Recovery of Lake Varese: reducing trophic status through internal P load capping

Lake Varese is one of the first and most evident examples of cultural eutrophication in southern Europe (Northern Italy). Although internal actions (hypolimnetic water withdrawal and injection of pure oxygen) were implemented following the construction of a sewage collection system in 1986, the complete reversal of the lake's eutrophic condition has not yet been achieved. Since the internal P load in Lake Varese, estimated to be 5.6 t yr(-1), is a determining factor in the continuation of the eutrophic status of the lake, in this study an application of a lanthanum-modified bentonite clay, able to bind phosphorus, was carried out in controlled enclosures for 12 months during 2009-2010. The results showed a sharp reduction (more than 80 %) of the P concentrations along the water column after the lanthanum-modified bentonite clay application and, from January onwards, the settled clay controlled the P release from the sediments, preventing a sharp increase in total P concentrations to values exceeding 0.28 mg P l(-1) that took place from August until October in untreated conditions. Though this action proved to be efficient in controlling the P fluxes from the lake sediments, low levels of dissolved oxygen were maintained in the bottom layer of the lake. A delay in the reduction of the anoxia could be observed in longer than the temporal window used for the enclosure trial. The time required to attain a significant reduction in anoxic conditions during the thermal stratification period depends on the reduction of total P and on several additional factors.