The results of comprehensive environmental studies of a marine discharge from a pulp mill is presented to illustrate a general strategy for environmental impact assessment and management of marine industrial wastewater discharges. The strategy includes the following elements: 1) a thorough chemical and ecotoxicological characterization of the wastewater which also involves degradability studies, 2) field monitoring, 3) cage studies with transplanted organisms, 4) hydrographical studies and subsequent dilution and fate modelling of wastewater and wastewater constituents, and 5) quantitative evaluation of the environmental impact as well as predictive evaluations of possible abatement measures by using the combined results of laboratory tests/chemical analyses and dilution/fate modelling to confine areas around the outfall, where adverse effects of various categories can be expected under various environmental settings. Results from field studies are used for comparison/verification. The present study deals with a discharge of unbleached semichemical sulphite pulp effluent which caused oxygen depletion and toxic effects, in particular towards phytoplankton algae. The usefulness of the general study approach was demonstrated and it was concluded in this case that biological field data alone gave limited clues to assessing nor to mitigating the pollution. This was due to the time and spacial variability of the data, and the limited possibilities of distinguishing effects of occasional oxygen depletion from toxic effects. Biological field studies were useful to assess the problem initially, however, and necessary to complement the calculated estimates both qualitatively and to give the study credibility.

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