This year at the beginning of the summer season – as is now almost traditional – the Mayor of Bolsena declared “with pride” that ARPA Lazio judged the quality of the water of the lake to be “excellent”. Yet, is not it the same ARPA which revealed that the global health status of the lake has worsened?

Both statements are correct, since they refer to two distinct characteristics of water. For bathing, the absence of faecal bacteria near the banks is important. For the global state of the lake, however, the concentration of nutrients (especially of phosphates) in the whole body of water is decisive (see “The lake’s conditions” and “Swimming conditions“).

When restoration work on the collector is completed and after further measures to prevent pollution have been taken, the quality of bathing water could indeed be excellent anywhere in the lake. The lake ecosystem quickly absorbs faecal bacteria, and the abundant rains and low temperatures of this spring have been beneficial.

However, this positive data does not change the fact that nutrient intake in the lake is excessive. These nutrients come from sewers (leaks and illegal discharges) and from fertilisers used in agriculture. They come into the lake either directly or through the pollution of the water table in the lake basin.

The nutrients cause a strong growth of plant and animal biomass, whose remains descend towards the bottom of the lake where their decomposition consumes oxygen. Under these conditions, the phosphorus trapped in the sediment is released causing a further increase in nutrient content. This, after reaching a critical threshold, can cause an irreversible deterioration (for a detailed discussion, see “eutrophication“).