Environmental risk assessment for discharge to the Glomma River from land-based salmon production

Summary

A discharge into the Glomma River should be positioned as deep in the freshwater layer as possible. There is no advantage to position the discharge in the salt wedge, because this can occasionally cause the particles to be carried upwards in the river in the compensation current. The risk for longer residence time for the waste water and sedimentation of particles will then be increased. Because the depth of the halocline varies during the tidal cycle and is affected by fluctuating water flow in the Glomma River, it is recommended that the discharge is positioned at about 8 meter depth. An assessment of a discharge in the Glomma River must both take local effects and effects out in the Glomma Estuary into account. The effects close to the discharge point will be mainly be associated with the particulate organic material that can sink down to the bottom and lead to increased oxygen consumption when this material is degraded. Effects outside in the estuary will be linked to eutrophication. Monitoring of the water bodies outside river mouth shows that these probably have moderate ecological state due to the relatively high nutrient concentrations.