Modeling Surface and Subsurface Pesticide Transport Under Three Field Conditions Using PRZM-3 and Gleams

Contaminant transport models should be evaluated over a wide range of conditions to determine theirlimitations. The models PRZM and GLEAMS have been evaluated many times, but few studies are available in whichpredicted movement in runoff and percolate were simultaneously evaluated against field data. Studies of this type areessential because pesticide leaching and runoff are mutually dependent processes. For this reason, PRZM-3 andGLEAMS were evaluated for their ability to predict metribuzin concentrations in runoff, sediment, subsurface soil, andpan lysimeters under three field conditions (yard waste compost amended, no-till, and conventional-till) on a Lowell siltloam soil. Sensitive input parameters were either site specific (climatic, soil, and chemical) or calibrated (K-factor,C-factor, curve number). In general, both models under-predicted metribuzin concentration in runoff water, runoffsediment, subplow layer soil (15-75 cm), and pan lysimeter water (75 cm). Contrary to field data, both models predictedthat a large percentage (> 50%) of metribuzin would move below the mixing zone (top 1 cm) during the first rainfallevent after application. Relatively little metribuzin was predicted to move beyond the plow layer (top 15 cm) into the panlysimeters or subsurface soil throughout the simulation period, possibly due to the lack of a macropore component in themodels. High metribuzin concentrations in sediment (field data) indicated that relatively little metribuzin moved below themixing zone, possibly because of hysteresis but much of the metribuzin that did move was quickly transported into thepan lysimeters, probably due to macropore flow. GLEAMS more accurately predicted pesticide concentration in sedimentand PRZM predicted subsurface soil concentration somewhat more accurately than GLEAMS. Little difference inaccuracy was detected between models on metribuzin concentration in runoff or metribuzin concentration in percolate.Although both models generally under-predicted metribuzin concentration in runoff, runoff transport (mass of metribuzinin runoff) for the study period was over-predicted by both models which emphasizes the importance of accuratelypredicting herbicide concentration and runoff volume soon after application when the surface pesticide concentrationsare highest.