CHEMISTRY AND CRITICAL LOADS OF ALPINE LAKES IN CANTON TICINO (SOUTHERN CENTRAL ALPS)

We studied the chemistry of atmospheric deposition and of 55 Alpine la kes in Canton Ticino (Switzerland). The chemistry of the precipitation and the deposition of ions from the atmosphere were characterized by a marked gradient, with higher values for sulphate, nitrate, ammonium and acidity in the southernmost part of the Canton, and a halving of t he values in the northern area. Results showed that most of the 55 lak es, mainly located in the north, are sensitive to acidification, 87 % and 33 % of them having alkalinity values of below 200 and 20 mu eq l( -1), respectively. The main processes determining water chemistry were carbonate and silicate weathering depending on the geo-lithology of t he watershed, while the input of atmospheric deposition was a source o f variability for nitrogen and chloride. A comparison between the crit ical and present loads of acidity to the lakes showed that a reduction of more than 50% of the acidifying compounds would be required to kee p a safe level of alkalinity in all the lakes. Ammonium plays an impor tant role in acidification processes and must be considered in the red uction of emissions.