Emilie Garel

Strontium isotopes as tracers of water-rocks interactions, mixing processes and residence time indicator of groundwater within the granite-carbonate coastal aquifer of Bonifacio (Corsica, France)

This study aims at identifying the water-rock interactions and mixing rates within a complex granite-carbonate coastal aquifer under high touristic pressure. Investigations have been carried out within the coastal aquifer of Bonifacio (southern Corsica, France) mainly composed of continental granitic weathering products and marine calcarenite sediments filling a granitic depression. A multi-tracer approach combining physico-chemical parameters, major ions, selected trace elements, stable isotopes of the water molecule and 87Sr/86Sr ratios measurements is undertaken for 20 groundwater samples during the low water period in November 2014. 5 rock samples of the sedimentary deposits and surrounding granites are also analysed. First, the water-rock interactions processes governing the groundwater mineralization are described in order to fix the hydrogeochemical background. Secondly, the flow conditions are refined through the quantification of inter aquifer levels mixing, and thirdly, the kinetics of water-rock interaction based on groundwater residence time from a previous study using CFCs and SF6 are quantified for the two main flow lines. A regional contrast in the groundwater recharge altitude allowed the oxygene-18 to be useful combined with the 87Sr/86Sr ratios to differentiate the groundwater origins and to compute the mixing rates, revealing the real extension of the watershed and the availability of the resource. The results also highlight a very good correlation between the groundwater residence time and the spatial evolution of 87Sr/86Sr ratios, allowing water-rock interaction kinetics to be defined empirically for the two main flow lines through the calcarenites. These results demonstrate the efficiency of strontium isotopes as tracers of water-rock interaction kinetics and by extension their relevance as a proxy of groundwater residence time, fundamental parameter documenting the long term sustainability of the hydrosystem.

Characterization of the aquifers of the Bangui urban area, Central African Republic, as an alternative drinking water supply resource

Abstract This paper presents the results of a survey carried out in 2010 aimed at evaluating the type and quality of the groundwater resources of the Bangui region of the Central African Republic. This work is the first step towards the development of groundwater resources in the Central African Republic in order to find alternatives to direct pumping from the Ubangi River and provide the population of the suburbs with a safer drinking water supply from deep boreholes. By combining both geological and hydrogeochemical approaches, it appears that the geology of Bangui is favourable to the development of a secure and sustainable water supply from groundwater provided that the conditions of exploitation would be constrained by the local authorities. The deep Precambrian carbonate aquifers, known as the Bimbo and Fatima formations, are identified as target resources in view of the relatively good quality of their water from the chemical point of view, and the semi-confined structure of the aquifers that prevents the mixing with shallow aquifers that are already strongly affected by domestic and industrial pollution. The main difficulty in terms of exploitation is to appreciate the depth of the resource and the more or less fractured/palaeo-karstified type of the porosity. Editor Z.W. Kundzewicz Citation Djebebe-Ndjiguim, C.L., Huneau, F., Denis, A., Foto, E., Moloto-a-Kenguemba, G., Celle-Jeanton, H., Garel, E., Jaunat, J., Mabingui, J., and Le Coustumer, P., 2013. Characterization of the aquifers of the Bangui urban area, Central African Republic, as an alternative drinking water supply resource. Hydrological Sciences Journal, 58 (8), 1760–1778.

Origin and recharge mechanisms of groundwater in the upper part of the Awaj River (Syria) based on hydrochemistry and environmental isotope techniques

The Barada and Awaj basin is the most important and extensively used water basin in Syria. Chemical and isotopic data of groundwater have been used to determine the spatial distribution of hydrogeological features in the upper part of Awaj River catchment area located southwest of this basin. Hydrogeochemical evolution of groundwater reveals the domination of dissolution/precipitation mechanisms in these very complex stratigraphic sequences. The dissolution of carbonate rocks as well as reverse cation exchange processes seem to be the main factors controlling groundwater mineralization. The isotopic composition of precipitation and groundwater indicate that the modern-day atmospheric precipitation is the main source of groundwater recharge before an important evaporation occurred. The isotopic data also imply an existence of hydraulic connection between the different aquifers system. The results obtained allowed us to delineate two main spatial groundwater zones within the study area with different flow components. The south, central and eastern parts are considered to be one zone which is characterised by a shallow horizontal flow associated with active interaction between groundwater and hosting rocks. The nitrate concentrations in this zone are attributed to anthropogenic sources. The second zone consists in south-western, western and north-western parts where the karstic features are well developed mainly in the Jurassic strata. This zone is characterized by a high vulnerability to pollution confirmed by high values of nitrate coming from sewage water. The deep vertical groundwater flow component seems to be dominant in this zone and is controlled by fractures and geological structures.

Hydrochemistry to delineate groundwater flow conditions in the Mogher Al Mer area (Damascus Basin, Southwestern Syria)

The hydrochemistry of groundwater from the Mogher Al Mer area, located in southwestern Syria, has been used as a tool to identify and assess the hydrogeological systems and associated conditions. In this arid region of Syria, groundwater is considered as the main source of water supply for both drinking and irrigation purposes. The detailed description of hydrogeochemical conditions, including major ions, physico-chemical and in situ field parameters, has underlined the very complex variability of the stratigraphic sequences and hence the numerous hydrogeological units within the study area. On the one hand, groundwater chemical signature is found to be mainly controlled by the water–rock interaction processes in the mountainous western part of the study area. On the other hand, anthropogenic influences are observed in the eastern plain. In terms of recharge mechanisms, the region can be considered as a part of a main intermediate or even regional flow system instead of a local one.

Delayed nitrate dispersion within a coastal aquifer provides constraints on land-use evolution and nitrate contamination in the past

Identifying sources of anthropogenic pollution, and assessing the fate and residence time of pollutants in aquifers is important for the management of groundwater resources, and the ecological health of groundwater dependent ecosystems. This study investigates anthropogenic contamination in the shallow alluvial aquifer of the Marana-Casinca, hydraulically connected to the Biguglia lagoon (Corsica, France). A multi-tracer approach, combining geochemical and environmental isotopic data (δ18O-H2O, δ2H-H2O, 3H, δ15N-NO3-, δ18O-NO3-, δ11B), and groundwater residence-time tracers (3H and CFCs) was carried out in 2016, and integrated with a study of land use evolution in the catchment during the last century. Groundwater NO3- concentrations, ranged between 2 mg/L and up to 30 mg/L, displaying the degradation of groundwater quality induced by anthropogenic activities (agricultural activities). Comparatively high δ15N-NO3- values (up to 19.7‰) in combination with δ11B values that were significantly lower (between 23‰ and 26‰) than the seawater background are indicative of sewage contamination. The ongoing deterioration of groundwater quality can be attributed to the uncontrolled urbanization development all over the alluvial plain, with numerous sewage leakages from the sanitation network and private sewage systems. Integration of contaminant and water-residence time data revealed a progressive accumulation of pollutants with time in the groundwater, particularly in areas with major anthropogenic pressure and slow dynamic groundwater flow. Our approach provides time-dependent insight into nitrogen pollution in the studied aquifer over the past decades, revealing a systematic change in the dominant NO3- source, from agricultural to sewage contamination. Yet, todays low groundwater quality is to large parts due to legacy pollution from land-use practices several decades ago, underlining the poor self-remediating capacity of this hydrosystem. Our results can be taken as warning that groundwater pollution that happened in the recent past, or today, may have dire impacts on the quality of groundwater-dependent ecosystems in the future.

Delineating the Aquifer Role in the Anthropogenic Fingerprint on the Groundwater-Dependent Ecosystem of the Biguglia Lagoon (Corsica, France)

The Biguglia lagoon is the largest wetland of Corsica Island and is located immediately south to the densely urbanized city of Bastia. A part of the ecological preservation of the lagoon is based on the quantity and the quality of the freshwater supply coming from groundwater flows unfortunately still underestimated and misunderstood. To evaluate the role of the groundwater connexion to the lagoon hydrological behaviour, different hydrogeological investigations were carried out with a multi-tracer geochemical approach (major ions, trace elements, δ18O, δD and 3H). Main results show a clear impact of the surrounding anthropogenic activities on the groundwater quality of the alluvial plain of the Biguglia lagoon. The presence of old groundwater dated with 3H indicates a current pollution of the groundwater due to former agricultural practice. The impact of the vegetable production, which is a recent land activity, is not yet visible. However, decennial residence times of groundwater indicate in the near future, a potential threat to the drinking water supply quality as well as to the lagoon water quality by the current agricultural activities, and particularly by nitrates. In the end, a resilience analyse was led considering the groundwater as a main node of the lagoon socio-ecologic system. This transdisciplinary approach has underlined a dynamic global system, where the link between the different social influences is clearly expressed in the groundwater quality. Taking into account the groundwater resilience factors is then one of the key issues for the global management of Mediterranean lagoons.

Multi-tracers Strategy to Define a Conceptual Model for the Coastal Aquifers of Mediterranean Islands, Case Study of the Bonifacio Aquifer (Corsica, France)

A hydrochemical and multi-isotope study was conducted to identify the flow paths, the recharge areas and the geochemical processes governing the evolution of groundwater in the Mediterranean carbonate coastal aquifer of Bonifacio (Corsica). The study is aimed at improving the hydrogeological conceptual model based on environmental tracer investigation tools to characterise and quantify the complex aquifer system. Hydrogeochemical parameters and isotope (δ2H, δ18O, 3H) surveys of rainwater and groundwater have been carried out monthly during two years. A local meteoric water line has been defined and marine, terrestrial and anthropogenic influences on the recharge water hydrochemistry have been described. Preferential recharge during autumn/winter of rainfall is observed and depletion in the isotopic signature for some groundwater samples suggests a recharge at higher altitude from the surrounding granites. A modification of the input signal during infiltration through the unsaturated zone appears and the groundwater hydrochemistry displays differential variation in time and space, with the presence of inertial water bodies in the lower aquifer mainly. Dissolved anthropogenic gases CFCs and SF6 were used to evaluate groundwater residence time. CFCs have been relevant despite the presence of a deep unsaturated zone and the computed rate of groundwater renewal is pluriannual to multi-decadal. Natural SF6 was found in granites and has been used as a direct tracer of groundwater origin, highlighting its role in the aquifer lateral recharge. Strontium isotopes (87Sr/86Sr) were used to improve the knowledge on groundwater mineralization and mixing processes, and are relevant to confirm and quantify the granitic contribution to the aquifer recharge. To improve the quantification of the aquifer water balance terms, submarine groundwater discharges were also studied using aerial infrared images in conjunction with Radon and Radium isotopes (222Rn, 223,224Ra).

An Analytical Method for Assessing Recharge Using Groundwater Travel Time in Dupuit-Forchheimer Aquifers: R. Chesnaux et al. Groundwater x, no. x: x-xx

An analytical solution to calculate the recharge of unconfined aquifers with Dupuit-Forchheimer type flow conditions is proposed. This solution is derived from an existing closed-form analytical solution initially developed to determine groundwater travel time when the recharge of the aquifer is known. This existing solution has been modified to determine recharge when groundwater travel time is known. An illustration is given with a field case example for the Bonifacio aquifer of the island of Corsica (France), in the Mediterranean. In this aquifer, previously established differences in groundwater residence time between two water samples were determined from anthropogenic atmospheric gas (chlorofluorocarbons and sulfur hexafluoride) measurements. The time difference is entered into the new analytical solution to determine recharge. The calculations yield a value of average recharge that agrees with the results obtained by several other methods that were presented in previous studies to assess the recharge of the Bonifacio aquifer. Also presented in this study is a sensitivity analysis of the new analytical solution, to quantify the influence of different parameters that control recharge: hydraulic conductivity, effective porosity and the groundwater travel time. This study illustrates how geochemical data can be combined with physical models to measure recharge. Such an approach could be adopted in other homogeneous aquifers worldwide that satisfy Dupuit-Forchheimer type flow conditions.