Persistence of heavy metal in river sediment

The first objective of this work is the assessment of the heavy metals accumulation and equilibrium phenomena in river sediment, in relation to the state of water quality. The variables necessary to estimate correctly the solid/liquid partition coefficient (Kd) are also defined, because it is a key parameter for evaluating the risk associated with contaminated sediments. The second objective is to assess metals lability, analyzing whether and how easily heavy metals are available to generic biota. The Diffuse Gradient in Thin film technique was used to asses the Effective Concentration (CE). The coefficient of solid­liquid distribution of the metals labile pool (Kdl) and the characteristic time for the system to approach equilibrium (Tc) were calculated to include them in risk analysis inside a more realistic frame. The Esino River was chosen as case study. The results confirmed the necessity to determine experimentally Kd: its values available in literature, widely used in risk analysis, resulted incorrect up with some orders of magnitude. The results confirm that both Kdl and Kd depend on Chemical Oxygen Demand and Specific Superface Area, although following different lows. Metals speciation, environmental conditions, type and texture of the sediment, water features, organic fraction, pH and salinity can influence the outcome. Dynamic speciation shows that although water quality was good, the risk associated with the metal mobility might be still high. Consequently, the concentration CE, which takes into account both the contribution of the water and the labile fraction in the sediment, exceeds from 8 to 100 times the concentration in the pore water and relatively hight values are more common in areas typically considered "clean".