Abstract :
[en] In the context of climate change, increasing water demand, and continuous anthropogenic pressure on groundwater resources, many aquifers are experiencing long-term declines in groundwater levels, threatening the sustainable state of these freshwater systems. Notably, a persistent groundwater deficit has been observed over recent decades in the Hesbaye chalk aquifer (Liège, Belgium), which plays a strategic role in drinking water production. In response to this situation, it is essential to evaluate mitigation measures aimed at limiting the impacts of climate change on groundwater resources. In this regard, managed aquifer recharge (MAR) has emerged as a promising adaptation strategy to enhance groundwater resilience by artificially increasing recharge during periods of water availability. However, setting-up effective and sustainable MAR operations requires a careful assessment of site-specific constraints, as its technical feasibility depends not only on target recharge volumes, but also on subsurface infiltration capacity and the chemical quality of the recharge water.
Urban runoff is increasingly considered as an alternative water source for MAR, as it provides significant and spatially distributed volumes that are often already collected by existing infrastructures. However, these waters may contain a wide range of contaminants originating from traffic, industrial, and operational activities, raising concerns regarding groundwater protection. While water quality considerations are commonly integrated in MAR studies, most investigations focus primarily on nutrients, metals, or conventional organic pollutants. Contaminants of emerging concern (CECs), and notably per- and polyfluoroalkyl substances (PFAS), are relatively less documented in the context of MAR, despite their persistence, mobility, and toxicity. As a result, the potential impact of MAR operations on the transfer of contaminants from runoff toward groundwater remains insufficiently studied, highlighting the need for a more comprehensive qualitative assessment of runoff-based recharge.
From a hydraulic efficiency perspective, most existing MAR studies focus on highly permeable geological settings such as alluvial or sandy aquifers, where infiltration capacity is not a limiting factor. Fine-grained sediments such as loamy soils are therefore generally excluded from classical MAR schemes due to their low permeability. In practice, MAR operations in regions dominated by such sediments rely on infiltration through low-permeability surface materials, whose hydraulic behaviour under MAR conditions is still insufficiently documented at the field scale. This limits the applicability of existing MAR concepts in fine-grained sediments settings and requires dedicated investigations of recharge processes in such geological background.
This thesis aims to address these gaps by evaluating the feasibility of MAR in the Hesbaye region from both quantitative and qualitative perspectives, with a particular focus on urban runoff as a potential recharge water source. An integrated approach combining field investigations, hydrogeochemical analyses, laboratory experiments, and numerical modelling was developed to assess recharge potential, contaminant occurrence, attenuation mechanisms, and associated risks.
From a quantitative perspective, infiltration rates through loess deposits representative of the Hesbaye plateau were assessed at a pilot infiltration basin. Field experiments combining conventional monitoring techniques and distributed temperature sensing with fiber optics (DTS-FO) allowed the estimation of infiltration rates under operational conditions, demonstrating that effective recharge can be achieved in loess sediments. At the regional scale, potential recharge water sources were identified and quantified, highlighting stormwater as a significant and spatially distributed resource, particularly in relation to traffic and airport infrastructures.
From a qualitative perspective, an extensive hydrochemical characterization of selected stormwater basins was conducted across multiple seasons to capture climatic and activity-related variability. Results reveal that runoff water quality is highly variable and reflects the influence of both traffic and other site-specific operational activities. Classical pollutants were generally present at moderate concentration levels, while CECs — including benzotriazoles, alkylphenols, and PFAS — were frequently detected and exhibited clear seasonal patterns linked to site-specific operational activities.
To investigate the fate of these contaminants during infiltration through loess sediments, laboratory batch experiments were performed using representative Hesbaye loess samples. The results demonstrate contrasting attenuation behaviours between contaminant families. Benzotriazoles and alkylphenols showed partial attenuation through sorption and biodegradation processes, whereas PFAS showed strong persistence and limited attenuation, with sorption potential decreasing for short-chain compounds.
Finally, a two-dimensional numerical model coupling variably saturated groundwater flow and reactive solute transport was developed to simulate MAR operations under realistic wetting–drying scenarios, while integrating field and laboratory results. Modelling results confirm that MAR can significantly enhance groundwater recharge even through fine-grained sediments, but also demonstrate that contaminant transfer is strongly controlled by unsaturated zone processes, CECs hydrogeochemical properties, and MAR operational conditions. In particular, short-chain PFAS emerge as the most critical contaminants in terms of groundwater vulnerability under MAR operations due to their persistence and high mobility.
Overall, this thesis demonstrates that MAR using urban runoff could contribute to the mitigation of groundwater depletion in the Hesbaye region, including in geological contexts traditionally considered unfavourable for recharge. At the same time, although the results indicate that natural attenuation processes during the infiltration through the loess layer is effective for many contaminants, they may be inefficient for persistent and weakly sorbing compounds, such as short-chain FPAS. These findings motivate the need to integrate appropriate pre-treatment strategies into MAR schemes when urban runoff is used, to avoid any groundwater contamination risks. By addressing underexplored geological settings and emerging contamination risks in the context of MAR, this work provides new insights to inform the safe design of MAR while explicitely accounting for associated groundwater quality risks, therefore contributing to sustainable groundwater management under future climatic conditions.
