Quantification of dissolved natural organic matter (DOM) mediated phototransformation of phenylurea herbicides in lakes

For many important classes of pesticides including phenylurea herbicides (P UHs) and triazines, photosensitized transformation may be the only relevant elimination process in surface waters. In this study, the dissolved organi c matter (DOM) mediated phototransformation of PUHs has been investigated i n laboratory and field experiments. The results indicate that, in surface w aters, the photosensitized transformation of PUHs may be significant and oc curs primarily by an initial one-electron oxidation most likely involving e xcited triplet states of DOM ((DOM)-D-3*) constituents. Using isoproturon a nd diuron as model compounds, it is shown that for a given DOM, quantum yie ld factors determined in the laboratory at a few selected wavelengths can b e used to quantify the overall DOM-mediated phototransformation of a given PUH under sunlight irradiation. Furthermore, it is demonstrated that this p rocess can be modeled for a given surface water, by applying the program GC SOLAR and a simple algorithm for cloud cover for quantification of average daily light intensities. Finally, the model has been successfully applied t o predict vertical concentration profiles of isoproturon and diuron in a sm all lake in Switzerland. To our knowledge, this is the first study in which DOM-mediated phototransformation of organic pollutants has been quantitati vely validated in the field.