Pluriannual evolution of the hydrochemistry of two Alpine lakes (Lake Paione Inferiore and Lake Paione Superiore, Ossola Valley) in relation to atmospheric loads.

Lakes Paione Inferiore and Paione Superiore (LPI, LPS) are extremely sensitive to acidification, so they are useful as indicators in studying changes in atmospheric pollutant fluxes on waterbodies. Regular trends observed in the last 3-4 years cannot merely be a consequence of seasonal or interannual variations. Increasing pH and alkalinity are mainly driven by a decrease in acidic inputs from the atmosphere, which have been halved over the last 10 years. This trend of atmospheric deposition chemistry has emerged in several sampling stations in the subalpine and Alpine area, in the watershed of Lake Maggiore. The decrease in deposition acidity is mainly related to a decrease in sulphate, while nitrate and ammonium deposition is still high, resulting in high nitrate concentrations in lake waters.