Photochemical consequences of prolonged hydrological drought: A model assessment of the Lower Lakes of the Murray-Darling Basin (Southern Australia)

The prolonged "Millennium" drought affecting Australia in the 2000s had important consequences on surface-water bodies, including the Lower Lakes (Lake Alexandrina and Lake Albert) located at the terminal end of the River Murray system. Shallower water depths, limited solute dilution and altered geochemical processes ensured that the concentration values of several water constituents increased considerably during drought, including the water parameters of photochemical significance (nitrate, bicarbonate, carbonate and the dissolved organic carbon, DOC). The aim of this study was to model the photochemical processes in the Lower Lakes during the drought and post-drought periods, to provide insight into the changes that photoinduced reactions can undergo in periods of water scarcity. Among the photochemical processes involved in the light-assisted transformation of dissolved compounds, an important role is played by indirect photochemistry where degradation is triggered by photogenerated transient species such as hydroxyl (OH) and carbonate (CO₃^-) radicals, and the triplet states of chromophoric dissolved organic matter (³CDOM*). Results of photochemical modelling suggest that the reactions induced by ³CDOM* would be enhanced during drought, while the processes triggered by OH and CO₃^- would be less modified. For compounds undergoing efficient degradation with ³CDOM*, enhanced photochemistry during drought could offset the higher concentration values resulting from lower dilution. In contrast, for compounds mainly degraded by OH or CO₃^- the drought period could produce a concentration increase not balanced by an increment in the photochemical reactivity of the water body.