Chemical characterization of urban waters aimed for managed aquifer recharge in the Hesbaye chalk aquifer (Liège, Belgium)

Managed Aquifer Recharge (MAR) is a viable method that has gained recognition for storing alternative waters in aquifers for subsequent recovery or environmental benefits. It has the potential to increase the supply of fresh water and protect aquifers from overexploitation and degradation, but it might also carry the risk of contaminating groundwater since the recharge water used may contain a wide range of organic and inorganic contaminants. Therefore, it is important to carefully assess the quality of these alternative sources of water (such as runoff water) used for MAR and implement appropriate treatment measures to remove or neutralize any contaminants that may be present. The purpose of this research is to conduct a preliminary feasibility study of MAR as a potential mitigation measure in the Hesbaye chalk aquifer since this major source of drinking water for the region of Liège (Belgium) is threatened both in terms of quantity and quality. In the first phase of the study, the quality of runoff waters collected from stormwater basins along national roads and in a national airport area was analysed and certain contaminants of emerging concerns were detected at concentrations close to drinking water limits or environmental safety guidelines. In particular, contaminations with PFAS compounds have been detected in stormwater basins in the airport area with maximum values reaching up 490, 330 and 250 ng/L for PFECHS, PFPeA and 6:2 FTS respectively. Other contaminants of emerging concerns such as alkylphenols and organophosphate flame retardants have been detected as well. In a second phase, estimates of expected recharge rates were determined through in-situ experimentation using a small infiltration pond with a pressure sensor and innovative active-DTS measurements with buried optical fiber cables to monitor the infiltration of water into the loess (eolian loam) sediments overlaying the Hesbaye chalk aquifer. Finally, these input data have been used to perform 1D transport modelling simulations in order to make a preliminary evaluation of the risk of groundwater deterioration in the case where these raw runoff waters are infiltrated without pre-treatment. Column infiltration tests are planned to get a better estimation of the soil attenuation capacity in the unsaturated zone. This study is unique in that i) it explores the feasibility of MAR in a country in which the method is not well-developed yet, ii) the use of airport runoff water as a potential source of recharge water is novel and has not been widely examined in previous MAR studies and iii) aquifer-soil treatment in loess sediments overlaying a chalky fractured aquifer is a unique hydrogeological setting to perform MAR operations.