Nutrients, mainly phosphorus, from industry, agriculture and other applications have run down and been stored at the bottom of Lake Norrviken for several decades. Phosphorus causes eutrophication.
Eutrophication results in formation of large quantities of microscopic algae, which turn the water green in summer. When the microalgae die and decompose, the lake bed becomes anoxic. And this causes even more phosphorus to be released into the water – the lake is over-fertilising itself, you could say.
The municipalities of Sollentuna and Upplands Väsby have helped the lake break this vicious circle of eutrophication. Injecting the bottom of the lake with aluminium chloride, a salt that is used to purify drinking water, accelerates the lake’s natural processes and binds phosphorus to the lake bed. Norrviken is regaining its equilibrium.
Just one year after treatment, the water is much clearer and bathers can now see their feet on the bottom of the lake. The lake is hugely important to everyone living nearby, and many people have said how delighted they are to see the improvement in the aquatic environment in Lake Norrviken.
