ACIDIFICATION AND WEATHERING PROCESSES IN HIGH-MOUNTAIN LAKES IN SOUTHERN ALPS

About 200 lakes in Southern Alps (Italy and Switzerland) were studied in order to quantify their acidification. Although samplings were carr ied out in summer, long after the acid shock caused by snowmelt, some lakes were found to be acidic and 47% of them show alkalinity values o f below 50 mueq l-1. Losses in alkalinity (acidification levels) were evaluated using a titration model with variable F-factor. Factor analy sis shows that the main factors influencing water chemistry are relate d to rock weathering and nitrate uptake by vegetation. In watersheds c ontaining carbonatic rocks the weathering of calcite is responsible fo r the most of the alkalinity production. We therefore focused on a set of 19 low-alkalinity lakes in the Maggia Valley lying in watersheds c ontaining mainly acidic rocks. Results show that the weathering of sil icate and calcite (present in small quantities, but highly soluble) an d nitrate uptake account for most of the alkalinity production. Other watershed and in-lake processes may be important in some cases, but th ey never account for more than 27% of the alkalinity production in the se lakes.