Photochemical transformation of agricultural pesticides promoted by natural water constituents in wetland surface waters

The effect that photochemical processes would have on improving the water quality of freshwater wetlands receiving drainage from agricultural watersheds was studied. Specifically, this research sought to elucidate how wetland water constituents would promote and inhibit direct and indirect photochemical processes.; Water samples were collected from several wetland field sites located in agricultural watersheds and from the Suwannee River. Natural organic matter (NOM) isolates (Suwannee River and Lake Fryxell Fulvic Acids) were also used. Characterization (UV/VIS, TOC, IC) of water samples helped to identify potential photosensitizers. The “target” non-point source (NPS) pollutants chosen were carbaryl and alachlor.; A solar simulator was used to irradiate parent compounds in unaltered and pH-adjusted (∼4) solutions. Samples were assayed by either HPLC or GC/ECD. Pseudo-first order photodegradation rate coefficients (k_obs) were determined from the kinetic data to serve as a measurable index of reaction efficiency to facilitate the comparison with other samples. When necessary, rate coefficients were corrected for the variation in source light intensity and inner filter effects.; At the ambient pH (∼8), direct photolysis and a dark reaction were effective in degrading carbaryl but not alachlor. For both compounds, waters containing high levels of nitrate led to significant photodegradation through nitrate photolysis. At low pH, indirect photolysis increased significantly for both molecules and is apparently related to the activation of certain photochemical pathways promoted by NOM and/or Fe-NOM complexes that produce reactive transients. Photobleaching studies provided support that indirect processes were promoted at low pH.; Moreover, the indirect photodegradation was seasonally and spatially dependent. In early season samples (high nitrate), k_obs corresponded to the nitrate level while in the latter season samples (low nitrate), to the levels of NOM and possibly iron.; The indirect photolysis also depended on the concentration of natural ROS scavengers (NOM and carbonate species). In these waters, the effect of carbonate species on scavenging was insignificant (<8%). The reactivity of NOM did not correlate to its hydrophobicity.