Photochemical reactions in sunlit surface waters: Influence of water parameters, and implications for the phototransformation of xenobiotic compounds

Photochemical reactions may be important transformation pathways for biorefractory xenobiotic compounds in surface waters. These reactions can be divided into direct photolysis and indirect photochemistry. In the former, a xenobiotic is transformed upon absorption of sunlight, in the latter sunlight is absorbed by natural photosensitisers (mainly chromophoric dissolved organic matter, nitrate and nitrite), which produce reactive transient species that induce transformation. Direct photolysis and indirect photochemistry are affected by sunlight irradiance, water chemistry and depth, and they can undergo seasonal and long-term changes. This chapter gives an overview of the main photoreactions of significance for surface waters, including their implications for pollutant transformation and the possible generation of harmful intermediates.