds remained relatively constant during 1987 and 1988, then numerically decreased in 1989; (3) coefficient of transformation (CT) values for F(x) and F(o) decreased from 1987 to 1989; and (4) CT values describing transformational changes from F(p) to F(x) were greater than those describing transformational changes from F(x) to F(o). Decreases in t1/2 and CT values for F(p), F(x), and F(o) with continued use over the three-year study is characteristic of enhanced microbial degradation.