FENAMIPHOS TRANSPORT, TRANSFORMATION, AND DEGRADATION IN A HIGHLY WEATHERED SOIL

Fenamiphos, a nematicide used on corn and sorghum, quickly oxidizes in to two metabolites which have similar activities and toxicities, yet a re more mobile and persistent than the parent compound. Given rite soi l and climatic conditions of the southeastern U.S., fenamiphos and its metabolites could be transported from the application site and contam inate off-site water bodies. A three-year study was conducted to evalu ate (1) degradation and transport of the fenamiphos parent (F-p) and i ts 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 respons e and simulating pesticide transport. Each year; fenamiphos was applie d 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 th e root zone at 50 mm intervals to a depth of 300 mm and from subsurfac e the outflow at selected times throughout each sweet corn (Zea mays L .) and hybrid pear 2 millet (Pennisetum glaucum (L.) R. Br) growing se ason. 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 measure d as runoff and LSF respectively. GLEAMS model simulations were correl ated with measured runoff (R-2 = 0.81) and LSF (R-2 = 0.89). Field hal f-lives (1(1/2)) were determined by comparing observed concentrations in soil by depth and time to those simulated with the GLEAMS model. Av erage t(1/2) values from measured field data were 5, 28, and 14 days f or F-p, F-x, and F-o, respectively For the three-yens 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 b eing F-x. Calibration of the GLEAMS model provided fit of the field da ta that indicated (1) F-p dissipated rapidly while the two metabolites (F-x and F-o) formed (average F-p t(1/2) 5.5 d); (2) t(1/2) values fo r all compounds remained relatively constant during 1987 and 1988, the n numerically decreased in 1989; (3) coefficient of transformation (CT ) values for F-x and F-o decreased from 1987 to 1989; and (4) CT value s describing transformational changes from F-p to F-x were greater tha n those describing transformational changes from F-x to F-o. Decreases in t(1/2) and CT values for F-p, F-x, and F-o, with continued use ove r the three-year study is characteristic of enhanced microbial degrada tion.