Clay layer

Probably the most controversial part of the restoration project of Lake Tuusula is the plan to cover the putrid mud bottom with a thin layer of clay. The basic idea of this is to prevent the phosphorus that is released from the bottom sludge from dissolving into the water. The clay layer would separate the putrid mud from the water masses and thus prevent it from using up the oxygen of the water. In this way the oxygen level of the water would increase and the internal loading of the lake diminish. This would also have an effect on the production of algae.

The plan would mean that the lake of the size of 1.2 square kilometres would get a ten-centimetre thick layer of clay to cover up the putrid mud at the bottom of its central deep basin. Technically the clay could easily be dug up from 2 to 3 meters depth at the west side of the basin where clean original varved clay can be found. Naturally, new matter will be deposited on top of the clay layer but that would not turn into putrid mud if the loading of the lake can be otherwise controlled. The method has already been tested on the lake. The results from the test areas of first about 40 acres in 1993 and of about 10 acres in 1996 were promising. The method also includes use of chemicals to decrease the emission of methane gases from the sludge.

The budget-estimate for the whole operation a couple of years ago was around 10 million marks, which would equal 1.7 euros. An alternative to the clay layer is dredging of the putrid mud, which however would cost double the amount of the clay layer plan. An additional problem with the dredging is the dislocation of the enormous amount of putrid mud that would be removed from the lake. The mass could not be spread on the fields due to its high readings of cadmium.

However, since the condition of Lake Tuusula has improved considerably as a result of the restoration efforts, the clay layer plan has been put aside at least for now.

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