Christian Fouillac

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.

An Overview of Dissolved and Suspended Matter Fluxes in the Loire River Basin: Natural and Anthropogenic Inputs

The spatial and temporal distributions of major elements were investigated in the surface waters and in associated suspended matter at two sites of the upper Loire basin (Orleans and Brehemont) between 1995 and 1998.According to geochemical and isotopic patterns, the dissolved load appears to result from a process of mixing rainwater inputs, weathering processes of carbonate and silicate bedrock, and agricultural and urban inputs. Natural inputs influence 60% of water chemical composition at both sites. Annual dissolved fluxes were estimated to be 1300 103 t/y at Orleans and 1620 103 t/y at Brehemont. Major elements are transported mainly in the dissolved fraction. After correcting for atmospheric and anthropogenic inputs, the silicate specific export rate was calculated to be 11 t/y/km2 throughout the basin and the carbonate specific export rate to be from 47 t/y/km2 at Orleans to 23 t/y/km2 at Brehemont.The suspended load appears to result from at least two particle reservoirs: a silicate reservoir and a carbonate reservoir. The silicate reservoir has a detrital origin, mainly during periods of high flow, while the carbonate reservoir has a detrital origin during periods of high flow and an authigenic origin during periods of low flow. Of the total annual flow of suspended matter, this authigenic material represents 16% at Orleans, 25% at Brehemont and 37% in the fluvial part of the estuary. After correcting authigenic inputs, the specific export rate due to mechanical weathering was estimated to be 8 t/y/km2 throughout the Loire basin.

The distribution of arsenic (III) and arsenic (V) in geothermal waters: Examples from the Massif Central of France, the Island of Dominica in the Leeward Islands of the Caribbean, the Valles Caldera of New Mexico, U.S.A., and southwest Bulgaria

Abstract Data for As (III), As (V) and total As for various types of geothermal spring water are given. The different species of As were separated at the time of sampling after filtration, acidification and dilution where necessary, using an anion-exchange method. Total As, As (III) and As (V) were determined by flameless atomic absorption spectrometry. The total As content ranges from 5 to 1800 μg l −1 . Samples from Dominica show evidence of different sulfur-arsenic oxidation kinetics, As (III) being the major component (85–99%) in fluids from deep levels of the geothermal reservoir and also (99%) in oxidized waters of the acid sulfate type. The bicarbonate spring water of the Valles Caldera, New Mexico, contains more As (III) than As (V), but there As (V) is the major component of the acid sulfate springs. As (III) proves to be the predominant form (60–100%) in the warm alkaline waters from southwest Bulgaria. A selection of CO 2 -rich spring waters from the Central Massif, France (Cezallier) is analysed. Two groups are distinguished on the basis of major-element chemistry and total As content. Oxidized forms are mainly encountered in the first group, including a well sample containing 0–5% As (III). The As (III) content of the second group ranges from 10% to 70%. These preliminary results indicate that the fluids directly related to hydrothermal systems contain mainly As (III), whereas acid sulfate springs have a variable As (III) content. Bicarbonate waters are generally enriched in oxidized As. In the absence of springs or sinks within a geothermal region, the total As concentration may display conservative properties, but both As (III) and As (V) may be reactive during mixing and cooling.

Sulphur and oxygen isotopes of dissolved sulphur species in formation waters from the Dogger geothermal aquifer, Paris Basin, France

Isotopic analyses of the S in sulphides and sulphates from formation waters were made at 35 geothermal wells exploiting the Dogger geothermal aquifer of the Paris Basin. Isotopic analyses of the O of the dissolved sulphates were also made on 12 of these. The results show a distribution of the δ34S of the sulphates according to geographic zones that also correspond to variations in the geochemistry of the fluids, in particular the sulphide content. Samples taken in the Seine-St-Denis area and in the areas north and west of Paris show high δ34S values ranging from +26.3 to +48.9%. Values of δ34S for sulphides range from −8.5 to + 5%, and are interpreted as reflecting the effect of bacterial reduction in a confined part of the basin. Modelling this reduction by a Rayleigh distillation mechanism made it possible to calculate a value of 1.038 ± 0.003 for the coefficient of fractionation between the sulphates and sulphides. We show that an important part of the sulphides formed by bacterial reduction has been subtracted from the fluid by deposition. South of Paris and in the Val de Marne, the δ34S values for the sulphates are more uniform, ranging between +22.4 and +26.6%, despite the existence of bacterial activity. This is most probably due to a higher rate of flow of the waters, entailing renewal of the sulphate stock. The isotopic composition of the O of the sulphates for the Seine-St-Denis area and the areas west and north of Paris is very uniform, between +16.1 and +17.1%, despite the wide range in δ34S (+26.3 to +48.9%) for the same samples. The calculated isotopic temperatures are close to those measured in situ. These results suggest either that sulphate-water isotopic equilibrium was established after the reduction, or that the coefficient of O fractionation is very low (<1.003) during bacterial reduction.

A strontium isotopic study of mineral and surface waters from the Cézallier (Massif Central, France): implications for mixing processes in areas of disseminated emergences of mineral waters

Abstract Surface and mineral waters from a geothermal area (Cezallier) in the centre of the Massif Central, France, were analysed for Sr isotopes, Sr and major elements in order to use Sr isotopes to investigate the discharge of a mineralised water area, especially in the case of disseminate mineralised water emergences. The 87 Sr 86 Sr ratios of mineralised waters ranged from 0.715541 to 0.716773. The variation in 87 Sr 86 Sr values was small for the 5 mineral springs sampled over two periods (5 × 10 −6 –14 × 10 −6 ). Regarding the reproductivity of the 87 Sr 86 Sr ratio measurements, no differences can be seen and the 87 Sr 86 Sr ratio of mineral waters can be considered to be constant with time. The 87 Sr 86 Sr ratios of surface waters collected along the Zagat River ranged from 0.705303 to 0.715091. The lowest values were always observed in the headwaters. The difference in the 87 Sr 86 Sr ratios between the two sampling periods was ∼ 22 × 10 −4 and can be related to the hydrological situation (low or high flow). The 87 Sr 86 Sr ratios of surface waters collected in the drain ranged from 0.715072 to 0.716585. The difference in the 87 Sr 86 Sr ratios between the two sampling periods is ∼ 11 × 10 −4 for the drain headwaters and ∼ 13 × 10 −5 at the outlet of this area. Likewise, these variations can also be related to the hydrological situation. Plots of 87 Sr 86 Sr vs. 1 Sr ratios show two distinct binary mixing trends and the existence of at least four end-members. A mixing parameter f giving the proportions of the two components can be estimated for assumed 87 Sr 86 Sr ratio and Sr content values of the pure end-members. An estimate of the mineral water discharge can be calculated using the results of the mixing model and the measured flow. In the drain area, the mixing model gives an estimate ∼ 0.5–1 l s −1 , or 10–13% of mineral water in the total flow. A similar calculation was done for the river where the mineral water flow was ∼ 0.96 l s −1 (3.5 m 3 h −1 100 m downstream from the bridge in April 1994, and 6 m 3 h −1 20 m downstream from the bridge in September 1994.

Occurrence of mineral water springs in the stream channel of the Allier River (Massif Central, France): chemical and Sr isotope constraints

The French Massif Central has long been recognized as an area with numerous mineral water springs. It is often very difficult to measure the flow rate of mineral springs since they are spread out over extentive areas. Water from the Allier River has been sampled monthly over a 13-month period and a spatial cross-section survey carried out at base flow conditions in order to estimate the discharge of mineral water in the Allier River streambed. For the monthly survey, the 87Sr86Sr ratios vary between 0.709511 and 0.712656. A good correlation exists between 87Sr86Sr and 1Sr ratios and defines binary mixing between mineral waters and the Allier River sensu-stricto. The mixing processes have been studied using the Sr isotopic systematic and a conservative species (chloride). Except for one sample, the mineral water input never exceeds 2%. However, the mineral water discharge into the Allier streambed could not be precisely estimated because effluent seepage of mineral water may occur in a part of the channel representing only a small volume of the cross-sections. Variations in the solute concentrations across the river channel were measured at 14 sites chosen as being representative of areas where: (a) mineral water emerges, (b) no mineral water emerges, and (c) mixing is complete. A model developed for the monthly study using Cl as a reference was used in the spatial survey. The input of mineral water in the Allier streambed ranges from 0.1–0.15% in most of the samples and is greater than 2% at two points. The general mass balance of the proportions of mineral water and Allier River water (sensu-stricto) using the sum of cations and anions was calculated and the input of mineral water into the Allier streambed was determined at the cross-section nearest the Allier gauging station (0.134%). The discharge of the Allier River at the gauging station was about 17.7 m3/s during the sampling period, which gives a discharge of mineral water of about 0.02 m3/s into the Allier channel.

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.

Field titrations of dissolved sulfur species in anoxic environments — Geochemistry of Puzzichello waters (Corsica, France)☆

Abstract The use of the Ag/1bAg 2 S membrane electrode in combination with Hg 2+ titrations under a N 2 atmosphere enables a rapid and accurate determination of numerous dissolved S species: total sulfide, polysulfide S, thiosulfate and sulfite. These species have been measured in the waters of Puzzichello (Corsica, France), which are characterized by atmospheric oxidation of an initially sulfide-rich water. In the early stage of the oxidation process mainly polysulfide and thiosulfate are produced; in the final stage, sulfate is the main product. The concentrations of Fe and Cu are influenced by the oxidation process.

Geochemical Monitoring of Drilling Fluids: A Powerful Tool To Forecast and Detect Formation Waters

This paper describes a method based on the difference between the chemical compositions of formation and drilling fluids for analyzing drilling mud to forecast fluid-producing zones. The method was successfully applied in three boreholes in crystalline rocks in France. Subsequent geophysical logs and hydraulic tests confirmed the occurrence of flowing fractures.

Relations entre les concentrations des ions alcalins dans les eaux thermales sulfurées sodiques du Midi de la France

Abstract Alkali ion concentrations were studied in hot sulphurated springs in the south of France. Strong correlations between elements (K-Rb, K-Cs and Na-Li) are found. The element concentrations are also closely related with deep temperature as estimated by classical geothermometers. A discussion of the origin of the interelement correlations is presented. Further studies on the behaviour of trace elements during mineral dissolution are needed and can yield interesting methods of investigation in geothermal exploration.