Rate of bentazone transformation in four layers of a humic sandy soil profile with fluctuating water table

The rate of transformation of a pesticide as a function of the depth in the soil is needed as an input into computations on the risk of residues leach ing to groundwater. The herbicide bentazone was incubated at 15 degreesC in soil materials derived from four layers at depths of up to 2.5 m in a humi c sandy soil profile with a fluctuating water table (0.8 to 1.4m), while si mulating the redox conditions existing in the field. Gamma-irradiation expe riments indicated that bentazone is mainly transformed by microbial activit y in the soil. The rate constant for transformation was highest in the humi c sandy top layer; it decreased with depth in the sandy vadose subsoil. How ever, material from the top of the phreatic aquifer had a higher rate const ant than that from the layers just above. The presence of fossil organic ma terial in the fluviatile water-saturated sediment probably stimulated micro bial activity and bentazone transformation. The changes in the transformati on rate constant with depth showed the same trend as those in some soil fac tors, viz organic carbon content, water-extractable phosphorus and microbia l density as measured by fluorescence counts. However, the (low) concentrat ion of dissolved organic carbon (DOC) in the top of the aquifer did not fit the trend. The rate constant for bentazone transformation in the layers wa s higher at lower initial contents of the herbicide. (C) 2001 Society of Ch emical Industry.