Hydrogeological investigations in the upper Ouémé catchment in Benin, West Africa

The scope of this PhD thesis was the hydrogeological conceptualisation of the Upper Ouémé river catchment in Benin. The study area exceeds 14,500 km**2 and is underlain by a crystalline basement. At this setting the typical sequence of aquifers - a regolith aquifer at the top and a fractured bedrock aquifer at the bottom - is encountered, which is found in basement areas all over Africa and elsewhere in the world. The chosen regional approach revealed important information about the hydrochemistry and hydrogeology of this catchment. Based on the regional conceptual model a numerical groundwater flow model was designed. The numerical model was used to estimate the impact of climate change on the regional groundwater resources. This study was realised within the framework of the German interdisciplinary research project IMPETUS (English translation: "Integrated approach to the efficient management of scarce water resources in West Africa"), which is jointly managed by the German universities of Bonn and Cologne. Since the year 2000 the Upper Ouémé catchment was the principal target for investigations into the relevant processes of the regional water cycle. A first study from 2000 to 2003 (Fass, 2004, http://nbn-resolving.de/urn:nbn:de:hbz:5n-03849) focused on the hydrogeology of a small local catchment (~30 km**2). In the course of this thesis five field campaigns were underdone from the year 2004 to 2006. In the beginning of 2004 a groundwater monitoring net was installed based on 12 automatic data loggers. Manual piezometric measurements and the sampling of groundwater and surface water were realised for each campaign throughout the whole study area. Water samples were analysed for major ions, for a choice of heavy metals and for their composition by deuterium, oxygen-18 and tritium. The numerical model was performed with FEFLOW. The hydraulic and hydrochemical characteristics were described for the regolith aquifer and the bedrock aquifer. The regolith aquifer plays the role of the groundwater stock with low conductivity while the fractures of the bedrock may conduct water relatively fast towards extraction points. Flow in fractures of the bedrock depends on the connectivity of the fracture network which might be of local to subregional importance. Stable isotopes in combination with hydrochemistry proved that recharge occurs on catchment scale and exclusively by precipitation. Influx of groundwater from distant areas along dominant structures like the Kandi fault or from the Atacora mountain chain is excluded. The analysis of tritium in groundwater from different depths revealed the interesting fact of the strongly rising groundwater ages. Bedrock groundwater may possibly be much older than 50 years. Equilibrium phases of the silicate weathering products kaolinite and montmorillonite showed that the deeper part of the regolith aquifer and the bedrock aquifer feature either stagnant or less mobile groundwater while the shallow aquifer level is influenced by seasonal groundwater table fluctuations. The hydrochemical data characterised this zone by the progressive change of the hydrochemical facies of recently infiltrated rainwater on its flow path into deeper parts of the aquifers. Surprisingly it was found out that seasonal influences on groundwater hydrochemistry are minor, mainly because they affect only the groundwater levels close to the surface. The transfer of the hydrogeological features of the Upper Ouémé catchment into a regional numerical model demanded a strong simplification. Groundwater tables are a reprint of the general surface morphology. Pumping or other types of groundwater extraction would have only very local impact on the available groundwater resources. It was possible to integrate IMPETUS scenario data into the groundwater model. As a result it was shown that the impact of climate change on the groundwater resources until the year 2025 under the given conditions will be negligible due to the little share of precipitation needed for recharge and the low water needs for domestic use. Reason for concern is the groundwater quality on water points in the vicinity of settlements because of contamination by human activities as shown for the village of Dogué. Nitrate concentrations achieved in many places already alerting levels. Health risks from fluoride or heavy metals were excluded for the Upper Ouémé area.

(Table 1) Cation exchange capacity and other characteristics of soils common in Lower Saxony, Germany

The chemical and biochemical processes associated with the filtration of rainwater through soils, a step in groundwater recharge, were investigated. Under simulated climatic conditions in the laboratory, undisturbed soil columns of partly loamy sands, sandy soils and loess were run as lysimeters. A series of extraction procedures was carried out to determine solid matter in unaltered rock materials and in soil horizons. Drainage water and moisture movement in the columns were analysed and traced respectively. The behaviour of soluble humic substance was investigated by percolation and suspension experiments. The development of seepage-water in the unsaturated zone is closely associated with the soil genetic processes. Determining autonomous chemical and physical parameters are mineral composition and grain size distribution in the original unconsolidated host rock and prevailing climatic conditions. They influence biological activity and transport of solids, dissolved matter and gases in the unsaturated zone. Humic substances, either as amorphous solid matter or as soluble humic acids play a part in diverse sorption, solution and precipitation processes.

Interstitial water chemistry at DSDP Leg 57 Holes

Analyses of water samples taken by means of an in-hole sampler generally show good agreement with analyses of samples collected by routine shipboard squeezing techniques. At Sites 438 and 439, a decrease in salinity with depth is related to former freshwater flow from an aquifer that crops out at an anticline on a deep sea terrace between Japan and the top of the trench slope of the Japan Trench. This former subaerial recharge suggests significant late Cenozoic subsidence of the terrace, because it now lies at a water depth of 1500 meters. Samples from the trench slope at Site 440 have extremely high values of alkalinity and ammonia, presumably because of a favorable combination of high sedimentation rate and organic carbon content. Diagenetic conditions on the trench slope favor formation of the Fe-Mg carbonate mineral, ankerite; at Site 440 it first occurs at a depth below the sea floor of only 29 meters in late Pleistocene strata. Undissolved diatoms persist to relatively great depth at the sites of Leg 57 because of a low geothermal gradient caused by subduction. Secondary silica lepispheres first appear at 851 meters at the most landward and warmest site, Site 438, in strata 16 million years old with an ambient temperature of 31 °C.

Isotopic and chemical compositions of rocks and minerals from the TAG hydrothermal mound, ODP Site 957

Strontium- and oxygen-isotopic measurements of samples recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Leg 158 of the Ocean Drilling Program provide important constraints on the nature of fluid-rock interactions during basalt alteration and mineralization within an active hydrothermal deposit. Fresh Mid-Ocean Ridge Basalt (MORB), with a 87Sr/86Sr of 0.7026, from the basement beneath the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member black smoker fluids. Pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater (>200°C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz+pyrite+paragonite stockwork cut by quartz-pyrite veins. Initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a Mg-bearing mixture of upwelling, high-temperature (>300°C), black smoker-type fluid with a minor (<10%) proportion of seawater. Continued high-temperature (>300°C) interaction between the wallrock and these Mg-bearing fluids results in the complete recrystallization of the wallrock to chlorite+quartz+pyrite. The quartz+pyrite+paragonite assemblage replaces the chloritized basalts, and developed by reaction at 250-360°C with end-member hydrothermal fluids having 87Sr/86Sr ~0.7038, similar to present-day vent fluids. The uniformity of the 87Sr/86Sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87Sr/86Sr ratio of end-member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork. Precipitation of anhydrite in breccias and as late-stage veins throughout most of the mound and stockwork, down to at least 125 mbsf, records extensive entrainment of seawater into the hydrothermal deposit. 87Sr/86Sr ratios indicate that most of the anhydrite formed from ~2:1 mixture of seawater and black smoker fluids (65%±15% seawater). Oxygen-isotopic compositions imply that anhydrite precipitated at temperatures between 147°C and 270°C and require that seawater was conductively heated to between 100°C and 180°C before mixing and precipitation occurred. Anhydrite from the TAG mound has a Sr-Ca partition coefficient Kd ~0.60±0.28 (2 sigma). This value is in agreement with the range of experimentally determined partition coefficients (Kd ~0.27-0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21°N on the East Pacific Rise. The d18O (for SO4) of TAG anhydrite brackets the value of seawater sulfate oxygen (~9.5?). Dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition, in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid-ocean ridges.

Strontium chemistry of anhydrite from DSDP/ODP Hole 504B

A unique record of the chemical evolution of seawater during hydrothermal recharge into oceanic crust is preserved by anhydrite from the volcanic sequences and sheeted dike complex in ODP Hole 504B. Chemical and isotopic analyses 87Sr/86Sr, delta18O, delta34S of anhydrite constrain the changing composition of fluids due to reaction with basalt. There is a general trend of decreasing 87Sr/86Sr of anhydrite, corresponding to the minor incorporation of basaltic strontium with depth in the volcanic rocks. 87Sr/86Sr ratios decrease rapidly with depth in the dikes to values identical to host basalt (0.7029). Sr/Ca ratios (<0.1 mmol/mol) suggest that recharge fluids have very low Sr concentrations and fluids evolve by first precipitating Sr-bearing phases before extensive exchange of Sr with the host basalt. There is a background trend of decreasing sulfate delta18O with depth from +12-13? in the lower volcanics to +7? in the lower sheeted dikes recording an increase in recharge fluid temperature from c. 150° to c. 250°C, and confirming the presence of sulfate in hydrothermal fluids at elevated temperatures. From the amount of anhydrite recovered from Hole 504B and the amount of seawater sulfur that has been reduced to sulfide, a minimum seawater recharge flux can be calculated. This value is 4-25 times lower than estimates of high-temperature fluid fluxes based on either thermal constraints or global chemical budgets and suggests that there is significant deficit of seawater-derived sulfur in the oceanic crust. Only a minor proportion of the seawater that percolates into the crust near the axis is heated to high temperatures and exits as black smoker-type fluids. A significant proportion of the axial heat loss must be advected at 200-250°C by sulfate-bearing hydrothermal solutions that egress diffusely from the crust. These fluids penetrate into the dikes and exchange both heat and chemical tracers without the extensive clogging of porosity by anhydrite precipitation, which would halt hydrothermal circulation for any reasonable fluid flux. The heating of the major proportion of hydrothermal fluids to only moderate temperatures (c. 250°C) reconciles estimates of hydrothermal fluxes derived from thermal models and global geochemical budgets. The flux of hydrothermal sulfate would be of a magnitude similar to the riverine input, and oxygen-isotopic exchange at 200-250°C between dissolved sulfate and recharge fluids during hydrothermal circulation provides a mechanism to continuously buffer seawater sulfate oxygen to the light isotopic composition observed.

Mineralogy, chemistry and stable isotope composition of hydrothermal altered sheeted dikes of ODP Hole 111-504B

During ODP Leg 111 Hole 504B was extended 212 m deeper into the sheeted dikes of oceanic Layer 2, for a total penetration of 1288 m within basement. Study of the mineralogy, chemistry, and stable isotopic compositions of the rocks recovered on Leg 111 has confirmed and extended the previous model for hydrothermal alteration at the site: axial greenschist hydrothermal metamorphism was followed by seawater recharge and subsequent off-axis alteration. The dikes are depleted in 18O (mean delta18O = +5.1 ? +/- 0.6 ?) relative to fresh mid-ocean ridge basalt. Oxygen isotopic data on whole rocks and isolated secondary minerals indicate temperatures during axial metamorphism of 250°-350°C and water/rock ratios about one. Increasing amounts of actinolite with depth in the dike section, however, suggest that temperatures increased downward in the dikes. Pyrite + pyrrhotite + chalcopyrite + magnetite was the stable sulfide + oxide mineral assemblage during axial alteration, but these minerals partly re-equilibrated later at temperatures less than 200°C. The dikes sampled on Leg 111 contain an average of 500 ppm sulfur, slightly lower than igneous values. The delta34S values of sulfide average 0?, which indicates the presence of basaltic sulfide and incorporation of little or no seawater-derived sulfide into the rocks. These data are consistent with models for the presence of rock-dominated sulfur in deep hydrothermal fluids. The presence of anhydrite at 1176 m within basement indicates that unaltered seawater can penetrate to significant depths in the crust during recharge.

Geochemistry of carbonate cements of ODP Leg 112 samples

The evolution of pore fluids migrating through the forearc basins, continental massif, and accretionary prism of the Peru margin is recorded in the sequence of carbonate cements filling intergranular and fracture porosities. Petrographic, mineralogic, and isotopic analyses were obtained from cemented clastic sediments and tectonic breccias recovered during Leg 112 drilling. Microbial decomposition of the organic-rich upwelling facies occurs during early marine diagenesis, initially by sulfate-reduction mechanisms in the shallow subsurface, succeeded by carbonate reduction at depth. Microcrystalline, authigenic cements formed in the sulfate-reduction zone are 13C-depleted (to -20.1 per mil PDB), and those formed in the carbonate-reduction zone are 13C-enriched (to +19.0 per mil PDB). Calcium-rich dolomites and near-stoichiometric dolomites having uniformly heavy d18O values (+2.7 to +6.6 per mil PDB) are typical organic decomposition products. Quaternary marine dolomites from continental-shelf environments exhibit the strongest sulfate-reduction signatures, suggesting that Pleistocene sea-level fluctuations created a more oxygenated water column, caused periodic winnowing of the sediment floor, and expanded the subsurface penetration of marine sulfate. We have tentatively identified four exotic cement types precipitated from advected fluids and derived from the following diagenetic environments: (1) meteoric recharge, (2) basalt alteration, (3) seafloor venting and (4) hypersaline concentration. Coarsely crystalline, low-magnesium (Lo-Mg) calcite cements having pendant and blocky-spar morphologies, extremely negative d18O values (to -7.5 per mil PDB), and intermediate d13C values (-0.4 per mil to +4.6 per mil PDB) are found in shallow-marine Eocene strata. These cements are evidently products of meteoric diagenesis following subaerial emergence during late Eocene orogenic movements, although the strata have since subsided to greater than 4,000 m below sea level. Lo-Mg calcite cements filling scaly fabrics in the late Miocene accretionary prism sediments are apparently derived from fluids having lowered magnesium/calcium (Mg/Ca) and 18O/16O ratios; such fluids may have reacted with the subducting oceanic crust and ascended through the forearc along shallow-dipping thrust faults. Micritic, high-magnesium (Hi-Mg) calcite cements having extremely depleted d13C values (to -37.3%c PDB), and a benthic fauna of giant clams (Calyptogena sp.) supported by a symbiotic, chemoautotrophic metabolism, provide evidence for venting of methane-charged waters at the seafloor. Enriched d18O values (to +6.6%c PDB) in micritic dolomites from the continental shelf may be derived from hypersaline fluids that were concentrated in restricted lagoons behind an outer-shelf basement ridge, reactivated during late Miocene orogenesis.

Hydrogen and oxygen stable isotope analyses of precipitation sampled at Erlangen (southern Germany) between 2010 and 2013

Shallow groundwater aquifers are often influenced by anthropogenic contaminants or increased nutrient levels. In contrast, deeper aquifers hold potentially pristine paleo-waters that are not influenced by modern recharge. They thus represent important water resources, but their recharge history is often unknown. In this study groundwater from two aquifers in southern Germany were analyzed for their hydrogen and oxygen stable isotope compositions. One sampling campaign targeted the upper aquifer that is actively recharged by modern precipitation, whereas the second campaign sampled the confined, deep Benkersandstein aquifer. The groundwater samples from both aquifers were compared to the local meteoric water line to investigate sources and conditions of groundwater recharge. In addition, the deep groundwater was dated by tritium and radiocarbon analyses. Stable and radiogenic isotope data indicate that the deep-aquifer groundwater was not part of the hydrological water cycle in the recent human history. The results show that the groundwater is older than ~20,000 years and most likely originates from isotopically depleted melt waters of the Pleistocene ice age. Today, the use of this aquifer is strictly regulated to preserve the pristine water. Clear identification of such non-renewable paleo-waters by means of isotope geochemistry will help local water authorities to enact and justify measures for conservation of these valuable resources for future generations in the context of a sustainable water management.

Uranium concentrations and isotope ratios Bahamas seawater and ODP holes pore-water

The geometry, timing, and rate of fluid-flow through carbonate margins and platforms is not well constrained. In this study, we use U concentrations and isotope ratios measured on small volumes of pore-water from Bahamas slope sediment, coupled with existing chlorinity data, to place constraints on the fluid-flow in this region and, by implication, other carbonate platforms. These data also allow an assessment of the behaviour of U isotopes in an unusually well constrained water-rock system. We report pore-water U concentrations which are controlled by dissolution of high-U organic material at shallow depths in the sediment and by reduction of U to its insoluble 4+ state at greater depths. The dominant process influencing pore-water (234U/238U) is alpha recoil. In Holocene sediments, the increase of pore-water (234U/238U) due to recoil provides an estimate of the horizontal flow rate of 11 cm/year, but with considerable uncertainty. At depths in the sediment where conditions are reducing, features in the U concentration and (234U/238U) profiles are offset from one another which constrains the effective diffusivity for U in these sediments to be c. 1-2 * 10**-8 cm**2/s. At depths between the Holocene and these reducing sediments, pore-water (234U/238U) values are unusually low due to a recent increase in the dissolution rate of grain surfaces. This suggests a strengthening of fluid flow, probably due to the flooding of the banks at the last deglaciation and the re-initiation of thermally-driven venting of fluid on the bank top and accompanying recharge on the slopes. Interpretation of existing chlorinity data, in the light of this change in flow rate, constrain the recent horizontal flow rate to be 10.6 ( 3.4) cm/year. Estimates of flow rate from (234U/238U) and Cl[-] are therefore in agreement and suggest flow rates close to those predicted by thermally-driven models of fluid flow. This agreement supports the idea that flow within the Bahamas Banks is mostly thermally driven and suggests that flow rates on the order of 10 cm/year are typical for carbonate platforms where such flow occurs.

Mineral content, major oxides and trace elements concentrations in clay from different IODP Holes of 331-C0013, 331-C0017, East China sea shelf and different Taiwan rivers

The Okinawa Trough (OT) in the East Asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic-hydrothermal activities. In this study, the clays collected during IODP Expedition 331 to the middle OT (Iheya North Knoll) were analyzed for mineralogical and geochemical compositions. By comparing with the clays from the East China Sea shelf and surrounding rivers, we examine different clay origins. The hydrothermal field in the mid-OT is dominated by Mg-rich chlorite, while the recharge zone has clay mineral assemblages similar to the shelf and rivers, showing high content of illite, subordinate chlorite and kaolinite and scarce smectite. Compared to the terrigenous clays, the hydrothermal clays in the OT have high concentrations of Mg, Mn and Zr but low Fe, Na, K, Ca, Ba, Sr, P, Sc and Ti, while the hydrothermal clays in the mid-ocean ridge are relatively enriched in Fe and V and depleted in Al, Mg, Zr, Sc and Ti. Different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays, diagnostic of variable clay origins. We infer that the OT hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids. The remarkably different compositions of hydrothermal clays between the sediment-rich back arc basin like OT and the sediment-starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid-rock/sediment reactions under various geologic settings.

(Supplemet S1) Silicon oxide and Potassium + Sodium oxide from the PACManus hydrothermal area

This study presents a systematic analysis and interpretation of autonomous underwater vehicle-based microbathymetry combined with remotely operated vehicle (ROV) video recordings, rock analyses and temperaturemeasurements within the PACManus hydrothermal area located on Pual Ridge in the Bismarck Sea of eastern Manus Basin. The data obtained during research cruise Magellan-06 and So-216 provides a framework for understanding the relationship between the volcanism, tectonismand hydrothermal activity. PACManus is a submarine felsic vocanically-hosted hydrothermal area that hosts multiple vent fields locatedwithin several hundredmeters of one another but with different fluid chemistries, vent temperatures and morphologies. The total area of hydrothermal activity is estimated to be 20,279m**2. Themicrobathymetrymaps combinedwith the ROV video observations allow for precise high-resolution mapping estimates of the areal extents of hydrothermal activity.We find the distribution of hydrothermal fields in the PACManus area is primarily controlled by volcanic features that include lava domes, thick andmassive blocky lava flows, breccias and feeder dykes. Spatial variation in the permeability of local volcanic facies appears to control the distribution of venting within a field.We define a three-stage chronological sequence for the volcanic evolution of the PACManus based on lava flow morphology, sediment cover and lava SiO2 concentration. In Stage-1, sparsely to moderately porphyritic dacite lavas (68-69.8 wt.% SiO2) erupted to form domes or cryptodomes. In Stage-2, aphyric lava with slightly lower SiO2 concentrations (67.2-67.9 wt.% SiO2) formed jumbled and pillowed lava flows. In the most recent phase Stage-3, massive blocky lavaswith 69 to 72.5wt.% SiO2were erupted throughmultiple vents constructing a volcanic ridge identified as the PACManus neovolcanic zone. The transition between these stages may be gradual and related to progressive heating of a silicic magma following a recharge event of hot, mantle-derived melts.

(Appendix) Petrographic information, trace element and Sr isotope ratios of Pito Deep samples

Fluid flow through the axial hydrothermal system at fast spreading ridges is investigated using the Sr-isotopic composition of upper crustal samples recovered from a tectonic window at Pito Deep (NE Easter microplate). Samples from the sheeted dike complex collected away from macroscopic evidence of channelized fluid flow, such as faults and centimeter-scale hydrothermal veins, show a range of 87Sr/86Sr from 0.7025 to 0.7030 averaging 0.70276 relative to a protolith with 87Sr/86Sr of ~0.7024. There is no systematic variation in 87Sr/86Sr with depth in the sheeted dike complex. Comparison of these new data with the two other localities that similar data sets exist for (ODP Hole 504B and the Hess Deep tectonic window) reveals that the extent of Sr-isotope exchange is similar in all of these locations. Models that assume that fluid-rock reaction occurs during one-dimensional (recharge) flow lead to significant decreases in the predicted extent of isotopic modification of the rock with depth in the crust. These model results show systematic misfits when compared with the data that can only be avoided if the fluid flow is assumed to be focused in isolated channels with very slow fluid-rock exchange. In this scenario the fluid at the base of the crust is little modified in 87Sr/86Sr from seawater and thus unlike vent fluids. Additionally, this model predicts that some rocks should show no change from the fresh-rock 87Sr/86Sr, but this is not observed. Alternatively, models in which fluid-rock reaction occurs during upflow (discharge) as well as downflow, or in which fluids are recirculated within the hydrothermal system, can reproduce the observed lack of variation in 87Sr/86Sr with depth in the crust. Minimum time-integrated fluid fluxes, calculated from mass balance, are between 1.5 and 2.6 * 10**6 kg/m**2 for all areas studied to date. However, new evidence from both the rocks and a compilation of vent fluid compositions demonstrates that some Sr is leached from the crust. Because this leaching lowers the fluid 87Sr/86Sr without changing the rock 87Sr/86Sr, these mass balance models must underestimate the time-integrated fluid flux. Additionally, these values do not account for fluid flow that is channelized within the crust.

Descriptions, isotopes and minerals of sediments at DSDP Leg 64 Holes

Mineralogical and oxygen isotopic analyses of samples from Deep Sea Drilling Project Sites 477, 481, and 477 in the Guaymas Basin indicate the existence of two distinct hydrothermal systems. In the first, at Sites 481 and 478, hot dolerite sills intruded into highly porous hemipelagic siliceous mudstones that were moderately rich in organic matter, thermally altered the adjacent sediments, and expelled hydrothermal pore fluids. The second, at Site 477 and active at present, is most probably caused by a recent igneous intrusion forming a magma chamber at shallow depth. In the first hydrothermal system, the main thermal reactions above and below the sills are dissolution of opal-A and formation of quartz, either directly or through opal-CT; formation of smectite; formation of analcime only above the sills; dissolution and recrystallization of calcite and occasional formation of dolomite or protodolomite. The d18O values of the hydrothermally altered sediments range from 9.9 to 12.2 per mil (SMOW). The d18O values of recrystallized calcites above the first sill complex, Site 481, indicate temperatures of 140° to 170°C. No fluid recharge is required in this system. The thickness of the sill complexes and the sequence and depth of intrusion into the sediment column determine the thickness of the alteration zones, which ranges from 2 or 3 to approximately 50 meters. Generally, the hydrothermally altered zone is thicker above than below the sill. In the second type, the sediments are extensively recrystallized. The characteristic greenschist-facies mineral assemblage of quartz-albite-chlorite-epidote predominates. Considerable amounts of pyrite, pyrrhotite, and sphene are also present. The lowest d18O value of the greenschist facies rocks is 6.6 per mil, and the highest d18O value of the associated pore fluids is +1.38 per mil (SMOW). The paragenesis and the oxygen isotopes of individual phases indicate alteration temperatures of 300 ± 50°C. On the basis of the oxygen isotopes of the solids and associated fluids, it is concluded that recharge of fluids is required. The water/rock ratio in wt.% is moderate, approximately 2/1 to 3/1 - higher than the calculated water/rock ratio of the hydrothermal system at the East Pacific Rise, 21 °N.

Chemical and isotopic compositions of groundwaters in the Badain Jaran Desert, Northern China

Despite its extreme aridity the Badain Jaran Desert is rich in groundwater. In the southeastern part of this desert it is characterized by coexistence of high megadunes and a great number of lakes. Deuterium and oxygen 18 isotope compositions as well as hydrochemistry of groundwater, lake water, soil water and river water were investigated in detail to gain an insight into their relationships and the origin of the groundwater. The results show that the groundwater and the lake water are genetically related, but unrelated to local precipitation and the leakage of Heine River at the northern slope of the Qilian mountain. dD and d18O values of deep soil water (deeper than 40 cm) and groundwater plot on the same evaporation line E11, which shows that they have the same recharge source. The point of intersection between E11 and LMWL suggests that the groundwater originates from a water resource, which has a weighted mean value that is lighter by some 6 per mil d18O than local precipitation in Badain Jaran Desert. 3H data of water samples show that the groundwater in the Badain Jaran Desert originates from water recharged after the nuclear test. The deep fault zone underground maybe a water circulation channel based on helium analysis of groundwater. The result has guiding significance to rational exploitation and utilization of the local groundwater.

(Table 1) Chloride concentrations in water obtained from observation wells on Helgoland Dune, North Sea

The hydrogeological conditions are unfavourable for a sufficient supply of drinking-water. The small size of the catchment area, the large hydraulic gradient inside the steep 'Buntsandstein'-cliff and the low geodetic level of the 'Dune Island' and the foreshore at the eastern foot of the cliff do not allow the formation and recharge of a sufficiently exploitable geodetic freshwater dome over the underlying saltwater. This means that until recently the provision of sufficient drinking-water for the island's inhabitants, for its garrison as well as for visiting ships was a problem. This problem has now been solved by the desalination of seawater.