Long term trends in Northern Italy alpine sites: deposition chemistry and surface water evolution (1997-2008)

Recent trends of monthly precipitation from three monitoring sites in Lombardy region were analyzed to reveal the long term changes in chemical composition. One site is located in Val Masino-SO, in the Retic Alps, in a remote forest area at 1190 m a.s.l., while the other two sites, Moggio-LC and Giovetto-BS (1220 and 1150 m a.s.l. respectively), are closer (50 and 90 km respectively) to Milan and relatively affected by emissions from urbanized areas. Thirteen years data are available for the Val Masino site (1995-2008) and from 2000 to 2006 for the other two sites. The statistical significances of systematic changes were calculated with the Seasonal Kendall test (SKT). The precipitation total declined for all the three sites until 2007. Like in several European sites we observed a sharp decline in sulphur concentrations, and a subsequent increase of pH values. Otherwise the pattern is less clear for reduced and oxidized nitrogen. In two of the three sites no evidence on trends for nitrogen concentration is observed, while in the other one (Moggio) a weak significant increase of nitrogen and ammonium seems to occurred. Data of basic cations, bicarbonate and organic nitrogen were also analyzed showing different temporal patterns for the three sites. For the Val Masino site, trends in deposition chemistry were compared to the chemical evolution (1997-2008) of Masino stream water representing the output of the study catchment. The discharge and sulphate concentrations of the stream shows a significant decrease, while a more complex pattern is evident for nitrogen compounds closely related to snow melt and biological retention processes. A detailed data analysis shows a clear seasonal pattern of NO3 concentration in response to hydrology and biological uptake in the growing season.