Le present rapport correspond au bilan de l’etat initial pre-arasement pour le volet Flux hydriques, sedimentaires et chimiques, divise en deux chapitres. Le premier chapitre etablit le bilan du suivi des flux hydriques, sedimentaires et chimiques sur les stations en amont et en aval des barrages. Le deuxieme chapitre presente deux etudes preliminaires realisees en 2015 et financees dans le theme Dynamique fluviale. La premiere etude s’interesse a la contamination en metaux lourds des sediments de l’Yvrande. La deuxieme etude concerne les caracteristiques geochimiques des eaux souterraines sur le bassin versant de la Selune.
Climate change affects groundwater availability and residence times, necessitating a thorough understanding of aquifer characteristics to define sustainable yields, particularly in regions where water is heavily exploited. This study focuses on the Volvic volcanic aquifer (Chaîne des Puys, France), where groundwater recharge has decreased due to climate change, raising concerns about water use sustainability. To address these challenges, this work proposes a multi-tracer approach, based on hydrogeological monitoring, including the estimation of groundwater ages, major elements chemistry and water stable isotopes to better characterise this resource decrease and more peculiarly its origin and its impact on the environment that has never been addressed. Relative fractions of ancient and modern water contributions (up to 20 %) to the aquifer have been thus estimated as well as the apparent ages of groundwaters (34 years). We highlight the complementarity of tracers used, allowing a better definition of recharge sources and transit times of groundwaters within the aquifer. These results led to the proposal of a hydrogeological conceptual model, highlighting a bi-modal recharge, distinguishing between a long-term recharge upon 30 years, supplemented by a recent component (≃ 1 year) related to annual precipitation. This study provides valuable information on groundwater circulation and the response of volcanic aquifers systems to climate change, while highlighting the importance of assessing residence times. By addressing the challenges posed by systems with contrasting permeability and recharge gradients, it improves understanding of volcanic hydrology and provides a basis for the development of (numerical) hydrological models to assess the impacts of global change.
Abstract. In recent decades, saline fluids have been sampled worldwide at great depths in continental basements. Although some of them have been attributed to marine transgressions the mechanisms allowing their circulation is not understood. In this paper, we describe the horizontal and vertical distribution of moderately saline fluids (60 to 1400 mg L−1) sampled at depths ranging from 41 to 200 m in aquifers at the regional scale of the Armorican Massif (northwestern France). The horizontal and vertical distributions of high chloride concentrations are in good agreement with both the altitudinal and vertical limits and succession of the three major transgressions between the Mio-Pliocene and Pleistocene ages. The mean chloride concentration for each transgression area is exponentially related to the time spanned until present. It defines the potential laws of leaching of marine waters by fresh meteoric waters. The results of the Armorican aquifers provide the first observed constraints for the time scales of seawater circulation in the continental basement and the subsequent leaching by fresh meteoric waters. The general trend of increasing chloride concentration with depth and the time frame for the flushing process provide useful information to develop conceptual models of the paleo-functionning of Armorican aquifers.
<p>Drinking water quality in agricultural rural areas remains locally a challenge even all the effort made by local authorities to restore the groundwater resources quality, especially regarding nitrates. In Plourhan, a ~2000 inhabitants, about 10 km from the sea, NW France, the drinking water is pumped in a natural spring emerging from the Brioverian basement. The nitrate concentrations exceed the 50 mg/L standard for drinking water supply, and thus needs to be diluted to be delivered to the population. Over the last 15 years, a large programme of measures was undertaken in order to reduce the NO<sub>3</sub> concentration, including the purchase of agricultural parcels around the spring, moving progressively from mixed farming and livestock to fallows and meadows, and thus drastically change the local land use. Despite all these efforts, nitrate concentrations only decrease very slowly and remain above the 50 mg/L standard.</p><p>In this context, the objective of this study is to better understand the transfer of nitrates at the basin scale, by studying flow paths, geochemical reactions, transit times that are key parameters to estimate the vulnerability and the recovery-time of the critical zone. In that way, a geochemical and isotopic approach is applied at the basin scale. Major elements analysis of the groundwater reflect the drained contrasted lithologies as metasediments (pelites & sandstones) and amphibolite, with a large spatial heterogeneity of the NO<sub>3</sub> concentrations, ranging from a few mg/L to more than 50 mg/L. Nitrogen and oxygen isotopes of nitrates (&#948;<sup>15</sup>N-NO<sub>3</sub> and &#948;<sup>18</sup>O-NO<sub>3</sub>) suggest that denitrification can occur locally in some wells presenting low or intermediate &#160;NO<sub>3</sub> contents, whereas other wells present high or low NO<sub>3</sub> concentrations without any evidence of denitrification processes. The mean residence time of groundwater is assessed through CFCs and SF6 dissolved gas measurements. Some wells preferentially in amphibolite, present water with low recharge temperature (around 6&#176;C while the mean recharge temperature in Britany is 11-12&#176;C) correlated with low CFCs/SF6 values indicating that some very old groundwater (last glaciation :&#160; -19/17 k yrs) exists in the reservoir. Other ones in metasediments have modern water or a mixing between an old and a present day recharge. These results, together with structural and lithological detailed geological field mapping, help to draw up the conceptual model of the aquifer functioning regarding nitrates transfer in the critical zone.&#160;&#160;</p><p>This work is part of the POLDIFF study that benefits from the funding of BRGM and the French Loire-Bretagne water Agency.</p>
In many areas around the world, phreatic coastal aquifers are vulnerable to contamination and saltwater intrusion. This study aims to explore the hydrodynamic functioning of the coastal phreatic aquifer of the Urabá region, Colombia, based on the identification of hydrogeochemical processes and groundwater residence and transit times. Hydrodynamic data showed a fast response of the piezometric levels to precipitation with a maximum time lag of 48 h. Mean annual recharge was estimated at 19% with respect to precipitation. Another recharge mode is linked to infiltration from surface water, according to the hydrodynamic results. The main geochemical process identified is hydrolysis due to dissolved carbon dioxide, inducing silicate weathering and resulting in predominant HCO3-Ca-Mg facies. Monitoring of δ18O and δ2H compositions combined with 3H, 14C, chlorofluorocarbons (CFCs) and SF6 contents showed a fast recharge in the aquifer. Groundwater dating indicates recent waters mixed with older (>40 years) waters.
The varied gaseous composition of thermo-mineral waters emerging in a non-active zone reflects the diversity and complexity of groundwater pathways and provides important insights into their hydrogeological behaviours. The investigated geochemical content of complex thermo-mineral springs revealed the need to use dissolved gas contents as part of a multi-tracer approach to discriminate processes, geogenic (water–gas-rock interactions), abiotic (geological confinement, flow paths) and biotic activity influencing geochemical of groundwater along regional pathways. Irrespective of the dissolved element content or the water type, examining the overall concentration of dissolved gases enables an effective delineation of regional groundwater flow paths. Using dissolved gas content further contributed to the circumvention of some analytical challenges associated with conventional isotopic or geochemical techniques, often linked to the high concentration of elements such as iron, sulfate, sulfide or other naturally occurring elements content. The primary objectives are to analyse the gas composition of individual springs, to identify the origin of these gases in the groundwater, and to use this gas composition to improve the understanding of the flow patterns contributing to the geochemical diversity observed at the surface of the groundwater. From field investigations in a geologically and structurally complex area of Eastern Corsica (France), three types of gas contents are identified: (type 1) CH4 & H2S-rich, (type 2) N2-rich and (type 3) CO2-rich. The study of these dissolved gases highlights that the wide geochemical diversity of thermo-mineral waters observed here is not only related to the mineralogical composition of the local aquifer but also involves strong and cumulative interactions along deep regional circulation pathways. This approach also reveals a common deep crustal gaseous influence characterised by N2 production, which interacts during up flow with groundwater and then with the local metamorphic or sedimentary rock matrix. The groundwater's isotopic and geochemical contents are then altered by local lithologies encountered through both abiotic and biotic interactions. Finally, at shallow depths, phreatic groundwater can add its geochemical and isotopic footprint and dilute this complex mixture before groundwater emerges as mineral spring. This paper answers the primary objectives yet further demonstrates that using dissolved gas as a tracer of groundwater flow paths allows a deeper interpretation of surface geochemical and isotopic observations, distinguishes local from regional flow paths, and provides information about processes at the origin of groundwater diversity. The combination of tools presented in this paper (i.e., geochemical, dissolved gas, and isotopic tools) allows the establishment of a reliable regional groundwater flow scheme for thermo-mineral waters in a non-active zone. This scheme is essential to improve thermo-mineral water management, and protection to ensure their sustainable quality in front of increasing anthropogenic and climatic pressures.
