Joël Casanova

Strontium isotope systematics used to decipher the origin of groundwaters sampled from granitoids: the Vienne Case (France)

Sr isotope data from surface, shallow and deep groundwaters from the granitoids of the Vienne District (France) are presented in this paper. In surface waters, the Sr contents in the rocks and groundwaters agree with previous data for groundwaters sampled from granitic and sedimentary rocks in France where a large range in 87Sr/86Sr ratios is observed. After correction for the Sr input from rain, the surface water samples plot within a mixing field that can be explained by three end-members, one anthropogenic (low 87Sr/86Sr and high Cl/Sr ratio) and two end-members characterised by low Cl/Sr ratios and a large range in 87Sr/86Sr ratios (from around 0.707–0.720). For deep groundwaters, the 87Sr/86Sr ratios and Sr contents are also determined by applying a correction to account for the influence of cleaning waters during drilling operations. The results are scattered amongst five different groups and the lack of a direct linear relationship between any of the samples implies that, as found for the shallow groundwaters, the results are due to mixing between more than two end-members. A model to determine the 87Sr/86Sr ratio (Irf) of groundwater after interaction with an actively weathering granite is developed. The results yield a low Irf value for waters associated with weathering of the tonalite (0.70463) and a higher one for waters associated with weathering of the monzogranite (0.70704). Given the much higher Irf values derived from the deep groundwater samples, these results indicate that the deep groundwaters analysed within the Vienne hydrosystem cannot be directly related to weathering of either tonalite or monzogranite. It is speculated that this high 87Sr/86Sr source originated from marine incursions during the Jurassic and have been diluted by mixing with former groundwaters produced by water–rock interaction (WRI) with the granites.

14C and Th/U Dating of Pleistocene and Holocene Stromatolites from East African Paleolakes

During recent humid episodes, stromatolites were built along paleolake margins, some 60 m above the modern water level of Lakes Natron and Magadi (southern Gregory Rift Valley). Three generations of stromatolites are observed, the more recent ones frequently covering pebbles and boulders eroded from the older ones. The youngest one yielded 14C ages ranging from approximately 12,000 to 10,000 yr B.P. Their δ13C values (≥2.6%) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO2, thereby lending credence to the reliability of the 14C. An initial 230Th232Th ratio in the detrital component was determined by ThU measurements on the 14C dated stromatolites. Using this value a 230Th234U chronology for the older stromatolites was calculated. Ages of ≥240,000 and 135,000 ± 10,000 yr were obtained for the first and second generations, respectively. A humid episode apparently characterized eastern Africa during each glacial-interglacial transition. 18O and 13C measurements on stromatolites, when compared to values on modern waters and carbonates, provide paleohydrological information. Long residence time of the paleolake waters and less seasonally contrasted regimes are inferred.

MICROBIAL CONTROL ON THE PRECIPITATION OF MODERN FERRIHYDRITE AND CARBONATE DEPOSITS FROM THE CEZALLIER HYDROTHERMAL SPRINGS (MASSIF CENTRAL, FRANCE)

Abstract Actively precipitating hydrous iron oxides and carbonates are commonly associated with cool CO 2 -rich spring waters in the Massif Central (France). Chemical, isotopic and mineralogical analyses, and thermodynamic calculations were undertaken to elucidate the processes involved in the formation of these deposits. Hydrous iron oxide particles rich in Al, Si, Ca and As are the first to form in the springs. Accompanying the iron oxides are layered carbonate deposits, composed mainly of well-crystallised calcite. SEM examination of both oxides and carbonates sampled at the outlet of the springs shows that they basically consist of grape-shaped agglomerates of small (300 nm to 1 μm) anhedral crystals. Light acid etching of these agglomerates reveals colonies of spherical nannobacteria. Water trace partitioning between oxide and carbonate favours the hydrous oxide with As, Pb, Zn, Th concentration. The microscopic features of both types of deposit indicate that microbial mediation probably contributes to their formation.

Variability of 87Sr/86Sr in water draining granite revealed after a double correction for atmospheric and anthropogenic inputs

Abstract In a geochemical characterization of surface waters draining granites of the Vienne district, France, isotopic signatures strictly linked to water—rock interactions were determined by applying a double correction for atmospheric and anthropogenic inputs; the former based on the conservative behaviour of Cl and the latter based on the Sr concentrations and isotopic signatures, which are the only variables with clearly identifiable characteristics. The double-correction concept was validated by comparing the results with the theoretical Sr isotopic signature of water in equilibrium with the parent rock. The weathering model is based on the preferential dissolution of the three main Sr-bearing mineral phases: plagioclase, potassium feldspar and biotite. The Sr isotopic signatures of the waters are in good agreement with the calculated ones. The relative scattering observed for waters draining the same plutonic intrusion can be explained by water—rock interactions within different layers of the weathering profile.

Environmental controls on perennial and ephemeral carbonate lakes: the central palaeo-Andean Basin of Bolivia during Late Cretaceous to early Tertiary times

Abstract In the central palaeo-Andean Basin (Potosi Basin) of Bolivia, the up to 450-m-thick El Molino Formation (Campanian-Maastrichtian to early Tertiary) includes two main carbonate lacustrine episodes each of different extent and duration. These episodes are separated by an extensive development of alluvial facies (floodplain deposits) and limestones deposited in playa lakes or isolated ponds. The El Molino Formation has been investigated in six different areas of the Potosi Basin and affords the opportunity to document climatic and tectonic controls on the development of perennial and ephemeral carbonate lacustrine systems. The first lacustrine episode originated predominantly in perennial lakes with wave-dominated low-gradient ‘ramp’-type margins. Outer marginal lacustrine facies include winnowed oolitic grainstones, ostracod packstones and microbialite bioherms. More agitated areas were characterized by nearshore oolitic bars with associated thrombolite mounds. The fossil content, and the lack of significant precipitation of evaporites, indicate that the lake waters were of low salinity (probably oligohaline) during most of this period. After the first lacustrine episode, the widespread development of terrigenous alluvial facies (floodplain deposits), with interspersed playa lakes or isolated ponds in depressions on the floodplain, may indicate a change in the net water budget. The second lacustrine episode is dominated by dark micritic limestones and, to a lesser extent, microbialite bioherms which developed under semiarid climatic conditions. This episode includes the deposits of ephemeral saline, locally alkaline, shallow lakes which were characterized by low-energy, low-gradient ‘ramp’-type margins. The stable isotopic analysis of 163 carbonate samples covering the different facies and depositional settings displays a wide range of values ( −14.2 18 O and −12.9 13 C ) that is typical of nonmarine environments. The distribution of the values indicates that, during the deposition of the El Molino Formation, the basin was hydrologically closed and experienced no strong hydrogeographical changes, except for variations in the palaeolake level related to fluctuations in the regional P/E ratio. These are recorded by two main isotopic trends related to changes between perennial and ephemeral lacustrine conditions. Lacustrine sedimentation was controlled predominantly by climatically driven hydrological changes with repeated oscillations of the water level (expansion and contraction) and subsequent fluctuations in the width of lacustrine-facies belts.

Boron isotope signatures in the coastal groundwaters of French Guiana

[1] Drinking water used by the population of French Guiana comes mostly from rivers and can induce major health impacts. Consequently, groundwaters are being studied in terms of an alternative drinking water supply. In this context, boron isotopes were applied to the coastal aquifers to better constrain the water-rock interactions. We found that the δ 1 1 B signatures vary between -2.98 and 49.35%o in the groundwaters and between 36.35 and 42.41%o in the river waters and that the local seawater gave a value of 40.76%o. We also noted an inverse correlation between groundwater pH and δ 1 1 B: this relationship, previously demonstrated experimentally, is particularly well defined in the French Guiana coastal aquifer because of the wide pH range (4.18 to 7.64) naturally present in the groundwaters. It would appear that the variations in δ 1 1 B values are linked to water-rock interaction processes (e.g., dissolution/precipitation of silicates).

Nd isotopes and water mixing phenomena in groundwaters from Palmottu (Finland).

The Palmottu uranium ore deposit, located within a granitic host rock in southern Finland, is an excellent setting for conducting analogue studies to assess radionuclide transport from the uranium deposit fractured crystalline host rock. In this context, Nd isotope ratios are used to establish the degree of water-rock interaction (WRI) and to clarify mixing processes within the groundwaters. Variations in Nd isotopes in selected water types from the Palmottu hydrosystem provide new information on the mixing history.

Managed Aquifer Recharge: An Overview of Issues and Options

As covered in Chap. 2, many of the world’s aquifers are rapidly being depleted. Nearly one quarter of the world’s population – 1.7 billion people – live in regions where more water is being consumed than nature can renew (Gleeson et al. 2012). Over-exploitation occurs when groundwater abstraction is too intensive, for example for irrigation or for direct industrial water-supply like extracting fossil fuels (Pettenati et al. 2013; Foster et al. 2013). When groundwater is continuously over-pumped, year after year, the volume withdrawn from the aquifer cannot be replaced by recharge. Eventually, the groundwater level is much lower than its initial level and even when pumping stops, the aquifer has trouble rising once again to its original level. In continental zones, over-exploitation can lead to groundwater drawdown and, ultimately, to subsidence through development of sinkholes when underground caverns or channels collapse. In coastal areas, the decrease in groundwater recharge results in saltwater intrusion into the aquifer formation (Petalas and Lambrakis 2006; De Montety et al. 2008). Preserving local groundwater resources is an environmental and economic issue in coastal zones and is vital in an island context. The increasing demand for water caused by a growing population can lead to the salinization of groundwater resources if these are systematically over-exploited. Limiting the salinization of coastal aquifers is consistent with the groundwater objective of the European Union Water Framework Directive, which is to achieve a good qualitative and quantitative status by 2015. The economic advantage of preserving these threatened water resources is that, when there is a growing demand, a local water resource is sustained and there is no need to import water. Transporting water can cost 2–10 times more than limiting the intrusion of saltwater into a coastal aquifer.

Challenges of Artificial Recharge of Aquifers: Reactive Transport Through Soils, Fate of Pollutants and Possibility of the Water Quality Improvement

The unsaturated zone acts indeed as a natural reactive filter and can reduce or remove microbial and organic/inorganic contaminants through biogeochemical processes enhancing mass transfer between phases (soil – water – gases). The performance of the soil to purify the infiltrated water is based on both chemical, geo-biochemical and hydrodynamic coupled processes in a porous medium. The geochemical reactivity of soil minerals and the biodegradation of organic matter involving microbial mediated redox-reactions are the key reactions characterizing the water cleaning capacity of a soil. The reactive transport mechanisms induced by aquifer recharge using secondary or tertiary treated wastewaters still containing metals, metalloids and organic matter as pollutants is studied through laboratory and pilot experiments. This technology targets the geochemical reactivity and dynamics of soil to improve water quality while maintaining environment quality and protecting other resources (aquifers, agricultural production, soil, etc.). Obviously, the dilemma to meet these both constraints becomes a real challenge. This study aimed to develop a general concept based on the control of the physical, chemical and microbial keys processes easy to integrate in the numerical predictive and quantitative tools. The reactive transport modeling is carried out in order to identify the relevant processes controlling the filtration capability of the soil. Some results of ongoing projects based on the understanding of reactive transport processes will be presented. The technologic challenges emerged from the environmental safety issue and from the artificial recharge study will be discussed. Artificial groundwater recharge of aquifers by percolation through the unsaturated zone (UZ) is a technique to enhance the water quality for drinking water supplies. The performance of the UZ to purify the infiltrated water is based on chemical, geobiochemical and hydrodynamic coupled processes in a porous medium. The geochemical reactivity of soil minerals and the biodegradation of organic matter involving microbial mediated redox-reactions are the key reactions characterizing the epuration capacity of a soil. In order to improve our understanding of the physical and chemical phenomena controlling the efficiency of such process, a series of projects in a coastal aquifer in south-eastern France are built between Veolia and BRGM. The projects are based on the integration of numerical simulations with calibrated parameters on laboratory, pilot experiments and field aquifer characterization. The site characterizations and numerical simulations tend to show the development of “filtrating zones” by combination of various physico-chemical and thermokinetic processes. On the other hand, the mixing between infiltrating recharge waters and seawater can have important impact on the dissolution of carbonate minerals and precipitations of sulphate minerals. The results will be extrapolated to the real (industrial) system to elaborate exploitation scenarios and sensitivity analysis.