Modelling photochemical transformation of emerging organic pollutants in surface waters: effect of water level fluctuations following outflow or evaporation,relevant to arid and semi-arid environments

Water bodies located in arid and semi-arid environments undergo important fluctuations of the water level, and water loss in the dry season may occur through phenomena such as outflow and evaporation. Water scarcity often exacerbates the impact of pollution, because harmful compounds can reach very high concentration values in the presence of relatively low water volumes. Among self-cleansing (depollution) processes of water bodies, photochemical reactions are expected to play an important role in pollutant attenuation, although sometimes they yield harmful intermediates. In this work, the effects of water-level fluctuations on photochemical reactions were studied by means of a modelling approach. The model applies to well-mixed water bodies, such as the shallow ones that are more likely to undergo fluctuations of the water level. We investigated how water outflow and evaporation (as well as a mixed scenario) might modify the steady-state concentrations of the most important photoreactive transients (^?OH, CO₃^–?, ¹O₂ and triplet states of chromophoric dissolved organic matter, ³CDOM*). Under the same conditions, the possible phototransformation of emerging pollutants such as carbamazepine, ibuprofen, 2,4-dichloro-6-nitrophenol and 2,4-dinitrophenol was also assessed. It is shown that water loss would generally enhance photochemical reactions, but to a variable extent depending on the actual pathway. Outflow would favour all photoprocesses, and particularly those involving ^?OH and CO₃^–?. Conversely, evaporation would enhance reaction with ¹O₂ and ³CDOM* while having practically no effect on ^?OH, CO₃^–? and direct photolysis. Enhancement of photochemical self-cleansing of water bodies would partially reduce the pollution impact, but in some cases harmful compounds may be generated.