Assessment of discharge of wastewater from the VEAS overflow pipe in Lysakerfjorden.

Sammendrag

The purpose of this report is to describe how discharge from the VEAS-tunnel to Lysakerfjorden is spread. The overflow starts when the tunnel is overloaded, for example due to heavy rain episodes. Emphasis was placed on the spread of E. coli and how this affects the bathing sites in the area. E. coli is an indicator of fecal contamination. At high concentrations of E. coli in bathing water is the increased probability of occurrence of pathogenic bacteria, viruses and parasites. To solve the problem, we have combined the use of models with observations in the field. Visual Plumes model was used to study the expiry beam behaves just after it leaves the pipe. Then we calculated the further spread of the fjord by the numerical current and dispersion model GEMSS. A major discharge from the VEAS tunnel, which typically occur a few times each bathing season, were modeled. The spill led to high concentrations of E. coli along the west side of Lysakerfjorden with bathing locations Telenorstranda and Rolfstangen. Also Sollerudstranda was significantly affected. On all these beaches it is necessary to issue warning against swimming a few days after such large discharges from the VEAS tunnel. Overflow does not seem to affect the water in Bestumkilen and near Bygdø sjøbad to the same extent, and during and after heavy rainfall, it is likely that Hoffsbekken and Mærradalsbekken (with their respective overflow and sewage leaks) affect bathing water quality at these sites more than discharge from the VEAS tunnel. Also Sollerudstranda and Telenorstranda are heavily influenced by local emissions, including overflow to Lysakerelva.