Assessment of metal/metalloid occurrence in rivers with their accumulation in macrophyte case study with Myriophyllum alterniflorum.

Water quality monitoring with integrative tools is a main issue of concern for environment assessment. Submerged aquatic macrophyte can be a good candidate for the evaluation of contaminant content in rivers. Indeed, owing to their habitat, aquatic macrophytes interact directly with surface water; they can absorb contaminants and thus allow to detect their presence in water. In situ studies were conducted over 28 days in five aquatic environments, affected by different levels of anthropogenic pressure (domestic wastewater plant, industrial activities), during two field campaigns. We have investigated whether the accumulation of some metals and a metalloid (As) in Myriophyllum alterniflorum could be used to detect their occurrence in river waters. Our results demonstrated that long time bioaccumulation was correlated with the contaminant levels in water. However, the water composition and the duration of exposure affected the studied pollutants' absorption. On an operational point of view, the optimal duration of exposure of Myriophyllum alterniflorum to assess the water quality is conditioned by the contaminant contents in waters that can induce different defense mechanisms as the reduction of pollutant absorption and their efflux. In addition, the nutrient concentration influenced the accumulation of pollutants since the higher the nutrient level, the higher the essential metal accumulation was observed.

Key parameters influencing metallic element mobility associated with sediments in a daily-managed reservoir.

In a hydroelectric reservoir, sediments are subject to remobilization events, water-level fluctuations and physicochemical changes. Depending on their associated metallic content, surficial oxic to suboxic sediments could constitute a major source of metals. To identify the key parameters that control metallic elements in terms of their mobility and sensitivity to reservoir management, sediments were subject to resuspension and drying/wetting cycle experiments over a wide range of pH values, solid/liquid ratios (S/L) and redox (Eh) conditions. During these tests, special attention was also paid to the influence of pretreatments on samples, i.e., drying, aeration and the leachate composition (ultrapure water vs. natural water); on the preservation of the sediment characteristics; and especially on metallic element release. The results of this study show that the pH, S/L ratio and Eh parameters are key variables in metal solubilization; the pH influences metal mobility primarily through sorption-desorption phenomena as well as the dissolution of metallic-bearing phases, the S/L ratio modifies the sorption-desorption equilibria, and the Eh primarily affects the reducible sensitive phases and associated metallic elements through dissolution-precipitation processes. Under environmental conditions, evolution of these parameters can lead to a >20% solubilization of the most mobile elements, i.e., As and Cd. These results are influenced by the sample pretreatment and experimental conditions. In fact, even if the solubilization patterns show no significant differences between dry and wet sediment depending on the physicochemical conditions, the magnitude of their release is significantly affected. Drying pretreatment induces changes in metal speciation, notably altering the distribution of the most weakly bound elements; there is almost half the amount of metallic elements associated with the exchangeable fraction in dry compared to wet sediments. The solubilization percentages were higher in the ultrapure phase than in reservoir water primarily due to the low pH, which influenced the sorption equilibria.