ABSTRACT
The substantial reduction in ammonium input since 1981 has led to a gradual recovery of the water quality of the lake. In order to forecast the time necessary for Lake Orta water to reach its original alkalinity, a simple input-output model, which takes account the acidity/alkalinity variation with time, was used. Lake Orta is one of the most important water bodies in the main Italian lacustrine area which is formed by the great marginal lakes Maggiore, Lugano, Como, Iseo, and Garda, south of the Alps. It occupies the south-western part of the Lake Maggiore drainage basin. Lake Orta has for decades suffered dramatic industrial pollution. The differences are not significant for any of the ions except ammonium and bicarbonate, both of which are consumed in the lake. The in-lake oxidation of the ammonium to nitrate started to produce acidity, which reduced the alkalinity of the water.
