Changes in metal biogeochemistry resulting from wetland creation: bioavailability, toxicity and risk "WETMAT": final report
Tack, F.; Janssen, C.; Meire, P. (2006). Changes in metal biogeochemistry resulting from wetland creation: bioavailability, toxicity and risk "WETMAT": final report. Belgian Science Policy: Brussel. 83 pp.
In the vicinity of rivers, flooding of available lowlands can be considered as a possibility for temporal storage of river water during high water regimes to minimize the flooding danger of inhabited areas. The creation of wetlands for controlled flooding may also contribute to an increased ecological and biological value of the ecosystem area. Due to industrial activities, river water, sediments and soils are often contaminated with metals. The aim of this study was to determine the changes in the metal biogeochemistry under different hydrological conditions and in the presence of reed plants and how this affects the uptake of metals by plants and invertebrates. Two experimental setups were used. Lab scale experiments consisted of 42 barrels filled up with three different soils and flooded with water of different salinities. For the field scale experiment, 4 tanks filled with two soil types were placed on a platform in the river Scheldt and flooded daily by river water. Results showed that the metal mobility did not depend on the total metal content. Other factors such as soil pH, CEC, redox status and carbonate, OM and clay contents however play an important role in mediating metal behaviour. To prevent the mobilization of the metals, it is advisable to reduce redox variations in the soil. The two-year period of the experiment was too short to detect any measurable changes in most of the soil properties. The measured metal (im)mobilization kinetics can however already be incorporated in models in the near future. These models should allow to predict metal mobility changes at the oxic-anoxic interface upon changing hydrological conditions, where the already existing geochemical models fail due to the fact that they are based on steady state conditions. In the two-year experimental period, large quantities of metal contaminated sediment were found to be deposited on the soils of the tanks which were placed in the river Scheldt. This sedimentation could pose a problem, as it minimizes the water storage capacity of a flood control area. Moreover, the contamination status of alluvial soils should be used as a criterion when selecting new flood control areas as uncontaminated flood control areas are expected to be contaminated when polluted sediments originating from the surrounding surface waters are being deposited or when the soil is contacted with metal-polluted surface water. The latter can be expected in the future, as upon increasing oxygen contents of the river waters due to decreasing organic matter loads, metals can be mobilised from metal-polluted river sediments. Reduction of metal mobility upon increasing oxygen contents of river waters therefore needs further investigation. The metal accumulation in benthic organisms like T. tubifex was found to strongly depend on the inundation period. A higher Zn and Cu accumulation was recorded when substrates were allowed to oxidise. For the talitrid amphipod O. gammarellus, no clear influence was noted. Photosynthetic performance of the reed plants seemed to be mainly determined by other factors than merely metal contamination levels. There was however a possible relation between the abundance of stress enzymes and the metal contamination, making it a more valuable parameter to assess the effect of metals on reed plants in wetlands. The reed itself did not have any measurable effect on the concentration of the metals in the soil or pore water. Also no differences in the concentrations of metals in the reed biomass or differences in the reed growth were observed between the contaminated and uncontaminated soil.
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