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1.
Arsenic (As) contamination in groundwater in mineralized areas typically results from the oxidation of As-rich sulfide minerals in aquifers, from hydrothermal alteration of geothermal systems, or as a result of anthropogenic influences such as mining activity. The primary goal of this study was to determine the spatial and temporal variance in As concentrations in shallow groundwater in a mineralized area and to identify the main As source controlling the concentration patterns. To this end, a combination of a geostatistical technique for space–time modeling of As concentrations and a numerical simulation, which models the transport of As in groundwater, is implemented. A study site in North Sulawesi, Sulawesi Island, Indonesia was selected as it was suitable for investigating the importance of fault lines and metal mining on As contamination. Initially, stable isotope analysis was used to ascertain the groundwater source and the mixing mechanism of the shallow and deep groundwater. Geostatistical modeling revealed consistent general patterns of As concentrations during the past 10 years, with high concentrations found along a NW–SE axis. By matching the geostatistical results with the distributions of As concentrations obtained through transport modeling, the deep-seated hydrothermal system along the fault zone was found to be the major As source. Wastewater from the mine was also observed to be a local As source. Another important influence on the As concentration pattern was a river, which acted as a boundary to separate the groundwater systems into two regions.  相似文献   

2.
Arsenic (As) concentrations as high as 179 μg/L have been observed in shallow groundwater in the Alberta’s Southern Oil Sand Regions. The geology of this area of Alberta includes a thick cover (up to 200 m) of unconsolidated glacial deposits, with a number of regional interglacial sand and gravel aquifers, underlain by marine shale. Arsenic concentrations observed in 216 unconsolidated sediment samples ranged from 1 and 17 ppm. A survey of over 800 water wells sampled for As in the area found that 50% of the wells contained As concentrations exceeding drinking water guidelines of 10 μg/L. Higher As concentrations in groundwater were associated with reducing conditions. Measurements of As speciation from 175 groundwater samples indicate that As(III) was the dominant species in 74% of the wells. Speciation model calculations showed that the majority of groundwater samples were undersaturated with respect to ferrihydrite, suggesting that reductive dissolution of Fe-oxyhydroxides may be the source of some As in groundwater. Detailed mineralogical characterization of sediment samples collected from two formations revealed the presence of fresh framboidal pyrite in the deeper unoxidized sediments. Electron microprobe analysis employing wavelength dispersive spectrometry indicated that the framboidal pyrite had variable As content with an average As concentration of 530 ppm, reaching up to 1840 ppm. In contrast, the oxidized sediments did not contain framboidal pyrite, but exhibited spheroidal Fe-oxyhydroxide grains with elevated As concentrations. The habit and composition suggest that these Fe-oxyhydroxide grains in the oxidized sediment were an alteration product of former framboidal pyrite grains. X-ray absorption near edge spectroscopy (XANES) indicated that the oxidized sediments are dominated by As(V) species having spectral features similar to those of goethite or ferrihydrite with adsorbed As, suggesting that Fe-oxyhydroxides are the dominant As carriers. XANES spectra collected on unoxidized sediment samples, in contrast, indicated the presence of a reduced As species (As(−I)) characteristic of arsenopyrite and arsenian pyrite. The results of the mineralogical analyses indicate that the oxidation of framboidal pyrite during weathering may be the source of As released to shallow aquifers in this region.  相似文献   

3.
This study examined the natural and anthropogenic pollution of groundwater at a national groundwater monitoring station (NGMS) in a dyeing industry complex, Korea. The arsenic contamination of a shallow well at the NGMS was noticed, starting from 22 months after the well installation. Possibilities of several mechanisms for As pollution of the groundwater were examined. The arsenical pyrite oxidation as a source mechanism in the shallow aquifer may be disregarded because of deficiency of pyrite in the shallow sediments, concomitant with depleted dissolved oxygen and very low levels of redox potentials of the As-polluted groundwater. The effect of wastewater from the general industrial area through a covered sewer stream was also considered as a possible source. Even though occurrence patterns of phenol and volatile organic hydrocarbons were very similar to those of the polluted shallow groundwater, As was not detected in the wastewater. One of the most plausible sources of the arsenic pollution was believed to be the reductive dissolution of Fe hydroxide. The As-polluted shallow groundwater had also very high levels of pH, HCO3, COD and very low levels of DO and NO3, which support the possibility of As pollution by the reductive dissolution. Consequently, the shallow groundwater in and around the NGMS has been polluted with various contaminants including As, phenol, chlorinated solvents, and petroleum hydrocarbons through multiple sources of contamination, such as natural reductive dissolution, dyeing wastewater, industrial wastewater, and municipal sewage.  相似文献   

4.
The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.  相似文献   

5.
The Tibetan plateau is characterized by intense hydrothermal activity and abnormal enrichment of trace elements in geothermal waters. Hydrochemistry and B isotope samples from geothermal waters in Tibet were systematically measured to describe the fractionation mechanisms and provide constraints on potential B reservoirs. B concentrations range from 0.35 to 171.90 mg/L, and isotopic values vary between −16.57 ‰ and +0.52 ‰. Geothermal fields along the Indus-Yarlung Zangbo suture zone and N–S rifts are observed with high B concentrations and temperatures. The similar hydrochemical compositions of high-B geothermal waters with magmatic fluid and consistent modeling of B isotopic compositions with present δ11B values imply that the B in high-B geothermal waters is mainly contributed by magmatic sources, probably through magma degassing. In contrast, geothermal fields in other regions of the Lhasa block have relatively low B concentrations and temperatures. After considering the small fractionation factor and representative indicators of Na/Ca, Cl/HCO3, Na + K and Si, the conformity between modeling results and the isotopic compositions of host rocks suggests that the B in low-temperature geothermal fields is mainly sourced from host rocks. According to simulated results, the B in some shallow geothermal waters not only originated from mixing of cold groundwater with deep thermal waters, but it was also contributed by equilibration with marine sedimentary rocks with an estimated proportion of 10%. It was anticipated that this study would provide useful insight into the sources and fractionation of B as well as further understanding of the relationships between B-rich salt lakes and geothermal activities in the Tibetan plateau.  相似文献   

6.
The intensity and distribution of hydrothermal alteration are frequently used during the exploration and assessment of a geothermal prospect to estimate the size, shape and temperature of a thermal system. Geochemical and petrographic observations used to characterize the hydrothermal alteration include the mapping of both trace- and major-element dispersion patterns and the distribution of secondary mineral assemblages.This paper describes the trace-element and mineralogical distributions common to many of the high-temperature systems (> 150°C) that we have studied. However, examples of important geochemical relationships are primarily drawn from our detailed investigations of the Roosevelt Hot Springs thermal system in southern Utah. The hydrothermal fluids at Roosevelt Hot Springs are enriched in sodium chloride and contain approximately 9000 ppm total dissolved solids. The reservoir, with a base temperature near 270°C, is located in fractured gneisses and granites.At Roosevelt Hot Springs, the surface discharges consist of opaline and chalcedonic sinter, and alluvium cemented by silica, calcite, Mn oxide and Fe oxide. The geochemistry of these surface deposits is extremely variable, but locally they contain up to 5.5 ppm Hg, 858 ppm As, 18.8% Mn, 230 ppm Cu, 290 ppm Sb, 294 ppm W, 17 ppm Li, 68 ppm Pb, 26 ppm Zn, 4.9% Ba and 100 ppm Be. High concentrations of Au and Ag, although not present in the sinters at Roosevelt Hot Springs, occur in hot spring deposits from other chemically similar systems such as Steamboat Springs, Nevada.Mercury and As are the most widely distributed trace elements in the surface samples. Their distribution in soils overlying the thermal system expands the area of interest and helps define the high-temperature portion of the system. The highest concentrations of Hg and As, of up to 5.5 and 26 ppm, respectively, occur in soils within 300 m of the thermal discharges. A broader area extending up to 1000 m from the surficial thermal activity also contains ppb. Mercury anomalies tend to mark the location of faults within the uppermost portions of the reservoir and areas where the thermal fluids move laterally away from the thermal system toward the adjacent valley.Depletions of Mn, Cu and Zn are found in the acid-altered soils and in alluvium associated with the hot spring deposits and fumaroles. The acid alteration occurs locally in areas of surficial thermal activity and persists to depths of less than 60 m. Alteration minerals within these zones include alunite, jarosite, native sulphur, opal, chalcedony, kaolinite, sericite, montmorillonite, and mixed-layer clays. The formation of acid waters occurs near the surface and results from the oxidation of H2S contained within gases evolving from the fumaroles or within waters discharged by the hot springs. The locally intense acid-sulphate alteration and scavenging of metals within the soils occurs as the fluids percolate downward.Alteration mineralogy at depth is determined through examination of down-hole samples which penetrate the geothermal system to depths in excess of 2 km. Reservoir rocks of temperatures below about 210°C contain an alteration assemblage with mixed-layer clays, montmorillonite, sericite, pyrite, hematite, magnetite, calcite, chlorite, quartz, and potassium feldspar. At higher temperatures, mixed-layer clays and montmorillonite disappear and anhydrite appears locally.Altered rocks within the high temperature portions of the thermal field are characterized by anomalous concentrations of As and Li. Selective chemical leaching of the altered rocks and electron microprobe analyses indicate that As is contained primarily in pyrite or iron oxides after pyrite whereas Li occurs in clays and micas.Mercury exhibits an inverse relationship with temperature and is concentrated in the cooler portions of the thermal system to depths marked approximately by the 200°C isotherm. This distribution is similar to the distribution of clay minerals in the reservoir rock. Heating experiments indicate that Hg occurs primarily as Hg° and that it is readily mobilized by the thermal system at temperatures in excess of 200–250°C.  相似文献   

7.
Magnetic susceptibility and petrographic studies of drilled rock cuttings from two geothermal wells (Az-26 and Az-49) of the important electricity-generating geothermal system, Los Azufres, Mexico, were carried out to determine the relation between the magnetic susceptibility of rocks, the concentration of magnetic minerals and hydrothermal alteration. For this purpose, low-frequency magnetic susceptibility (χ lf) was measured and compared its distribution trends with those of magnetic and Fe–Mg silicate minerals, and with the extent of hydrothermal alteration in rocks of the two geothermal wells. The study indicates a decrease in χ lf values with depth in the two geothermal wells corresponding with: (1) an increase in the reservoir temperature and hydrothermal alteration; and (2) a decrease in the concentrations of Fe–Mg silicates and opaque minerals. The data suggest that ferromagnesian minerals and opaque minerals like ilmenite are the main contributors to the χ lf of rocks. The decrease in χ lf, ilmenite, and Fe–Mg mineral contents with an increase in the hydrothermal alteration degree, pyrite and haematite contents suggests the hydrothermal alteration of ilmenite and Fe–Mg minerals (characteristic of high χ lf values) to pyrite, haematite and other opaque minerals (with low χ lf values). The interaction of hydrothermal fluids with rocks results in the hydrothermal alteration of primary minerals. In a geothermal area, an anomaly of low magnetic susceptibility values of rocks in a homogenous litho unit characterized by high magnetic susceptibility may suggest hydrothermal alteration. Magnetic susceptibility can be a useful parameter, during the initial stages of geothermal exploration, in identifying hydrothermally altered rocks and zones of hydrothermal alteration both at the surface and from drilled wells in geothermal systems.  相似文献   

8.
The present study highlights the first evidence of hydrothermal mineral Thenardite (Na2SO4) from Puga geothermal area, North-western Himalayan belt in Ladakh Geothermal Province, India, which is unequivocal evidence for the presence of high-temperature hydrothermal fluid activity from one of the thickest crust areas of the Earth. The Puga geothermal belt illustrates a fault-bounded hydrothermal system with a clearly defined conductive zone, coinciding with Kiagar Tso fault typically exemplifying a shallow-level medium enthalpic geothermal reservoir. The hydrogeochemistry suggests that thermal and non-thermal waters are of Na-Cl-HCO3 and Ca-Mg-HCO3 type, respectively, with neutral to near alkaline pH. The silica and cation geothermometry reveal sub-surface temperatures around 150 °C and 250 °C, respectively, at shallow depth; however, >250 °C is anticipated at the deepest levels (~3 km). Stable isotope (δD and δ18O) studies explicate depletion of isotopic content for thermal waters over Puga river water and radiogenic isotope (3H) suggests matured thermal waters with ongoing water-rock interactions. The recharge altitude estimation and physiographic studies put forth that geothermal reservoir is recharged with the ice masses located at an altitude of 6458 m above mean sea level (msl) in the west of Puga valley, probably from the highest peak of Polokong La mountain. The two key processes participating in regulation of proportions of the dissolved salts in the thermal waters are silicate weathering and ion-exchange kinetics. The powder X-ray diffraction study reveals a major occurrence of hydrothermal mineral thenardite in the hot spring deposits for the first time along with huge encrustations of trona, borax, calcite and elemental sulfur. The high-temperature fluids encounter thenardite, pyrite, and jarosite-bearing minerals in basement rock causing enrichment of SO42− and Cl− in geothermal waters. The temperature-dependent speciation modelling (50 °C–200 °C) for major ion Na+ reveals the composition of the reservoir fluid (~150 °C): Na+ > NaCO3− > NaSO4− > NaHCO3 > NaF > NaOH. A conceptual evolution model of thermal waters involving the recharge-deep circulation-mixing-discharge of thermal springs is hence put forth in the study using various hydrogeochemical insights.  相似文献   

9.
Geothermal resources are very rich in Yunnan, China. However, source of dissolved solutes in geothermal water and chemical evolution processes remain unclear. Geochemical and isotopic studies on geothermal springs and river waters were conducted in different petrological-tectonic units of western Yunnan, China. Geothermal waters contain Ca–HCO3, Na–HCO3, and Na (Ca)–SO4 type, and demonstrate strong rock-related trace elemental distributions. Enhanced water–rock interaction increases the concentration of major and trace elements of geothermal waters. The chemical compositions of geothermal waters in the Rehai geothermal field are very complicated and different because of the magma chamber developed at the shallow depth in this area. In this geothermal field, neutral-alkaline geothermal waters with high Cl, B, Li, Rb Cs, As, Sb, and Tl contents and acid–sulfate waters with high Al, Mn, Fe, and Pb contents are both controlled by magma degassing and water–rock interaction. Geothermal waters from metamorphic, granite, and sedimentary regions (except in the Rehai area) exhibit varying B contents ranging from 3.31 mg/L to 4.49 mg/L, 0.23 mg/L to 1.24 mg/L, and <0.07 mg/L, respectively, and their corresponding δ11B values range from −4.95‰ to −9.45‰, −2.57‰ to −8.85‰, and −4.02‰ to +0.06‰. The B contents of these geothermal waters are mainly controlled by leaching host rocks in the reservoir, and their δ11B values usually decrease and achieve further equilibrium with its surrounding rocks, which can also be proven by the positive δ18O-shift. In addition to fluid–rock reactions, the geothermal waters from Rehai hot springs exhibit higher δ11B values (−3.43‰ to +1.54‰) than those yielded from other areas because mixing with the magmatic fluids from the shallow magma. The highest δ11B of steam–heated waters (pH 3.25) from the Zhenzhu spring in Rehai is caused by the fractionation induced by pH and the phase separation of coexisting steam and fluids. Given the strong water–rock interaction, some geothermal springs in western Yunnan show reservoir temperatures higher than 180 °C, which demonstrate potential for electricity generation and direct-use applications. The most potential geothermal field in western Yunnan is located in the Rehai area because of the heat transfer from the shallow magma chamber.  相似文献   

10.
Geochemical reactive transport modeling was coupled to bench-scale leaching experiments to investigate and verify the mobilization of geogenic arsenic (As) under a range of redox conditions from an arsenic-rich pyrite bearing limestone aquifer. Modeling and experimental observations showed similar results and confirmed the following: (1) native groundwater and aquifer matrix, including pyrite, were in chemical equilibrium, thus preventing the release of As due to pyrite dissolution under ambient conditions; (2) mixing of oxygen- and nitrate-rich surface water with oxygen-depleted native groundwater changed the redox conditions and promoted the dissolution of pyrite, and (3) the behavior of As along a flow path was controlled by a complex series of interconnected reactions. This included the oxidative dissolution of pyrite and simultaneous sorption of As onto neo-formed hydrous ferric oxides (HFO), followed by the reductive dissolution of HFO and secondary release of adsorbed As under reducing conditions. Arsenic contamination of drinking water in these systems is thus controlled by the re-equilibration of the system to more reducing conditions rather than a purely oxidative process.  相似文献   

11.
This study investigates the origin and chemical composition of the thermal waters of Platystomo and Smokovo areas in Central Greece as well as any possible relationships of them to the neighboring geothermal fields located in the south-eastern part of Sperchios basin. The correlations between different dissolved salts and the temperature indicate that the chemical composition of thermal waters are controlled by, the mineral dissolution and the temperature, the reactions due to CO2 that originates possibly by diffusion from the geothermal fields of Sperchios basin and the mixing of thermal waters with fresh groundwater from karst or shallow aquifers. Two major groups of waters are recognized on the basis of their chemistry: thermal waters of Na–HCO3–Cl type and thermal waters mixed with fresh groundwater of Ca–Mg–Na–HCO3 type. All thermal waters of the study area are considered as modified by water–rock interaction rainwater, heated in depth and mixed in some cases with fresh groundwater when arriving to the surface. Trace elements present low concentrations. Lithium content suggests discrimination between the above two groups of waters. Boron geochemistry confirms all the above remarks. Boron concentration ranges from 60 μg L?1 to 10 mg L?1, while all samples’ constant isotopic composition (δ11B ≈ 10 ‰) indicates leaching from rocks. The positive correlation between the chemical elements and the temperature clearly indicates that much of the dissolved salts are derived from water–rock interactions. The application of geothermometers suggests that the reservoir temperature is around 100–110 °C. Chalcedony temperatures are similar to the emergent temperatures and this is typical of convective waters in fault systems in normal thermal gradient areas.  相似文献   

12.
Field and laboratory investigations were performed to identify the principal mechanisms of the hydrochemical groundwater evolution among low mineralised groundwater in the Triassic Bunter sandstone aquifer of the Odenwald low mountain range, central Germany. Hydrochemical composition comprises low pH, SO4-rich shallow groundwaters issued by springs (Ca-Mg-SO4-type) grading to SO4-poor deep groundwaters with near-neutral pH (Ca-HCO3-type). Batch experiments of the original rock were run to determine primary mineral alteration reactions and the origin of dissolved ions. Principal experimental reactions comprise the decomposition of anorthite, K-feldspar, biotite and jarosite as mineral components of the original sandstone rock and the formation of clay minerals of the smectite group (Ca-montmorillonite, beidellite), and iron hydroxides as secondary minerals. Mobilisation of fluid inclusion in quartz grains contributes to Na and Cl concentrations in the leachates. The evolution of deep groundwater circulation proceeds by mineral alteration reactions calculated by the inverse modelling of both primary and secondary minerals to produce low-T mineral phases. The dissolution of K-feldspar converts Ca-montmorillonite to illite (illitisation). The formation of Na-beidellite correlates with decreasing concentration of Na in solution. Mineral reactions further proceed to the formation of kaolinite as stable mineral phase. As indicated by modelled adsorption curves, the decrease of SO4 concentrations during groundwater evolution relates to the adsorption of SO4 on iron hydroxides. The leaching of calcite indicated for individual groundwaters relates to the distribution of loess in the appropriate catchment areas.  相似文献   

13.
This study documents the environmental impacts of borate mines in Bigadiç district, which are the largest colemanite and ulexite deposits in the world. Borate-bearing formations have affected the concentrations of some contaminants in groundwater. Groundwater quality is directly related to the borate zones in the mines as a result of water–rock interaction processes. Calcium is the dominant cation and waters are Ca–SO4 and HCO3 type in the mine (Tülü borate mine) from which colemanite is produced. However in the Simav and Acep Borate Mines, ulexite and colemanite minerals are produced and waters from these open pit mines are Na–HCO3–SO4 types. High SO4 concentrations (reaching 519 mg/L) might be explained by the existence of anhydrite, gypsum and celestite minerals in the borate zone. Groundwater from tuff and borate strata showed relatively low pH values (7–8) compared to surface and mine waters (>8). EC values ranged from 270 to 2850 μS/cm. Boron and As were the two important contaminants determined in the groundwaters around the Bigadiç borate mines. Arsenic is the major pollutant and it ranged from 33 to 911 μg/L in the groundwater samples. The concentrations of B in the study area ranged from 0.05 to 391 mg/L. The highest B concentrations were detected at the mine areas. The extension of the borate zones in the aquifer systems is the essential factor in the enrichment of B and As, and some major and trace elements in groundwaters are directly related to the leaching of the host rock which are mainly composed of tuffs and limestones. According to drinking water standards, all of the samples exceed the tolerance limit for As. Copper, Mn, Zn and Li values are enriched but do not exceed the drinking water standards. Sulfate, Al and Fe concentrations are above the drinking water standard for the groundwater samples.  相似文献   

14.
The Complex Terminal (CT) confined aquifer of the Djerid basin, southwestern Tunisia, was studied using major ion concentrations and stable isotope contents in order to (1) investigate the changes on its hydrodynamic functioning due to the long-term over-pumping and the large-scale flood irrigation practices, (2) determine the principal mineralization processes of its fossil groundwater, and (3) examine the mode of recharge of this aquifer and whether it contains part of modern hydrological regime. The observed geochemical patterns indicated that the main mineralization processes affecting the CT groundwater water/rock interactions and mixing. The native Na > Cl and Cl > SO4 > Ca > Na waters, resulting from the dissolution of halite and gypsum and from pyrite oxidation, interacted with those of the underlying and the overlying aquifers without changing their chemical facies. Stable isotope data provided evidences about upward and downward leakage into the CT aquifer and their relationships with anthropogenic activities. They demonstrated that the long-term over-pumping of the CT aquifer, which contributed to the loss of its potentiometric pressure, favored the upward leakage of the artesian deep groundwater along parts of the major faults. Moreover, the large-scale flood irrigation practices in the oases domain, which ensured the recharge of the shallow water table by return flow, enhanced the downward leakage toward the CT aquifer.  相似文献   

15.
The near-surface water cycle in a geologically complex area comprises very different sources including meteoric, metamorphic and magmatic ones. Fluids from these sources can react with sedimentary, magmatic and/or metamorphic rocks at various depths. The current study reports a large number of major, minor and trace element analyses of meteoric, mineral, thermal and mine waters from a geologically well-known and variable area of about 200 × 150 km in SW Germany. The geology of this area comprises a Variscan granitic and gneissic basement overlain in parts by Triassic and Jurassic shales, sandstones and limestones. In both the basement and the sedimentary rocks, hydrothermal mineralization occurs (including Pb, Cu, As, Zn, U, Co and many others) which were mined in former times. Mineral waters, thermal waters and meteoric waters flowing through abandoned mines (mine waters) are distributed throughout the area, although the mine waters concentrate in and around the Schwarzwald.The present analyses show, that the major element composition of a particular water is determined by the type of surrounding rock (e.g., crystalline or sedimentary rocks) and the depth from which the water originates. For waters from crystalline rocks it is the origin of the water that determines whether the sample is Na–Cl dominant (deeper origin) or Ca–HCO3 dominant (shallow origin). In contrast, compositions of waters from sedimentary rocks are determined by the availability of easily soluble minerals like calcite (Ca–HCO3 dominant), halite (Na–Cl dominant) or gypsum (Ca–SO4 dominant). Major element data alone cannot, therefore, be used to trace the origin of a water. However, the combination of major element composition with trace element data can provide further information with respect to flow paths and fluid–rock interaction processes. Accordingly, trace element analyses showed, that:
  • −Ce anomalies can be used as an indicator for the origin of a water. Whereas surface waters have negative or strongly negative Ce anomalies, waters originating from greater depths show no or only weak negative Ce anomalies.
  • −Eu anomalies can be used to differentiate between host rocks. Waters from gneisses display positive Eu anomalies, whereas waters from granites have negative ones. Waters from sedimentary rocks do not display any Eu anomalies.
  • −Rb and Cs can also be indicators for the rock with which the fluid interacted: Rb and Cs correlate positively in most waters with Rb/Cs ratios of ∼2, which suggests that these waters are in equilibrium with the clay minerals in the rocks. Rb/Cs ratios >5 indicate reaction of a water with existing clay minerals, whereas Rb/Cs ratios <2 are probably related to host rock alteration and clay mineral formation.
The chemical compositions of carbonate precipitates from thermal waters indicate that rare earth elements (REEs), Rb and Cs concentrations in the minerals are controlled by the incorporation of clay particles that adsorb these elements.  相似文献   

16.
This paper describes the mobilization and speciation of As found in hydrothermally altered rock under oxic column conditions. The altered rock sample was obtained from a tunnel project located in the Nakakoshi area of Hokkaido, Japan, whose geology is represented by slate, shale and sandstone. This area has undergone silicification, pyritization and argillic alteration resulting in As-enrichment of the rock. Results of the column experiments show that the infiltration rate, bulk density and rock bed thickness affected the duration of water residence, which in turn influenced the pH of the rock–water system. Coexisting ions most notably Ca2+ at amounts greater than ca. 50 mg/L retarded the mobilization of As. Mobilization of As from the rock with time occurred in two stages: stage 1 (weeks 1–20) with higher As leaching and stage 2 (weeks 20–76) characterized by nearly constant As release. In addition, pore water As concentrations revealed that the columns developed into two regions: the top half where most of the leaching occurred and the bottom part dominated by adsorption. Thus, the mechanism controlling the mobilization of As from the rock is a combination of one or more of the following processes: dissolution of soluble As-bearing fractions, pyrite oxidation and adsorption reactions. Arsenite (As[III]) was the dominant species in the effluent at the start of the experiment in columns with shorter water residence time and lower pH conditions (<8). On the other hand, arsenate (As[V]) was the major inorganic species released from the rock at higher pH (8–9.5) and when the system was close to equilibrium. Speciation of As with depth also indicated that As[III] disappeared around the bottom half of the columns, probably as a result of adsorption and/or oxidation. Arsenic speciation is partially controlled by the pH dependent adsorption of As species. The important adsorbent phases in the rock included Fe–Al oxides/oxyhydroxides, clay minerals and organic matter, which permitted the columns to attenuate additional As loadings including As[III]. Implications of these results on the design of a novel disposal method for these altered rocks include the enhancement of As adsorption through the addition of natural or artificial adsorbents and the utilization of a covering soil with low permeability to minimize rainwater infiltration into the rock.  相似文献   

17.
The disseminated gold deposit of Agdarreh (24.5 t at 3.7 g/t Au) is hosted in hydrothermally leached Miocene reefal limestone in the Takab geothermal field, which is part of the Cenozoic Urumieh–Dokhtar volcanic arc of NW Iran. Alteration and mineralisation are largely bedding controlled blanket-like and include: (1) pre-ore decalcification; (2) first-stage silicification associated with pyrite (early pyrite with 3–4 wt% As, late pyrite with <1–3 wt% As) and sphalerite; (3) second-stage silicification with precipitation of galena, Pb–Sb–As sulphides, sulphosalts, tellurides and native bismuth; (4) late-stage cinnabar and barite in vugs; (5) oxide ore stage and carbonate alteration (complex Mn–Fe-rich oxyhydroxides, arsenates, sulphates, APS minerals and rutile in residual leached rock and infill of karstic cavities). Gold occurs invisibly in the jasperoids and is enriched in the Mn–Fe oxyhydroxide surface cap of the jasperoids. Gold mineralisation is associated with the hydrothermal metal suite of As, Sb, Hg, Te, Se, Tl, Ba, Zn, Ag, Cd, Bi and Pb, and is characterised by very low Cu contents. Arsenian pyrite probably carried most of the primary (invisible) gold. Native gold occurs in association with the late-stage cinnabar and the oxide ore. The Agdarreh deposit shows many similarities with Carlin-type ore and is interpreted to have resulted from near-surface hydrothermal activity related to the Cenozoic arc volcanism that developed within the extensional Takab graben. The extensive oxidation at Agdarreh may be partly due to the waning stages of hydrothermal activity. Active H2S-bearing thermal springs are locally depositing extremely high contents of Au and Ag, and travertine is present over large areas, suggesting that ore-forming hydrothermal activity occurred periodically from the Miocene to Recent in the Takab geothermal field. The present paper deals with the geological framework, host rocks, characteristic features of hydrothermal alteration and mineralisation, and genesis of the Agdarreh deposit. The results of fluid inclusion and stable isotope studies are in progress and will be given in a forthcoming paper.  相似文献   

18.
The present study has examined the relationship of groundwater arsenic (As) levels in alluvial aquifers with topographic elevation, slope, and groundwater level on a large basinal-scale using high-resolution (90 m × 90 m) Shuttle Radar Topography Mission (SRTM) digital elevation model and water-table data in Bangladesh. Results show that high As (>50 μg/l) tubewells are located in low-lying areas, where mean surface elevation is approximately 10 m. Similarly, high As concentrations are found within extremely low slopes (<0.7°) in the country. Groundwater elevation (weekly measured by Bangladesh Water Development Board) was mapped using water-table data from 950 shallow (depth <100 m) piezometers distributed over the entire country. The minimum, maximum and mean groundwater elevation maps for 2003 were generated using Universal Kriging interpolation method. High As tubewells are located mainly in the Ganges–Brahmaputra–Meghna delta, Sylhet Trough, and recent floodplains, where groundwater elevation in shallow aquifers is low with a mean value of 4.5 m above the Public Works Datum (PWD) level. Extremely low groundwater gradients (0.01–0.001 m/km) within the GBM delta complex hinder groundwater flow and cause slow flushing of aquifers. Low elevation and gentle slope favor accumulation of finer sediments, As-carrying iron-oxyhydroxide minerals, and abundant organic matter within floodplains and alluvial deposits. At low horizontal hydraulic gradients and under reducing conditions, As is released in groundwater by microbial activity, causing widespread contamination in the low-lying deltaic and floodplain areas, where As is being recycled with time due to complex biogeochemical processes.  相似文献   

19.
Groundwater chemistry and isotope data from 44 public supply wells in the Napa and Sonoma Valleys, California were determined to investigate mixing of relatively shallow groundwater with deeper hydrothermal fluids. Multivariate analyses including Cluster Analyses, Multidimensional Scaling (MDS), Principal Components Analyses (PCA), Analysis of Similarities (ANOSIM), and Similarity Percentage Analyses (SIMPER) were used to elucidate constituent distribution patterns, determine which constituents are significantly associated with these hydrothermal systems, and investigate hydrothermal contamination of local groundwater used for drinking water. Multivariate statistical analyses were essential to this study because traditional methods, such as mixing tests involving single species (e.g. Cl or SiO2) were incapable of quantifying component proportions due to mixing of multiple water types. Based on these analyses, water samples collected from the wells were broadly classified as fresh groundwater, saline waters, hydrothermal fluids, or mixed hydrothermal fluids/meteoric water wells. The Multivariate Mixing and Mass-balance (M3) model was applied in order to determine the proportion of hydrothermal fluids, saline water, and fresh groundwater in each sample. Major ions, isotopes, and physical parameters of the waters were used to characterize the hydrothermal fluids as Na–Cl type, with significant enrichment in the trace elements As, B, F and Li. Five of the wells from this study were classified as hydrothermal, 28 as fresh groundwater, two as saline water, and nine as mixed hydrothermal fluids/meteoric water wells. The M3 mixing-model results indicated that the nine mixed wells contained between 14% and 30% hydrothermal fluids. Further, the chemical analyses show that several of these mixed-water wells have concentrations of As, F and B that exceed drinking-water standards or notification levels due to contamination by hydrothermal fluids.  相似文献   

20.
This paper reports new geochemical data on dissolved major and minor constituents in surface waters and ground waters collected in the Managua region (Nicaragua), and provides a preliminary characterization of the hydrogeochemical processes governing the natural water evolution in this area. The peculiar geological features of the study site, an active tectonic region (Nicaragua Depression) characterized by active volcanism and thermalism, combined with significant anthropogenic pressure, contribute to a complex evolution of water chemistry, which results from the simultaneous action of several geochemical processes such as evaporation, rock leaching, mixing with saline brines of natural or anthropogenic origin. The effect of active thermalism on both surface waters (e.g., saline volcanic lakes) and groundwaters, as a result of mixing with variable proportions of hyper-saline geothermal Na–Cl brines (e.g., Momotombo geothermal plant), accounts for the high salinities and high concentrations of many environmentally-relevant trace elements (As, B, Fe and Mn) in the waters. At the same time the active extensional tectonics of the Managua area favour the interaction with acidic, reduced thermal fluids, followed by extensive leaching of the host rock and the groundwater release of toxic metals (e.g., Ni, Cu). The significant pollution in the area, deriving principally from urban and industrial waste-water, probably also contributes to the aquatic cycling of many trace elements, which attain concentrations above the WHO recommended limits for the elements Ni (∼40 μg/l) and Cu (∼10 μg/l) limiting the potential utilisation of Lake Xolotlan for nearby Managua.  相似文献   

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