Formation of transient polymetric Al-species in mixing zones and the subsequent deposition of Al on fish gills

Summary

In acidic water aluminium is mostly present as low molecular mass cationic species (LMM Ali). When acidic water is limed, pH increases and polymerisation of LMM aluminium species takes place instantaneously in the resultant mixing zone and transient, reactive aluminium polymers are produced (HMM Ali). These Al-species are extremely reactive to fish gills. Following high amounts of deposition of Al onto fish gills, severe biological effects (including acute fish mortality) have been observed (Rosseland et al 1992, Lydersen et al. 1994, Pôleo et al. 1994, Salbu et al. 2000). To follow the time dependent transformation processes influencing the size distribution and reactivity of Al-species in mixing zones, development of in situ fractionation techniques is essential. In the present work, the results are produced using an in situ hollow fibre system (molecular size discrimination) with an on line cation exchange column (reactivity discrimination) applied in both field and experimental mixing zone studies. In mixing zone experiments where the water pH is increased from 5 to pH 6.2-6.5 by NaOH, results demonstrated the formation of high molecular mass cationic Al-species (HMM Ali) within seconds after the increase in pH, whilst the concentration of LMM Ali species were reduced. With time, the transient HMM Ali species transform into inert colloidal Alc which is no longer gill-reactive. When fish are present in the mixing zone, reactive Al-species are deposited on their gills. When the rate of deposition is higher than the rate of depuration, severe biological effects can occur. Once formed, the HMM Ali species have only a certain lifetime, and the severity of the biological response decreases with increasing time after pH is increased. Thus the most critical periods with respect to unstable mixing zones and effects on fish are high flow snowmelt episodes during spring where smolts are emigrating towards sea.