CHEMICAL RESPONSE OF LAKE ORTA (NORTHERN ITALY) TO LIMING

Lake Orta is a subalpine lake, with a maximum depth of 143 m and a volume of 1.3 10(9) m3. In 1926 the lake began to be heavily polluted by effluents rich in copper and ammonium sulphate discharged by a rayon factory. In the sixties, metals (Cu, Cr, Ni and Zn) released from plating factories became a further source of pollution. The negative effects of the metal pollution were aggravated by the acidification of the lake water caused by the oxidation of the ammonium. pH values decreased from values around 7.0 in 1948 to the minimum values of 3.8-4.0 over the whole water column which were reached in 1984. The ammonium discharge was greatly reduced in 1981, but in-lake concentrations both of ammonium and hydrogen ion remained high, because of the low alkalinity load from the watershed and the long water renewal time (8.5 years). From May 1989 to June 1990 the lake was limed with 10,900 tons of CaCO3, the first instalment of the 18,300 tons necessary to reach a target alkalinity of 50-mu-eq l-1, after the complete oxidation of the ammonium. The effect was a decrease in the mean acidity of the lake by 173-mu-eq l-1, i.e. a variation which would have taken five years with the natural load of alkalinity from the watershed. The effects were more marked in the 0-40 m water layer, where pH between 6.0 and 8.0 was measured during summer and autumn 1989. A marked decrease in Cu, Zn, Ni, Cr, Al concentrations was observed in the neutralized water, while values remain high in the part of lake which is still acid.