The composition of dissolved organic matter (DOM) dictates its reactivity. Upon irradiation, DOM generates a collection of photochemically-produced reactive intermediates (PPRIs), including hydroxyl radical, singlet oxygen, and triplet DOM. These species participate in indirect photolysis reactions and thus affect the fate of persistent organic contaminants in sunlit surface waters. We collected diverse water samples along the St. Louis River in order to investigate how the photochemical reactivity of DOM with selected contaminants changes as it moves downstream and into Lake Superior. Experiments were conducted under simulated sunlight with each water sample for four target compounds:  atorvastatin (a statin), carbamazepine (an anticonvulsant), venlafaxine (an antidepressant), and DEET (an insect repellent). Direct and indirect photolysis rates were quantified; a significant rate enhancement due to indirect photolysis was observed for all four contaminants. Rates normalized to dissolved organic carbon concentration revealed the general trend that terrestrially-derived DOM in the upper watershed is less efficient at contributing to contaminant degradation than the microbially-derived DOM that dominates downstream in the estuary and lake. A series of quencher experiments was also performed to quantify the contributions of individual PPRIs to observed indirect photolysis rates for each contaminant.

Impact of dissolved organic matter composition variability on indirect photolysis of contaminants in the St. Louis River

255th National Meeting (2018)

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