Biogeochemical processes controlling density stratification in an iron-meromictic lake

摘要

In many mining regions on earth, pit lakes have been indentified where a permanent stratification creates an anoxic reductive deep water layer, the monimolimnion, with increased concentrations of dissolved gases and undesired ionic substances, e.g. heavy metals. We investigate one of the processes responsible for permanent stratification formation, the cycle of precipitation and re-dissolution of iron species due to changing redox conditions in different water layers. Hence, evaluating the relevant processes in these so called iron-meromictic lakes on their impact on density stratification helps to predict future water quality and to forecast potential turnovers.We present an approach of quantifying the impact of iron-redox reactions on lake stability of a small well documented iron meromictic Lake called Waldsee in the lower Lusatian Mining district by summing up in-situ measured vertical electrical conductivity profiles. We used the derived "summed conductivity" as a measure reflecting the amount and seasonal mass-balance dynamics of electro-active substances in the lake. Furthermore we conducted a unique aeration experiment to physically reproduce the assumed chemical reactions in the lake.Our findings showed that calculating "summed conductivity" revealed a seasonal oscillation in phase with the chemocline depth which indicates the transition zone between the two water layers. However, induced stratification was sustained throughout the observation period of four years. Mass-balance calculations revealed that the internal iron redox cycle alone was able to maintain the conductivity gradient. However, an inflow of significant amounts of ion rich groundwater seemed to be needed for the recovery of summed conductivity during times of rising chemocline in Waldsee. Aeration of deep water from the lake by the experiment lead to a change in water properties close to surface water layer properties of Waldsee and hence confirmed the occurrence of internal geochemical processes: iron removal, pH depression, DOC removal and CO_2 loss.