INCREASED MORTALITY OF FISH DUE TO CHANGING AL-CHEMISTRY OF MIXING ZONES BETWEEN LIMED STREAMS AND ACIDIC TRIBUTARIES

The present study is mainly focusing on mortality variations of fish d ue to changing Al-chemistry of mixing zones. An artificial mixing zone was made by pumping water from a limed stream and an acidic tributary into a mixing channel. Atlantic salmon (Salmo salar L.) parr were exp osed to the mixed water, limed stream water, and acidic tributary wate r. Mortality, blood haematocrit and plasma Cl- -concentration were rec orded. Neither mortality, nor changes in haematocrit and plasma Cl- we re observed when fish were exposed to limed water, while in both acidi c and mixed water, mortalities and loss of plasma Cl- were observed. T he highest mortality rates were found within the initial part (0 to 20 s) of the mixing zone. Blood haematocrit increased only in fish expos ed to acidic tributary water. Our results shows that changes in Al-che mistry and subsequent Al-polymerization occur when acidic tributary wa ter is mixed with limed stream water. We have also demonstrated that t he toxicity which can arise in mixing zones are greater than in the or iginal acidic water before mixing. The variations in mortality observe d are associated with the quality and quantity of Al-polymerization as well as ageing of the polymers.