Modelling of discharge of chemicals in the Drammen River

Summary

In a model simulation with the numerical model GEMSS, discharge of chemicals to the Drammen River has been simulated, where the aim is to exterminate fish infected with the salmon parasite Gyrodactylus salaris. By releasing an amount of 32000 L of a rotenone solution of strength of 3.3%, from three discharge points, from Hellefoss to Drammen center, it is possible to achieve a high enough concentration (above 0.7 mg/L) in nearly the entire river. There will be a challenge to control the concentration in various small and large back waters of the river. In one arm of the river inside the island Fallagsøya (near station E36), the increase in the concentration appears a little later than in the rest of the river. In this arm is also the location where the concentration remains high for the longest time after the discharge ends. This area of the river should be investigated close before any measures against G. salaris commences. The predicted no effect concentration in the environment (PNEC) of the rotenone solution can be calculated from experiments where invertebrates are exposed to rotenone for a period typically for 24 to 48 hours. The LC50 value is the concentration at which 50 % of the test population die, which is about 3 mg/L for the most sensitive species. PNEC value can be calculated using a safety factor of 100, giving a value of 0.03 mg/L. The salinity in the Drammen Fjord is continuously very low in the top 5 meters. During floods in the river the freshwater layer is up to 10 m thick. The salinity is lower than 5 psu in most of the fjord. A discharge of 32000 L of rotenone solution, gives a concentration in the surface layer of the Drammen Fjord exceeding the PNEC value for some hours in the central parts and along the south-western coast of the inner fjord. The highest concentrations occur in the inner fjord nearest the river mouth and the fjord's western coast towards Solumstranda. The concentration does not exceed 0.25 mg/L south of Solumstranda. To achieve a concentration of dissolved aluminum (Ali) of 30 μg/L throughout the Drammen River, it is necessary discharge approximately 0.4 L/s of an aluminum solution of strength of 4.3 %. Concentrations will exceed 7 μg/L in most of the inner parts of the fjord all the way to Steinsbråten Fyr, and exceed 20 μg/L in the areas closest to the river mouth and along the coast down to Solumstrand.