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Dissertation
Hydrogeophysical Characterisation of an Aquifer Area in Spa, Belgium
Authors: --- --- --- ---
Year: 2021 Publisher: Liège Université de Liège (ULiège)

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Abstract

The region of Spa has been well-known for the quality of its waters for a long time. This region is still a vast resource for groundwater nowadays. Prospection for water resource has been guided by local geological knowledge and the use of drillings to identify suitable spots for water production. Geophysical methods were implemented in order as a time-saving and cost-efficient method I order to guide the location of future drillings. 
This work focuses on the implementation, processing and interpretation of the electrical resistivity tomography (ERT) geophysical method. This method measures the underground response to an electrical current passing through. This response, known as electrical resistivity, is essentially linked to the abundance of clay material in the underground, the interconnectivity of the solid grains within the rocks, and, most remarkably, to the amount of free water.
This study takes place in the cambro-ordovician context of the Stavelot-Venn Massif. The field of investigation is divided into two distinct geological regions: on the north-east lies the first district, called Area 1, characterized by quartzite-rich environment; on the south-west lies the second area, called Area 2, where clay-rich geological materials such as clay and phyllite are abundant. Both regions have been affected by two orogenetic episodes and carry the scars of such events. These take the form of faults and folds, as well as various degree of fracturing within rock layers. Features related to faults and intense fracturing are key to the water production prospect. In this region, faults act as lifts for deep waters, and fractured layers may form suitable water reservoirs.
ERT is particularly appropriate in this context for it shows strong contrasts of between massive rock layers and free waters. Faults and highly fractured area bearing water appear clearly on profiles. Some ambiguities may still appear due to occasional artifacts and loss of resolution, particularly at depth.
The method was implemented in four distinct regions. The first region, PleinFays, revealed the presence of two faults in a transition area between the clay-rich Area 1 and a quartz-rich area in the south-east. The second region, Américains, revealed the presence of a fault deep enough to be explored for water production. The third region, Belle Hé, revealed a deep area that could correspond to a water reservoir, but suffers from ill-resolution. The fourth area, Fontaines, revealed the presence of a fault that could be exploited for water production as well.
Eventually, ERT proved to be an efficient tool to identify geological structures related to water production. Further research could be carried out in order to map the horizontal and vertical expansion of identified faults.


Dissertation
Push-Pull tests for the characterization of heterogeneity in alluvial aquifers
Authors: --- --- --- --- --- et al.
Year: 2018 Publisher: Liège Université de Liège (ULiège)

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Abstract

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.

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