Lake Trehörningen restoration project. Changes in water quality after sediment dredging

Abstract

An increased load of domestic wastewater to Lake Trehörningen induced oxygen-poor water conditions and the development of a reduced sulphide-rich sediment layer. Severely polluted, the lake did not recover, even after advanced wastewater treatment and sewage diversion. Restoration measures with suction dredging and macrophyte elimination were applied in 1975 and 1976. The loose topmost sediment was pumped into an embanked and overgrown bay which was used as a settling pond. The activities also included a restoration of the shorelines. This project is the largest restoration programme carried out in Sweden on a single lake, corresponding to a cost of about US $2 000 000.

The restoration of Lake Trehörningen was followed by a highly intensive research programme which included water chemistry and algal assays. The concentrations of phosphate and total phosphorus decreased by 73 and 50% respectively, as summer average values, two years after the restoration. However, the concentrations of phosphorus are still too high to permit this element to act as a prime algal growth-limiting nutrient. The algal biomass has also remained at the same magnitude as before the restoration. Nitrate-N concentrations showed a tenfold increase, based on average values for the summer period. However, based on the results of the algal assays, a rapid and marked response was obvious, with a drastic decline in the algal growth potential.

In addition, the water quality of the tributaries was frequently of an objectionable character (0.1–0.2 g P m⁻³). The nutrient loading from these sources exceeds the critical level for the lake, and measures have now been carried out to treat all the inflowing waters for the removal of phosphorus.