A new approach was developed to account for the contribution of indirect photolysis of pesticides and pharmaceuticals in which laboratory test conditions are similar to those prevalent in the aqueous environment. Rates of photolysis as a function of water composition were investigated for several aquatic contaminants. Using the laboratory-based test system, PhotoFate, the dependence of phototransformation rates on concentrations of natural water constituents that are radical producers and scavengers (nitrate, colored dissolved organic matter, bicarbonate) was studied. Mean half-lives of the model compounds in the presence of water constituents were compared to their direct photolysis half-lives to assess the contribution of photosensitized reactions to their fate in surface waters. Reactions mediated by .OH were predominant in waters with high nitrate concentrations. Colored dissolved organic matter (cDOM) acted mainly as a radiation filter and had a more important role in scavenging radicals than in their production. However, in low nitrate waters, the contribution of cDOM-derived reactive intermediates to the degradation of parent compounds became more apparent

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