Date of defense :
01 December 2004
Thesis Supervisor(s) :
van der LEE Jan
Ecole Doctorale :
GEOSCIENCES ET RESSOURCES NATURELLES
Summary : In order to maintain navigation channels, periodic dredging of the bed sediments is carried out. In industrialised areas, these sediments may have been contaminated by discharged effluents. Therefore, significant quantities of contaminated sediments may be deposited, generally on the banks of the watercourse. Consequently, physicochemical alteration of these sediments, especially oxidation, plays a significant role in determining the mobility and hence eventual distribution of these contaminants. These mobilised toxic elements may represent a hazard for the soils and the aquifers in the vicinity of the deposited sediments. The objective of this study was to characterize and model the mechanisms responsible for the mobilization of the inorganic pollutants in the studied sediment. Both laboratory experiments and field studies were performed.
The laboratory experiments consisted of performing leaching test under controlled and simplified conditions, on sediments contaminated with Pb, Zn and Cd, sampled after 5 years of ageing. To characterize both the mobile components (major elements and metals) and the mechanisms controlling their mobility, the leaching tests were performed as both static tests in the form of batch kinetic desorption tests and dynamic tests. The dynamic tests were performed as both batch, in the form of shaking cascade test and in column in order to account for the hydrodynamic factors. All the tests found that zinc and cadmium were reversibly bound to the sediment and underlined the importance of calcium concentration in determining their mobility by ionic exchange. After characterising both the composition of the sediment and the mechanisms controlling the metals motilities, a conceptual model of the sediment was devised in order to simulate the experiments. The model accounted for buffering capacity, mineral dissolution kinetics and ionic exchange. Simulations performed using the geochemical code CHESS gave good agreement with the results obtained from the batch experiments. The reactive transport code HYTEC was used to simulate the column experiments, using the same reaction parameters as for the batch simulations.
The field studies were primarily related to the characterization of the sediment deposit. The thirty year old deposit contains a subsurface layer of very highly contaminated materials (Pb, Zn, Cd, As). On the surfaces of the deposit, oxidation of the sediment with precipitation of secondary phases such as carbonates and sulphates as well as (hydr)oxides is evident. However, due to the thickness of the deposit, the major phases of the system are sulphides. A piezometric study of the deposit identified three zones, differing in their texture, total metals content, hydrochemistry and hydrogeology. From these data in combination with geochemical modelling, it was possible to determine the mechanisms responsible for controlling the mobility of the metals. In order to evaluate the environmental impact of the sediment deposits the study also considered the subjacent horizons and aquifer. The high sorption capacity of the subjacent horizons was fount to be responsible for the retention of the investigated metals (Pb, Zn, Cd). Simulations of both saturated and unsaturated zones were performed by HYTEC to reproduce the site hydrogeology, coupled with a geochemical model accounting for ionic exchange and mineral precipitation. These simulations were used to predict the infiltration of metals into the subjacent aquifer. Moreover a survey of this aquifer was set up and confirmed the absence of contamination. Lastly, hydrogeological simulations were performed to investigate tracer dispersion in the aquifer, accounting for the local hydrogeology.
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