Lake Orta is a small and deep mountain lake in the north-Italian lake zone, in which large glacial lakes such as Maggiore, Como, Isco and Garuda are arranged along the southern margin of the Alps, but is particularly noteworthy for its serious pollution and acidification due to the inflow of industrial wastewater during the past decades. It is located in the southwestern part of the larger drainage basin of Lake Maggiore. Both the lake and its catchment area are long and narrow extending in the north south direction. The lake water is drained from its northern end to the River Niguglia which successively joins the River Strona and then River Toce that enters L. Maggiore. The catchment area ranges between 290 m and 1,643 m in altitude and its lower slopes are covered by deciduous hardwood forests and higher parts by conifer forests.

The pollution of the lake started as far back as in 1926, when the Bemberg factory was constructed on the lake southern shore. This plant produced (and is still producing) rayon, and required a considerable quantity of water taken from the lake. When the water is returned to the lake, it was adulterated with ammonium sulphate and copper of high concentrations. During the first few years, disastrous effects on the lake biota had already been apparent, probably owing to an accidental discharge of copper in large amounts. This initial stage was followed by a chronic change in the lake water chemistry, brought about by the in-lake oxidization of ammonium which produced hydrogen ion at a molar ratio of 1:2 when completely oxidized into nitrate. The lake water, originally poor in buffer capacity because of the prevailing geology of the watershed (gneiss, mica schists, granites), became progressively more acid reaching pH values of 3.8 - 4.3 in the whole water column in 1984 - 1986.

As for the metal pollution, the copper load from the Bemberg plant was considerably reduced in 1958 by the installation of a recovery plant using ion-exchange resins, and has decreased further since 1980, when a new treatment plant came into operation for copper and ammonium removal. The effects of this intervention was evident: the ammonium concentration was halved in two years and pH began to decrease. On the other hand, there was an increase in the load of heavy metals (Cu, Zn, Cr, Ni) since the mid-seventies due to the emission from small bathroom-accessory factories mainly in the watershed of a tributary river, the Lagna. Although some of the factories began wastewater treatment to some extent in the late 70's, the level of pollution is still sufficiently high to prevent effective operation of an activated sludge plant at the mouth of the Lagna which is mainly intended to treat domestic sewage. The once lost lake ecosystem is now recovering steadily after the large-scale liming operation started in 1980.

L. Orta is also known for its scenic and historical landscape with old churches and monastery on an island and along the shore, and attracts a number of sightseeing visitors (1, 2, 3, 4).