Cycling pp'DDT and pp'DDE at a watershed scale: the case of Lago Maggiore (Italy)

A point source of DDT pollution of industrial origin, discovered in 1996 in Lago Maggiore, the second largest (212 km2) and deepest (373 m) lake in Italy, created concern for wildlife and human health because contamination in some fish species exceeded the threshold for human consumption, even though the concentrations measured in the water were much lower than the legal requirements for drinking purposes. Some precautionary measures were undertaken to prevent DDT runoff from the industrial site to the lake but soil restoration had not yet been carried out. The first study to quantify land based sources of DDT homologue pollution was performed in 1998 by sampling all the main tributaries monthly in order to evaluate the annual load to the lake. From May 2001 to May 2002, the study was repeated in order to evaluate the degree of recovery. In this work we compared the results of that survey with those of a more recent campaign carried out from May 2001 to May 2002, concluding that land based sources are still relevant for pp’DDE, the most stable metabolite of pp’DDT, while they are negligible for the parent compound. Furthermore, the Toce River, receiving the soil runoff of the industrial area, was found to give the greatest contribution of DDT and DDE load to the lake in the past campaign, while its load drastically decreased in 2001-2002 in the case of pp’DDT, being comparable to that of Ticino River(the second biggest river of the Lago Maggiore basin, which rises in the St. Gotthard Massif of the Swiss Alpes). Air transport and cold condensation are very likely responsible for trapping DDT and DDE in high mountain glaciers, which represent a secondary pollution source.