In Wallonia (south part of Belgium), approximately 80 % of drinking water supply is provided by groundwater catchments. Many of these are located in rural areas and are therefore most often threatened or impacted by pollutants of agricultural origin, such as nitrates and pesticides. However, another significant portion of groundwater catchments are in urbanised environments (residential areas, economic activity zones, proximity to waterways, etc.). These peri-urban areas can be associated with a wide variety of pollution sources ranging from accidental punctual spills to more continuous pollution, hidden and sometimes larger-scale pollution linked to faults/leakage in drainage networks, known or unknown landfills, treated or untreated housing and industrial wastewater, storage systems for products for private or industrial use (fuel oil tanks, etc.). These catchments may also interact strongly with superficial watercourses. The problem linked to the complexity of those urban/sub-urban catchments points out the diversity of land use occupation reflected in the relevant diversity of pollution sources linked to that.

In this context, the CASPER project aims to develop an integrated operational methodology to optimise the protection of groundwater resources and water catchments from urban pollution in their recharge areas. More specifically, this requires (1) identifying their origin(s) which is (are) not always known (2) characterising their composition and their importance in terms of pollutant load, (3) assessing the current and future risks for the environment, (4) assessing the current and future risks they represent for the catchments and, as a result, the need for remediation. To do so, firstly the challenge is on being able to discriminate the different pollution sources using a combination of specific tracers and approaches (such as isotopes ratio (Nikolenko et al., 2018; Widory et al., 2005), occurrence of pharmaceautical substances (Neufcourt, 2017), Gd analysis (Boester and Rüde, 2020; Petelet-Giraud et al., 2009) etc.). And then, on being able to build a decision-making reference system to prioritize pollutions and concerned remediation measures.

This project takes place in the well-known context of the modification of the “urban water cycle”, mainly influenced by the increase in the number of impermeable surfaces due to the construction of houses, traffic lanes, car parks, etc.… and the increase in – antropic sources (drinkwater supply networks – domestic water consumption, despite the use of water-efficient machines, due to the number of households/house and comforts like swimming pools and gardens) and sewage. Nowadays there are many different inputs and outputs interacting in the cycle (Fig. 1).

The suburban context is more challenging because it presents many different pollution sources and contaminants (Fig. 2): the ones typical of an urban context, the ones which characterize the industrial activities and the ones linked to agricultural practice.

Having in mind that complexity in terms of contamination, it is unrealistic to talk about ideal conservative tracer unique to a specific source and pathway. The same substance in fact could have many different origins (the nitrate, for example, could be from agricultural practices and uses and from human contamination). Therefore, nowadays has been proven the better efficiency of using a multi-component and multi-tracer approach to distinguish between mixtures of pollutants.

The first pilot site chosen for the project is a case of set of 4 abstraction wells located in Boussu, in the western Wallonia, in Belgium. The wells pump water from a semi-confined chalk aquifer: partly unconfined when it is outcropping, and partly covered by alluvial deposits and sandy-clay layers. The groundwater is flowing from S/SE to N/NW in the investigated portion of the chalk aquifer, and, on the basis of that observation and the historical data available on the site, a list of possible contamination sources is made (Fig. 3a). The network of monitoring wells and surface waters to be checked and studied in qualitative and quantitative terms, is determined afterwards, in function of the suspected contaminations and the potential pollution sources' locations. Investigations are limited to the so-called CAA (Catchment Area of the Abstraction) corresponding to the perimeter of the surface in which abstracted groundwater is recharged, and in particular to the PAFA (the Portion/Part of Aquifer Feeding the Abstraction) given the fact that the extensions were only linked to some safety reasons concerning the nature of surface waters flowing in the site. Those areas are already much larger than actual determined prevention zones (Fig. 3b).

Three different sampling campaigns were planned for a semi-circular panel of investigation in the W-S-E sides of the sites, in both aquifers, the chalk (points represented by circles in Fig. 3b) and the upper sand aquifer (triangle-shape points), and also in the surrounding surface water which might be infiltrating in the area (square-shape). Every time the surveys were more focused (and displaced) on the most impacted areas and sides of the site.

In the map it is possible to notice how the 1st and 2nd campaigns were more extended in the site, and samples were taken from surroundings surface waters and both chalk and sand aquifer. While the 3rd campaign mainly concentrates on the south-east side of the site, and only investigates the chalk aquifer.

The analyses performed are:

Physical parameters (ph, redox potential, dissolved O2, electrical conductivity and temperature);

Table 1 summarises the details of the procedures (filtration, preservative and conservation, type of bottle) for the analyses chosen and indicates, for each, the laboratory in charge.

Stable isotopes of nitrate and boron are done to mainly distinguish urban effluents from agricultural fertilisers and manure (Nikolenko et al., 2018; Widory et al., 2005). Stable isotopes of sulphate are analysed to distinguish anthropic activity, sulphide minerals oxidation, sulphur-carbon compounds mineralisation, lixiviation, dissolution of evaporates, or alteration of carbon mines waste (Knöller et al., 2011). Those results are coupled with the occurrence of pharmaceutical (carbamazepine, etc.) and lifestyle (caffeine, nicotine) substances to identify anthropogenic contamination (Neufcourt, 2017) or agricultural pesticides-products and manure contamination substances; and also, with the occurrence of Gd (REE) which is used as a contrast agent in magnetic-resonance imaging, therefore it is suitable to verify contamination by anthropic wastewater and hospitals (Boester and Rüde, 2020; Petelet-Giraud et al., 2009). Furthermore, isotopes of chlorinated solvents are studied to understand if natural attenuation/degradation and/or other biochemical reactions are happening, and finally try to get closer to the location of their point source/origin (Åkesson et al., 2021). Microbiological and bacteriological analysis are also undertaken to make an inventory of all the microbial/bacterial species present in the samples, identifying populations associated with specific sources of pollution and verifying hypothesis on the occurrence of specific biochemical reactions (under peculiar conditions) in soil and groundwater, such as denitrification, sulphur reduction/oxidation, chlorinated solvents degradation, etc. (Kanohin et al., 2018; Krumar et al., 2014). Thus, first links between the abundance of different bacterial species and their affinity/resistance to some minerals/substances/conditions could be studied. The methodology used for the microbiological analysis is based on molecular taxonomy (linked to the 16S RNA structure). The procedure's steps are: (1) extraction of the total DNA on a filter; (2) amplification of the genetic sequences that constitute the intended target, (3) identification of the bacteria present on the basis of existing libraries, (4) identification of the most similar samples by nonmetric dimensional scaling.

Interpretation using multivariate methods and clustering (e.g.: SOMs, t-SNE, PCA, …) will be done later. The aims would be (1) to confirm some hypothesis already elaborated looking at the spatial distribution of measured concentrations of pollutants and the localisation of different contamination sources, and (2) the classification/subdivision of the sampled points in groups linked to their behaviour in the subsoil and the land use.

The main pollutants impacting the quality of the groundwater abstracted are: NO3, SO4 and Chlorinated Solvents. For each one of these pollutants, some specific analyses are performed in order to try to be able to differentiate their origins.

To briefly show some of the first results concerning the analysis cited above, following there are few graphs related to the 2021 summer sampling campaign (no. 2), which, when compared to the results obtained in autumn 2020 (campaign no. 1), show very similar trends, both in terms of major elements, specific substances occurrence and isotopes quantification. Figure 4 is showing the isotopes of nitrate and boron, as a function of NO3 concentrations (mg L-1). Groundwater chalk's samples are located among the manure, agricultural contamination (non-organic fertilisers) and atmospheric deposition boxes, which confirms that the nature of their nitrate concentrations is rather linked to agricultural origin. While, the surface water points fall into the wastewater box, which confirms their nature as sewers. Finally, there is only one point, corresponding to one of the abstraction wells, which is shifted to the right and that indicates possible denitrification (which might be confirmed by the increasing presence of nitrite and manganese and decreasing content of dissolved oxygen).

Sulphate stable isotopes results (Fig. 5) show that the majority of the groundwater samples related to chalk aquifer are located between the boxes of sulphide oxidation, the presence of slag-heaps (located all around the site, especially in the SE side, where the chalk is outcropping) and the mineralisation of carbon and sulphur in the soil. The points that are most excluded are those of surface water and sand aquifer directly linked to the landfill present in the site: in fact they show a rather anthropogenic origin of SO4 contamination (mainly from the residential area east located). Finally, there are no obvious trends of bacterial sulphate reduction at this stage.

The pharmaceutical substances are also analysed: a selection of few substances for each of the 4 categories analysed using their occurrence mainly as a proof of hypothesis contamination's origin (Fig. 6). Those results will be also coupled with Gd occurrence and anomalies, once the data will be available.

There are no remarkable trends shown by the concentrations and spatial distributions, but few observations are pointed out:

The 4 pumping wells together with the closer piezometers do not have any “life-style” substances.

Chlorinated solvents are mainly found in the chalk aquifer samples, with the highest concentrations measured in the east side of the site. Therefore, the source of that pollution seems to be located to the east, very close to a new hospital/ancient fuel station. The spring 2022 campaign shows a focus on the wells located closer to the area where the source is supposed to be and where possibly some degradation reactions are happening. To go further on this interpretation, analysis on gradient concentrations, molar ratio, plume behaviour and isotopes of Carbon, Hydrogen and Chloride will be performed, together with more research of historical and possibly former pollutants activities impacting the site.

Data from the 3rd sampling campaign (spring 2022) are still not available. Microbiological analysis results are also not showed in this manuscript, because interpretation are still going on.

The approach illustrated is a rigorous methodology to assess pollution sources. The analysis and interpretation done until now are just part of the investigations which are still going on.

Clustering and multivariate analysis will be performed on the chalk aquifer, to prove some of the previous reflexions and while collecting more information on how to group and categorized the points of the area while looking at the collected data.

A hydrogeological model will be optimized to simulate present/future contamination from the known/discovered sources of pollution.

Finally, to confirm the hypotheses, especially on the origin of sulphates and chlorinated solvents, additional field investigations must be carried on: an expansion of the sampling network at the level of the chalk is planned (drilling of new piezometers, use of additional wells in the proximity of the detected possible sources) together with the intention to measures the groundwater fluxes via FVPDM technique (Brouyère et al., 2018). The fluxes measures will also help in the hydrogeological model calibration and optimization and will be coupled with hydrogeochemical results.

As last, the development of useful tools to prioritize investigation and remediation measures should be generalized from that specific case to a more general one, in order to try to have a referential procedure to follow upon qualitative and quantitative studies in an aquifer.