By means of simulated sunlight, the influence of natural organic matter (NOM) on the photochemical degradation of halogenated pesticides in the absence and presence of
nitrate as a precursor of the highly reactive
OH radicals in aqueous solutions and freshwater was investigated. Solutions of
dichlorprop or
terbutylazine (a) in
phosphate-buffered demineralized water containing
nitrate and/or NOM and (b) in natural freshwaters were irradiated by a 1000-W Xe short-
arc lamp. The collimated beam was filtered using a combination of optical filters (WG 320 and WG 295) to simulate the solar spectrum under summer midday conditions. In the absence of
nitrate and NOM, the pesticides were degraded photolytically by simulated sunlight. The degradation rates depended on the absorption spectrum in the UVB range and the quantum yield of the degradation. The photochemical degradation of the pesticides was faster in the presence of
nitrate due to the sunlight-induced formation of
OH radicals. In the absence of
nitrate, low concentrations of NOM of a brownwater lake accelerated the degradation due to the formation of reactive species by NOM. At higher concentrations of NOM, the inner filter effect of NOM lowered the degradation rates. In the presence of 50 mg l-1
nitrate, NOM decreased the degradation rate significantly. In case the natural water samples were used as a matrix for the experiments (
nitrate concentrations between 2 mg l-1 and 15 mg l-1, DOC concentrations below 2.3 mg l-1), NOM acted mainly as a radiation filter and as a scavenger of
OH radicals. As a consequence, in most freshwater systems, the accelerating effect of NOM by the formation of reactive species is of minor importance compared to the inner filter effect and to radical scavenging.