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The major objective of this study is to investigate simplified risk assessment models for pollutant leaching to and migration across groundwater. Data for this study were gathered through bibliographic research and experts’ advice. Three European countries were selected for this investigation: Italy, United Kingdom and Walloon Region. In the first part, the procedures are compared from a theoretical point of view. The Thesis then identifies the tools used by countries and applies them on a synthetic and real case. A sensitivity analysis is performed as well. The cases have highlighted differences in terms of decision-making and shown which parameters mostly affect results. Basing on the results of this research, it can be concluded that all the countries perform Concentration based risk assessment relying on simplified analytical equations and that the way of assessing risk is quite similar. Major differences could be noticed in the choice of factors and relative adjustments for modelling leaching in the vadose zone and in the way to obtain the remedial objectives. Types of solutions for modelling the transport of contaminant across groundwater differ as well. The synthetic case study brings to light some noticeable aspects. Firstly the difficulty in choosing parameters, particularly for the saturated zone, so as to respect the mass balance between saturated and unsaturated condition. Secondly, the fact that some factors, used by countries to simplify the movement of pollutants (i.e. dilution factor), actually have a great influence on results. Therefore, their physical consistency and reliability should be further investigated by comparing results of traditional RA tools with numerical models. Finally, the sensitivity analysis has shown that Mass flux approaches may bring additional contribution to the way the presence of Risk is assessed and that research should evolve in this direction.

Groundwater --- Concentration based Risk assessment --- analytical equations --- leaching --- migration --- Synthetic case --- Remedial objectives --- Mass flux --- Ingénierie, informatique & technologie > Géologie, ingénierie du pétrole & des mines

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In the scope of polluted site management, the heterogeneity of the subsoil is an important factor
to take into account in the sizing of a remediation plan. This thesis aims at assessing the heterogeneity of an alluvial aquifer through the use of push-pull tests. Push pull tests are single-well tracer tests consisting in the injection of a tracing substance in the aquifer ("push"), followed by the extraction of the mixture of tracing substance and groundwater from the same well ("pull").
These tests are especially suited to polluted sites considering their ease of implementation and
the small volume of potentially polluted water involved. Four tests were performed, during which a potassium chloride tracing solution was injected and then withdrawn from the aquifer. The differences between the tests was either the resting time that separated the injection and extraction phases, the injection rate or the injected volume. The influence of the resting period, of the injected volume and of the injection rate were studied. The mass recovery ratio was computed for each test and ranged between 70 and 80 %, ensuring a good representativity of the results. An analytical expression of a retardation coefficient was established, taking into account the natural flow in the aquifer. This expression is based on the recovery of the centre of mass of the tracer. Considering that the chosen tracer does not sorb on solid surface and that the dispersion effect is minimised in push-pull tests due to the flow reversal, the computed retardation highlights the importance of diffusion or immobile water effects. Indeed, the tracer recovery will be delayed as mass exchanges develop between high permeability zones and immobile water zones. Theses effects will be greater as the heterogeneity in the medium is important. The estimated retardation coefficients were comprised between 1 and 2.3 for the different tests but are expected to stabilise to a value of 2.4 as the elapsed time between the injection and extraction period increased. The stabilised value of the retardation is expected to correspond to the reaching of the equilibrium between the tracer concentration in high and in low permeability zones. The retardation coefficient is however highly dependant on the estimation of several parameters that are strongly correlated: the pore water velocity, the effective porosity and the dispersivity coefficient. Several propositions to make these estimations more robust have been presented.

Push-pull tests --- Single well injection-withdrawal tests --- Single well tracer tests --- Heterogeneity --- Pollutant mass fluxes --- Site remediation --- Alluvial aquifer --- Ingénierie, informatique & technologie > Géologie, ingénierie du pétrole & des mines