Hélène Pauwels

Chemistry and isotopes of deep geothermal saline fluids in the Upper Rhine Graben: Origin of compounds and water-rock interactions

Abstract Deep boreholes (⩽870 m) in the Upper Rhine Graben produce medium-temperature (120–150°C) saline fluids that circulate through the granitic basement and/or the overlying sedimentary rocks. The salinity of these deep fluids, sampled from both the granite and the sedimentary rock, can be explained by a three-step model: 1. (1) evaporation of seawater which produces a primary brine; li(2) mixing between a dilute fluid and the primary brine; and 2. (3) dissolution of halite by the later fluid. The thermal waters sampled at shallower depths are the result of mixing of the deep saline fluid and surface water. Geothermometer calculations indicate that some of the deep fluids did reach high temperatures (up to 220–260°C). During cooliug, reactions between fluid and rock took place, but the fluids did not have enough time to reach complete equilibrium with the surrounding rock.

Denitrification and mixing in a schist aquifer: influence on water chemistry and isotopes

Nitrate concentrations in groundwater close to the water table in the upper weathered zone of the Coet-Dan catchment (Brittany, France) reach 200 mg/l. With intensive agriculture covering 86.5% of the catchment area, the pollution results principally from the spreading of livestock manure. The rapid decrease of NO3− concentrations with depth in the fractured part of the aquifer is partly the result of dilution by older groundwater that was probably never polluted, as deduced from the isotopic data of water (3H, δ2H, δ18O) and sulphate (δ34S, δ18O) molecules. However, the extent of denitrification is demonstrated by N2 concentrations, as well as by mass balance calculations showing that denitrification may contribute to a loss of at least 50–70 mg l−1 of NO3− just below the upper zone of the aquifer. Several electron donors may be involved in the denitrification reactions; for example, ferrous iron and sulphides of the pyrite and organic matter, with at least the last two processes being aided by the presence of bacteria. Autotrophic denitrification is considered to be the predominant process in the unweathered schist, whereas heterotrophic processes may occur in the overlying weathered zone. Autotrophic reactions tend to liberate SO4, but concentrations of sulphate are probably controlled by precipitation of sulphate amorphous phases and also minerals such as jarosite.

Water-rock interaction processes in the Triassic sandstone and the granitic basement of the Rhine Graben: Geochemical investigation of a geothermal reservoir

Saline fluids have been collected in the Rhine Graben over the last two decades, both from the Triassic sandstone aquifer and the granitic basement down to a depth of 3500m. Their salinities and location are compared in order to distinguish the respective influences of temperature and host-rock mineralogy in the water-rock interaction processes. The comparison shows that sulphates in the sedimentary formations were dissolved by the fluids, which also led to Br enrichment. Mica dissolution has strongly increased the Rb and Cs contents, which then provide an indication of the degree of water-rock interaction. The Sr isotopic ratios are used to compare the fluids with the granite minerals. Two relationships are revealed for the fluids in the sandstone and the granite, one related to widespread mica dissolution, which could have affected both the Buntsandstein and the granite, and the other to subsequent plagioclase dissolution, which is observed only in the granite. Computations showed that 12.5g of mica and 1.658 of plagioclase per liter of fluid have been dissolved. The nature of these two relationships suggests two different evolutions for the fluids and the individualization of the two reservoirs during the grabens history. The cation concentrations are mainly controlled by temperature, and are independent of the type of host rock. Equilibrium with the rock mainly caused Ca and K concentration variations, which has induced clear CaK and Ca-δ18O, K-δ18O correlations. Geothermometric computations indicate that with increasing depth, the cations, the silica and the δ18O(SO4) geothermometers evolve towards a value close to 230δC. This demonstrates the existence of a hot reservoir in the granite of the graben, at a depth estimated at 4.5–5 km.

Temporal variability of nitrate concentration in a schist aquifer and transfer to surface waters

Nitrate concentrations monitored for 2.5 a in the stream water and groundwater of a small catchment, 86.5% of which is devoted to intensive agriculture, show temporal variations with a maximum during winter (as much as 200 mg l−1 in groundwater and 100 mg l−1 in stream water) and a minimum at the end of summer/beginning of autumn. Variations were also observed in the stream water and shallow groundwater after rainfall. The processes involved to explain these variations, determined mainly from NO3− Cl−, SO42−, piezometric and streamflow data, are: (a) variability of the relative contributions to stream water and shallow groundwater by upward fluxes of deeper groundwater which, as demonstrated previously, is denitrified mainly as a result of reaction with pyrite. (b) Denitrification of shallow groundwater during summer with organic matter acting as the electron donor. (c) Dilution by rain water. Nitrate concentrations in both stream water and shallow groundwater depend on the amount of precipitation, with an increased contribution from deep denitrified groundwater during dry periods. The temporal variations in NO3− concentration observed several metres below the water table are related to the preferential and rapid movement of NO3−-polluted water through fractures and large fissures, which has been estimated at 1 m day−1. Nitrate pollution in the catchment, because of the interaction with pyrite, also increases the net chemical weathering rate to values exceeding the world average.

Origins and processes of groundwater salinization in the urban coastal aquifers of Recife (Pernambuco, Brazil): A multi-isotope approach

In the coastal multilayer aquifer system of a highly urbanized southern city (Recife, Brazil), where groundwaters are affected by salinization, a multi-isotope approach (Sr, B, O, H) was used to investigate the sources and processes of salinization. The high diversity of the geological bodies, built since the Atlantic opening during the Cretaceous, highly constrains the heterogeneity of the groundwater chemistry, e.g. Sr isotope ratios, and needs to be integrated to explain the salinization processes and groundwater pathways. A paleoseawater intrusion, most probably the 120 kyB.P. Pleistocene marine transgression, and cationic exchange are clearly evidenced in the most salinized parts of the Cabo and Beberibe aquifers. All (87)Sr/(86)Sr values are above the past and present-day seawater signatures, meaning that the Sr isotopic signature is altered due to additional Sr inputs from dilution with different freshwaters, and water-rock interactions. Only the Cabo aquifer presents a well-delimitated area of Na-HCO3 water typical of a freshening process. The two deep aquifers also display a broad range of B concentrations and B isotope ratios with values among the highest known to date (63-68.5‰). This suggests multiple sources and processes affecting B behavior, among which mixing with saline water, B sorption on clays and mixing with wastewater. The highly fractionated B isotopic values were explained by infiltration of relatively salty water with B interacting with clays, pointing out the major role played by (palaeo)-channels for the deep Beberibe aquifer recharge. Based on an increase of salinity at the end of the dry season, a present-day seawater intrusion is identified in the surficial Boa Viagem aquifer. Our conceptual model presents a comprehensive understanding of the major groundwater salinization pathways and processes, and should be of benefit for other southern Atlantic coastal aquifers to better address groundwater management issues.

Groundwater Isolation Governs Chemistry and Microbial Community Structure along Hydrologic Flowpaths

This study deals with the effects of hydrodynamic functioning of hard-rock aquifers on microbial communities. In hard-rock aquifers, the heterogeneous hydrologic circulation strongly constrains groundwater residence time, hydrochemistry, and nutrient supply. Here, residence time and a wide range of environmental factors were used to test the influence of groundwater circulation on active microbial community composition, assessed by high throughput sequencing of 16S rRNA. Groundwater of different ages was sampled along hydrogeologic paths or loops, in three contrasting hard-rock aquifers in Brittany (France). Microbial community composition was driven by groundwater residence time and hydrogeologic loop position. In recent groundwater, in the upper section of the aquifers or in their recharge zone, surface water inputs caused high nitrate concentration and the predominance of putative denitrifiers. Although denitrification does not seem to fully decrease nitrate concentrations due to low dissolved organic carbon concentrations, nitrate input has a major effect on microbial communities. The occurrence of taxa possibly associated with the application of organic fertilizers was also noticed. In ancient isolated groundwater, an ecosystem based on Fe(II)/Fe(III) and S/SO4 redox cycling was observed down to several 100 of meters below the surface. In this depth section, microbial communities were dominated by iron oxidizing bacteria belonging to Gallionellaceae. The latter were associated to old groundwater with high Fe concentrations mixed to a small but not null percentage of recent groundwater inducing oxygen concentrations below 2.5 mg/L. These two types of microbial community were observed in the three sites, independently of site geology and aquifer geometry, indicating hydrogeologic circulation exercises a major control on microbial communities.

Water-rock interactions during experiments within the geothermal Hot Dry Rock borehole GPK1, Soultz-sous-Foreˆts, Alsace, France

Abstract Hydraulic injection-backflow tests were carried out in borehole GPK1 at a site designed for geothermal “Hot Dry Rock” experiments, located in the Rhine Graben near Soultz-sous-Foreˆts, France. The injected fresh water was in contact with granite at ∼137°C for a few hours. Through the monitoring of the chemical composition of the production fluids, it was noticed that substantial mixing with the indigenous brine from the granite, and chemical exchanges between the granite and the fluids (release of Co 2 , SO 4 , F, Ba and SiO 2 ) took place. The mineralogical origin of these components is discussed. It appears that at least calcite, biotite, sulphide minerals and feldspar were dissolved from the rock. For most of the components, the dissolution kinetics are very fast during the first steps of the interaction and then slow down after a few minutes. Sulphate concentration data indicate that the fluid entered into different fractures. The chemical compositions of both the indigenous brine and the injected fresh water are relevant to determine the behaviour of the minerals.

Comparison of long-term geochemical interactions at two natural CO2-analogues: Montmiral (Southeast Basin, France) and Messokampos (Florina Basin, Greece) case studies

Publisher Summary This chapter assesses the impact of long-term CO2 accumulations in two sandy reservoirs of different origin: the high-temperature and high-pressure reservoir at great depth at Montmira and the shallow, low-temperature and low-pressure reservoir at Messokampos. Petrographic characterization of the reservoirs enabled the identification of both the effects of CO2-induced geochemical interactions as well as their impact on reservoir lithologies. Subsequently, geochemical modeling was applied to reproduce the observed effects, identify their driving parameters and to assess their impact in terms of potential mineral trapping and porosity changes. It indicates that the porosity increase attributed to this reactivity requires that the sediment is flushed intensively with CO2-rich pore waters and that a flow regime in the reservoir must have been in place at a certain point in the reservoirs geological history. The impact of these reactions is minor and does not seem to influence the porosity of the sediment. Comparison of the geochemical interactions at the two sites shows that a reservoirs temperature and pressure conditions determine the impact of CO2 interactions, with elevated temperatures significantly increasing the reaction rates of mineral-trapping reactions.

Impact of climate changes during the last 5 million years on groundwater in basement aquifers

Climate change is thought to have major effects on groundwater resources. There is however a limited knowledge of the impacts of past climate changes such as warm or glacial periods on groundwater although marine or glacial fluids may have circulated in basements during these periods. Geochemical investigations of groundwater at shallow depth (80–400 m) in the Armorican basement (western France) revealed three major phases of evolution: (1) Mio-Pliocene transgressions led to marine water introduction in the whole rock porosity through density and then diffusion processes, (2) intensive and rapid recharge after the glacial maximum down to several hundred meters depths, (3) a present-day regime of groundwater circulation limited to shallow depth. This work identifies important constraints regarding the mechanisms responsible for both marine and glacial fluid migrations and their preservation within a basement. It defines the first clear time scales of these processes and thus provides a unique case for understanding the effects of climate changes on hydrogeology in basements. It reveals that glacial water is supplied in significant amounts to deep aquifers even in permafrosted zones. It also emphasizes the vulnerability of modern groundwater hydrosystems to climate change as groundwater active aquifers is restricted to shallow depths.

The French Carbogaseous Province: An Illustration of Natural Processes of CO2 Generation, Migration, Accumulation and Leakage

Underground storage of industrial quantities of carbon dioxide in deep saline aquifers, depleted oil and gas fields, or deep coal beds is an emerging technology that could lead to significant reductions in our greenhouse gas emissions to the atmosphere. Key issues that still need to be addressed are the long term security and safety of these man-made underground CO 2 storage facilities, as well as their public acceptability. Natural underground CO 2 accumulations, encountered in many countries throughout the world and existing for hundreds of thousands to millions of years, offer a unique opportunity to address these issues. The existence of naturally occurring underground CO 2 accumulations is extremely important for the social acceptance of geological CO 2 sequestration technology and the assessment of long-term storage effects. The peri-Alpine province exhibits a variety of geological environments and illustrates different contexts of CO 2 generation, migration, accumulation, and leakage linked to the regional geological history. Several natural CO 2 accumulations were discovered during gas and oil exploration in the 1960s. They show that CO 2 can remain trapped underground for millions of years at various sites characterized by specific geological contexts. Many springs and boreholes currently being exploited for their carbogaseous water prove that, in other geological contexts, CO 2 can migrate from the mantle towards the overlying formations and even up to the surface leading to natural CO 2 leakage without major risk. The geological characteristics of these sites, where CO 2 accumulations or carbogaseous waters are found, are compared to assess which geological structures have effectively trapped CO 2 .