Photodegradation potential of selected non-steroidal anti-inflammatory drugs in a middle-order Alpine river downstream of a wastewater treatment plant, during a year of enduring water scarcity.

The year 2022 was characterised by significant water shortages and droughts in Italy, with the most pronounced impact observed in the North-Western regions, including Piemonte. In conditions of water scarcity, treated wastewater undergoes little dilution by natural flows and this can deeply affect the chemistry of water-poor rivers and streams. However, increased pollution by wastewater would be partially offset by fast photodegradation of pollutants in shallow water and by the longer time allowed to photochemical reactions if water flows more slowly. We assessed the latter phenomena in the Stura di Lanzo, a middle-order Alpine river tributary of the largest Italian river, the Po, and affected by a wastewater treatment plant (WWTP). In 2022, the concentration values of the photochemically significant parameters nitrate, nitrite, and DOC were usually higher downstream of the WWTP outlet, which could slightly favour indirect photodegradation reactions. Direct and indirect photodegradation was assessed for the non-steroidal anti-inflammatory drugs paracetamol, diclofenac, and naproxen, all undergoing rather fast photoreactions. Photochemistry model results show that the three compounds would undergo 10-40 % photodegradation in spring and summer along the stretch separating the wastewater outlet from the confluence of Stura into the Po. Photodegradation would continue in the latter, but other WWTPs might contribute additional pollution in the meanwhile. Albeit significant, photodegradation could only partially promote the elimination of the contaminants.