Rapid evaluation of the hydrochemistry of a sedimentary basin using only ‘standard’ formation water analyses: example from the Canadian portion of the Williston Basin

The Canadian portion of the Williston Basin is used as an example of how > 15,000 ‘standard’ formation water analyses in about 15 aquifers can be evaluated rapidly using easily available culling and data distribution display software. Techniques and software used include a program for culling erroneous and missing data in ‘standard’ formation water analyses, electronic grids of stratigraphic surfaces and unit boundaries from the new Geological Atlas of the Western Canada Sedimentary Basin, grid manipulation software, and a geochemical modelling program (SOLMINEQ.88).Intra- and inter-basinal hydrochemical continuity at the regional scale was matched visually by comparing salinity maps on the Canadian side of the basin with salinity maps from the American side, and with the adjacent Alberta Basin. Inter-aquifer continuity to identify aquifer systems and the relative strength of aquitards was determined using a combination of. (1) salinity distribution maps; (2) maps showing salinity differences between aquifers and across intervening aquitards; and (3) the relation among SO₄ in formation waters, the distribution of anhydrite, and the saturation of the formation waters with respect to anhydrite. Evaporites strongly influence the composition of adjacent formation waters, even to the extent that in one aquifer the normal increase of salinity with depth is reversed. The process of freshwater influx into the western part of the basin is demonstrated by a series of salinity maps; they suggest that the process is current and has included removal of halite from the major basinal aquitard. This regional-scale evaluation required an elapsed time of only about 10 working days, hence a fairly rapid study.     

Model for the formation of fluorine-bearing rocks in the Carboniferous deposits of the Moscow artesian basin

The paper presents the results of the statistical and thermodynamic analysis of hydrogeochemical information on the genesis of F-bearing waters in the Carboniferous deposits of the Moscow artesian basin. The F concentration is demonstrated to increase with increasing salinity of the aqueous solution. As follows from the analysis of mineral equilibria, the saturation concentrations of the aqueous phase with respect to fluorite in association with calcite and gypsum is less than 2–3 mg of F/l. At the saturation of the aqueous phase with respect to fluorite in association with dolomite, the equilibrium concentration of F increases with increasing Mg concentration and decreasing equilibrium partial CO₂ pressure and can reach 8–10 mg of F/l. The main reason for this enrichment of the aqueous phase in F is certain features of mineral equilibria in the system of aqueous solution with Ca and Mg carbonates. An increase in the Mg²⁺ concentration in the aqueous phase decreases the Ca²⁺ concentration in the solubility equilibrium of dolomite, and this, in turn, decreases the F^? concentration in the solubility equilibrium of fluorite.     

Hydrochemical trends,palaeorecharge and groundwater ages in the fissured Chalk aquifer of the London and Berkshire Basins,UK

The hydrochemical, radiochemical, stable isotope, ¹⁴C and dissolved noble gas composition of groundwaters has been determined along two profiles across the confined, fissured Chalk aquifer of the London Basin of southern England, and for selected sites in the adjacent Berkshire Basin. During downgradient flow in the London Basin aquifer, the groundwater chemistry is modified by water–rock interactions: congruent and incongruent reaction of the carbonate lithology resulting in enhanced Mg/Ca and Sr/Ca ratios and ¹³C contents with increased residence times; redox and ion exchange reactions; and towards the centre of the Basin, mixing with a residual saline connate water stored in the Chalk matrix. There is evidence from anomalous water chemistries for a component of vertical leakage from overlying Tertiary beds into the confined aquifer as a result of historical dewatering of the aquifer. Dissolved noble gas contents indicate the climate was up to 4.5°C cooler than at present during recharge of the waters now found in the centres of both Basins; stable isotope (²H and ¹⁸O) depletions correspond to this recharge temperature change. For evolved waters having δ¹³C > −8‰ PDB a negative linear correlation is demonstrated between derived recharge temperatures and δ¹³C values, which is interpreted as mixing between relatively warm, light isotopic, fracture-borne waters and cooler stored waters of the matrix having a ¹³C signature more or less equilibrated with the Chalk. From geochemical (¹⁴C, ⁴He) age estimates, the abstracted water is interpreted as being either of wholly Holocene/post-Devensian glacial origin, or an admixture of Holocene and Late Pleistocene pre-glacial (cold stage interstadial) recharge. Devensian pleniglacial stage waters of the Last Glacial Maximum are not represented.     

Recovery of trace metals in formation waters using acid gases from natural gas

The maximum contents of Pb (360 mg l⁻¹), Zn (360 mg l⁻¹) and Ag (7.9 mg l⁻¹) in formation waters from the Alberta basin were high enough to suggest that it would be of interest to test the concept of recovering these metals by passing natural gas through the water, thereby precipitating the metal sulphides as the result of contact with hydrogen sulphide. The idea was to see if these metals could be recovered from formation water co-produced with crude oil prior to disposal of the water in deep formations, with the possibility of the sale of the metals partially offsetting the cost of disposal. It was proposed to use natural gas with a relatively small amount of hydrogen sulphide (insufficient for sulphur recovery) that must be removed by flaring before the gas is utilized. Accordingly, a database of 694 formation waters with major, minor and trace components was searched for appropriate analyses for detailed study. Of the nine analyses selected the majority were from Devonian and Granite Wash aquifers in the Peace River Arch area of northern Alberta, Canada. Modelling with PATH.ARC showed that there is a consistent set and order of precipitation reactions, in spite of the differences among the formation waters. As would be expected intuitively, acid gas addition makes the formation water more acidic, and metallic sulphide minerals are precipitated. Depending on the initial composition, the end minerals are any of galena, sphalerite, acanthite, covellite and pyrite. These are the minerals that must be beneficiated to recover the metals. A preliminary evaluation of the dollar value of the recovered metals shows that although the absolute values are low, there may be an advantage to recovering the metals if the waters are already being handled at the surface.     

Aquifer disposal of acid gases: modelling of water–rock reactions for trapping of acid wastes

The pore space of deep saline aquifers in the Alberta (sedimentary) Basin offers a significant volume for waste storage by “hydrodynamic trapping”. Furthermore, given the slow regional fluid flow in these deep saline aquifers, ample time exists for waste-water/rock chemical reactions to take place. A geochemical computer model (PATHARC) was used to compute the interaction of industrial waste streams comprising CO₂, H₂SO₄ and H₂S with the minerals in typical carbonate and sandstone aquifers from the Alberta Basin. The results support the idea that these acids can be neutralized by such reactions and that new mineral products are formed, such as calcite, siderite, anhydrite/gypsum and pyrrhotite, thereby trapping the CO₃, SO₄ and S ions that are formed when the acid gases dissolve in the formation water. Siliciclastic aquifers appear to be a better host for “mineral trapping” than carbonate aquifers, especially with regard to CO₂. Carbonate aquifers may be more prone to leakage due to high CO₂ pressures generated by reaction with H₂SO₄ and H₂S. Even though permeability decreases are expected due to this “mineral trapping”, they can be partially controlled so that plugging of the aquifer does not occur.     

Pleistocene recharge to midcontinent basins: effects on salinity structure and microbial gas generation

The hydrogeochemistry of saline-meteoric water interface zones in sedimentary basins is important in constraining the fluid migration history, chemical evolution of basinal brines, and physical stability of saline formation waters during episodes of freshwater recharge. This is especially germane for interior cratonic basins, such as the Michigan and Illinois basins. Although there are large differences in formation water salinity and hydrostratigraphy in these basins, both are relatively quiescent tectonically and have experienced repeated cycles of glaciation during the Pleistocene. Exploration for unconventional microbial gas deposits, which began in the upper Devonian-age Antrim Shale at the northern margin of the Michigan Basin, has recently extended into the age-equivalent New Albany Shale of the neighboring Illinois Basin, providing access to heretofore unavailable fluid samples. These reveal an extensive regional recharge system that has profoundly changed the salinity structure and induced significant biogeochemical modification of formation water elemental and isotope geochemistry.New-formation water and gas samples were obtained from Devonian-Mississippian strata in the Illinois Basin. These included exploration wells in the New Albany Shale, an organic-rich black shale of upper Devonian age, and formation waters from over- and underlying regional aquifer systems (Siluro-Devonian and Mississippian age). The hydrostratigraphic relations of major aquifers and aquitards along the eastern margin of the Illinois Basin critically influenced fluid migration into the New Albany Shale. The New Albany Shale formation water chemistry indicates significant invasion of meteoric water, with δD values as low as −46.05‰, into the shale. The carbon stable isotope system (δ¹³C values as high as 29.4‰), coupled with δ¹⁸O, δD, and alkalinity of formation waters (alkalinity ≤24.08 meq/kg), identifies the presence of microbial gas associated with meteoric recharge. Regional geochemical patterns identify the underlying Siluro-Devonian carbonate aquifer system as the major conduit for freshwater recharge into the fractured New Albany Shale reservoirs. Recharge from overlying Mississippian carbonates is only significant in the southernmost portion of the basin margin where carbonates directly overlie the New Albany Shale.Recharge of dilute waters (Cl⁻ <1000 mM) into the Siluro-Devonian section has suppressed formation water salinity to depths as great as 1 km across the entire eastern Illinois Basin margin. Taken together with salinity and stable isotope patterns in age-equivalent Michigan Basin formation waters, they suggest a regional impact of recharge of δ¹⁸O- and δD-depleted fluids related to Pleistocene glaciation. Devonian black shales at both basin margins have been affected by recharge and produced significant volumes of microbial methane. This recharge is also manifested in different salinity gradients in the two basins because of their large differences in original formation water salinity. Given the relatively quiet tectonic history and subdued current topography in the midcontinent region, it is likely that repeated cycles of glacial meltwater invasion across this region have induced a strong disequilibrium pattern in fluid salinity and produced a unique class of unconventional shale-hosted gas deposits.     

Chemical Composition and Geologic History of Saline Waters in Aux Vases and Cypress Formations, Illinois Basin

Seventy-six samples of formation waters were collected from oil wells producing from the Aux Vases or Cypress Formations in the Illinois Basin. Forty core samples of the reservoir rocks were also collected from the two formations. Analyses of the samples indicated that the total dissolved solids content (TDS) of the waters ranged from 43,300 to 151,400 mg/L, far exceeding the 35,400 mg/L of TDS found in typical seawater. Cl-Br relations suggested that high salinities in the Aux Vases and Cypress formation waters resulted from the evaporation of original seawater and subsequent mixing of the evaporated seawater with concentrated halite solutions. Mixing with the halite solutions increased Na and Cl concentrations and diluted the concentration of other ions in the formation waters. The elemental concentrations were influenced further by diagenetic reactions with silicate and carbonate minerals. Diagenetic signatures revealed by fluid chemistry and rock mineralogy delineated the water-rock interactions that took place in the Aux Vases and Cypress sandstones. Dissolution of K-feldspar released K into the solution, leading to the formation of authigenic illite and mixed-layered illite/smectite. Some Mg was removed from the solution by the formation of authigenic chlorite and dolomite. Dolomitization, calcite recrystallization, and contribution from clay minerals raised Sr levels significantly in the formation waters. The trend of increasing TDS of the saline formation waters with depth can be explained with density stratification. But, it is difficult to explain the combination of the increasing TDS and increasing Ca/Na ratio with depth without invoking the controversial 'ion filtration' mechanism.     

Geochemistry of deep formation waters in the Canning Basin,Western Australia,and their relationship to Zn‐Pb mineralization

The Canning Basin contains several Mississippi Valley‐type Zn‐Pb sulphide prospects and deposits in Devonian carbonate reef complexes on the northern edge of the Fitzroy Trough, and in Ordovician and Silurian marine sequences on the northern margin of the Willara Sub‐basin. This study uses the ionic composition and 5D, δ¹⁸O, δ³⁴S, ⁸⁷Sr/⁸⁶Sr isotopic data on present‐day deep formation waters to determine their origin and possible relationship to the Zn‐Pb mineralizing palaeofluids.

The present‐day Canning Basin formation waters have salinity ranging from typically less than 5000 mg/L up to 250 000 mg/L locally. The brines are mixtures of highly saline water, formed by seawater which evaporated beyond halite saturation (bittern water), with meteoric water ranging in salinity from low (<5000 mg/L) to hypersaline water (up to about 50 000 mg/L) formed by re‐solution of halite and calcium sulphate minerals. The original marine chemical composition of the bittern‐dominated brines was changed to that of a Na‐Ca‐Cl water by addition of Ca and removal of Mg and SO₄, initially by bacterial sulphate reduction and later by dolomitization of carbonate. Other reactions with terrigenous components of the sediment have provided additional Ca and Sr, including a small proportion of ⁸⁷Sr‐rich material. The δ³⁴S values of the bittern‐containing waters are within the range over which marine sulphate has fluctuated from the Ordovician to the Holocene, although one of the hypersaline waters has a value of +6.8%, indicating SO₄ of non‐marine origin. The pH of the bittern‐containing waters is low (about 5) and they contain significant concentrations of dissolved Fe (up to 120 mg/L).

The Canning Basin bitterns appear similar in origin and chemical composition to highly saline marine brines in the Mississippi Salt Dome Basin, USA, which are known to be either metal or sulphide‐rich depending on the organic content of the host rock. In the Canning Basin, mixing of the bittern water with the various types of meteoric water has resulted in decreases in salinity, Na, Ca, Mg, K, Sr, Li and Fe, and increases in HCO₃, SO₄ and pH.

Mixing of the bitterns with other types of metalliferous fluids and/or with sulphate‐containing hypersaline meteoric waters formed from the same marine evaporite sequence should produce ore‐precipitating fluids which are relatively hot and saline, and the resulting ore deposit should be of high grade and contain abundant sulphate minerals. In the southern Canning Basin, this type of mixing and the corresponding style of ore deposit is evident in the evaporite‐associated areas of Zn‐Pb mineralization near the Admiral Bay Fault. If the bitterns mix with low salinity HCO₃‐waters in near‐surface environments, then the ore‐precipitating fluids should have relatively low salinities and carbonate minerals would precipitate during later stages of mixing. In the Lennard Shelf, the present‐day formation waters, the style of the Zn‐Pb deposits, and range of salinity and temperature of the ore‐forming palaeofluids are consistent with this type of mixing.     

Hydrochemical relationships in groundwater from central São Paulo State, Brazil

Groundwater samples from different aquifers occurring at center/northeast portion of São Paulo State, Brazil, were collected and chemically analyzed. The waters leaching Mesozoic sediments are generally more acid (pH_average=5.9) and have lower values for total dissolved solids (TDS_average=105 mg/L) than those obtained for waters leaching Paleozoic sediments of Tubarão Group. First-degree trend surfaces revealed that the deeper tubular wells occur towards east/southeast and exploit Paleozoic sediments as well fractured/fissured diabases/basalts, whereas the tubular wells in the west/northwest region are shallower. Piper diagrams indicated that the majority of the waters are a blend of waters from different lithologies. Significant correlations were found among nitrate, chloride and bicarbonate, suggesting the occurrence of some anthropogenic inputs, whereas elevated alpha activity of geogenic ²²⁶Ra indicated the need of a broad radiometric survey in the area.     

“Rust” contamination of formation waters from producing wells

In the Alberta Basin there is a significant difference in the content of Fe between formation waters from drillstem tests and formation waters from producing wells. This was demonstrated using a data set comprising 525 formation waters from drillstem tests and 107 formation waters from producing wells. Both a cross-plot of Mn and Fe in the two sets of formation waters and box plots of the same data sets showed that formation waters from producing wells have dominantly Fe>Mn, compared with those from drillstem tests which are characterized by Mn>Fe. Suspecting that “rust” contamination from well casing and ancillary equipment was the cause, the pH values of the samples were compared to see if the two data sets also differed in pH. It was demonstrated that not only are formation waters from drillstem tests less acidic than those from producing wells, but there is a systematic statistical trend of increasing acidity with age of the strata (temperature, depth, and increased amounts of the acid gases — H₂S and CO₂). The difference between the pH of formation waters from drillstem tests and producing wells is attributed to the partial scrubbing of the acid gases from the fluids produced during the drillstem test; this results in less acidic formation waters. Vanadium may also be enhanced in formation waters from producing wells. This note reports these differences and cautions too much reliance on values for Fe in waters from producing wells.     

Preliminary hydrogeological assessment of Late Cretaceous–Tertiary Ardley coals in part of the Alberta Basin,Alberta, Canada

A preliminary hydrogeological evaluation was undertaken on the gas potential of shallow coals in the Pembina–Warburg exploration area in the Alberta Basin, Alberta Canada. Regional data for the Late Cretaceous–Tertiary Ardley Coal Zone (ACZ) were compiled and supplemented with site-specific data collected from a key test- well drilled as part of a regional exploration program. Limited regional pressure data suggest hydraulic communication between the uppermost Ardley with the overlying Paskapoo Formation. A comparison between hydraulic head and topography suggests that flow, at least in part of the Ardley–Paskapoo, is gravity driven. However, a decoupling of the hydraulic regime appears evident from pressure test data in beds stratigraphically below the uppermost Scollard (Ardley) and above the base of Scollard at least in the eastern part of the study area where the test-well was drilled. The decoupling is evident in regional pressure data but the precise stratigraphic position may vary.Regionally, formation waters typically are Na–HCO₃ type with salinities (as TDS) of less than 2000 mg/L. Anomalously high bicarbonate (dissolved inorganic carbon or DIC) concentrations exceeding 1500 mg/L with δ¹³C_DIC + 22.50‰ and dissolved methane identified in formation waters collected directly from Ardley coal in test-well 103 point to the presence of secondary biogenic gas. The ¹³C isotopes for DIC, coupled with ¹⁸O and ²H isotopes for associated groundwater and regional hydraulic data, suggest that the uppermost Ardley Coal Zone in the eastern part of the study area is part of a regional, topographically driven, dynamic flow system in which methanogenic processes are modifying groundwater chemistry and gas charging parts of the area. Whether or not biogenic gas-charging in the Ardley is pervasive is uncertain. The relatively small coal data set requires that further exploration in the study area should consider the presence of microbial gas and the potential for hydrogeological controls on its distribution. However, further detailed testing will be necessary to develop a consistent and useful database for exploration and development.     

Spatial variation in arsenic and fluoride concentrations of shallow groundwater from the town of Shahai in the Hetao basin,Inner Mongolia

Twenty-nine wells were selected for groundwater sampling in the town of Shahai, in the Hetao basin, Inner Mongolia. Four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2.5–20 m. Results show that groundwater As exhibits a large spatial variation, ranging between 0.96 and 720 μg/L, with 71% of samples exceeding the WHO drinking water guideline value (10 μg/L). Fluoride concentrations range between 0.30 and 2.57 mg/L. There is no significant correlation between As and F⁻ concentrations. Greater As concentrations were found with increasing well depth. However, F⁻ concentrations do not show a consistent trend with depth. Groundwater with relatively low Eh has high As concentrations, indicating that the reducing environment is the major factor controlling As mobilization. Low As concentrations (<10 μg/L) are found in groundwater at depths less than 10 m. High groundwater As concentration is associated with aquifers that have thick overlying clay layers. The clay layers, mainly occurring at depths <10 m, have low permeability and high organic C content. These strata restrict diffusion of atmospheric O₂ into the aquifers, and lead to reducing conditions that favor As release. Sediment composition is an additional factor in determining dissolved As concentrations. In aquifers composed of yellowish-brown fine sands at depths around 10 m, groundwater generally has low As concentrations which is attributed to the high As adsorption capacity of the yellow–brown Fe oxyhydroxide coatings. Fluoride concentration is positively correlated with pH and negatively correlated with Ca²⁺ concentration. All groundwater samples are over-saturated with respect to calcite and under-saturated with respect to fluorite. Dissolution and precipitation of Ca minerals (such as fluorite and calcite), and F⁻ adsorption–desorption are likely controlling the concentration of F⁻ in groundwater.     

Hydrogeochemical genesis of groundwaters with abnormal fluoride concentrations from Zhongxiang City,Hubei Province,central China

The groundwaters from Zhongxiang City, Hubei Province of central China, have high fluoride concentration up to 3.67 mg/L, and cases of dental fluorosis have been found in this region. To delineate the nature and extent of high fluoride groundwaters and to assess the major geochemical factors controlling the fluoride enrichment in groundwater, 14 groundwater samples and 5 Quaternary sediment samples were collected and their chemistry were determined in this study. Some water samples from fissured hard rock aquifers and Quaternary aquifers have high fluoride concentrations, whereas all karst water samples contain fluoride less than 1.5 mg/L due to their high Ca/Na ratios. For the high fluoride groundwaters in the fissured hard rocks, high HCO₃ ⁻ concentration and alkaline condition favor dissolution of fluorite and anion exchange between OH⁻ in groundwater and exchangeable F⁻ in some fluoride-bearing minerals. For fluoride enrichment in groundwaters of Quaternary aquifers, high contents of fluoride in the aquifer sediments and evapotranspiration are important controls.     

Fluoride dynamics in the granitic aquifer of the Wailapally watershed,Nalgonda District,India

High concentrations of fluoride (up to 7.6 mg/L) are a recognized feature of the Wailapally granitic aquifer of Nalgonda District, Andhra Pradesh, India. The basement rocks provide abundant sources of F in the form of amphibole, biotite, fluorite and apatite. The whole-rock concentrations of F in the aquifer are in the range 240–990 mg/kg. Calcretes from the shallow weathered horizons also contain comparably high concentrations of F (635–950 mg/kg). The concentrations of water-soluble F in the granitic rocks and the calcretes are usually low (1% of the total or less) but broadly correlate with the concentrations observed in groundwaters in the local vicinity. The water-soluble fraction of fluoride is relatively high in weathered calcretes compared to fresh calcretes.Groundwater major-ion composition shows a well-defined trend with flow downgradient in the Wailapally aquifer, from Na–Ca–HCO₃-dominated waters in the recharge area at the upper part of the catchment, through to Na–Mg–HCO₃ and ultimately to Na–HCO₃ and Na–HCO₃–Cl types in the discharge area in the lowest part. The evolution occurs over a reach spanning some 17 km. Groundwater chemistry evolves by silicate weathering reactions, although groundwaters rapidly reach equilibrium with carbonate minerals, favouring precipitation of calcite, and ultimately dolomite in the lower parts of the watershed. This precipitation is also aided by evapotranspiration. Decreasing Ca activity downgradient leads to a dominance of fluorite-undersaturated conditions and consequently to mobilisation of F. Despite the clear downgradient evolution of major-ion chemistry, concentrations of F remain relatively uniform in the fluorite-undersaturated groundwaters, most being in the range 3.0–7.6 mg/L. The rather narrow range is attributed to a mechanism of co-precipitation with and/or adsorption to calcrete in the lower sections of the aquifer. The model may find application in other high-F groundwaters from granitic aquifers of semi-arid regions.     

Geochemistry of the high-PCO2 waters in Logudoro,Sardinia, Italy

A geochemical study of the high-PCO₂ waters in Logudoro, northern Sardinia, was carried out starting from regional hydrogeochemical prospecting for geothermal energy, based on the major dissolved components and some minor elements. This preliminary investigation led to the identification of five different lithologies marking the different aquifers. The high-PCO₂ waters can be divided into the less saline (TDS < 1g/l) with high tritium unit (T.U.) values and the more saline ones (TDS= 1–3.5 g/l) with T.U. values close to zero. The water-rock interaction process affecting the major components is shown to be the result of interaction between CO₂-rich waters and aluminosilicates; the process takes place at different degrees depending on the depth at which CO₂ interacts with different aquifers while migrating upward from the mantle. Consideration of the SO₄/Cl and F/Cl ratios in the solution allowed the deep ciruits of S. Martino and Abbarghente in the Oligo-Miocene volcanic rocks and S. Lucia in the carbonate-schistose-granitic basement of the Goceano Mountains to be located.     

New insights into the origin and migration of brines in deep Devonian aquifers, Alberta, Canada

Analysis of hydraulic heads and chemical compositions of Devonian formation waters in the west central part of the Alberta Basin, Canada, characterizes the origin of formation waters and migration of brines. The Devonian succession in the study area lies 2000–5000 m below the ground surface, and has an approximate total thickness of 1000 m and an average slope of 15 m/km. Four Devonian aquifers are present in the study area, which form two aquifer systems [i.e., a Middle–Upper Devonian aquifer system (MUDAS) consisting of the Elk Point and Woodbend–Beaverhill Lake aquifers, and an Upper Devonian aquifer system (UDAS) consisting of the Winterburn and Wabamun aquifers]. The Ireton is an effective aquitard between these two systems in the eastern parts of the study area. The entire Devonian succession is confined below by efficient aquitards of the underlying Cambrian shales and/or the Precambrian basement, and above by overlying Carboniferous shales of the Exshaw and Lower Banff Formations.The formation water chemistry shows that the Devonian succession contains two distinct brine types: a ‘heavy brine,’ located updip, defined approximately by TDS >200 g/l, and a ‘light brine’ with TDS <200 g/l. Hydraulic head distributions suggest that, presently, the ‘light brine’ attempts to flow updip, thereby pushing the ‘heavy brine’ ahead. The interface between the two brines is lobate and forms large-scale tongues that are due to channeled flow along high-permeability pathways. Geological and hydrogeochemical data suggest that the following processes determined the present composition of the ‘light’ and ‘heavy’ brines: original seawater, evaporation beyond gypsum but below halite saturation, dolomitization, clay dehydration, gypsum dewatering, thermochemical sulfate reduction (TSR), and halite dissolution. The influx of meteoric (from the south) and metamorphic (from the west) waters can be recognized only in the ‘light brine.’ Albitization can be unequivocally identified only in the ‘heavy brine.’ The ‘heavy brine’ may be residual Middle Devonian evaporitic brine from the Williston Basin or the Elk Point Basin, or it may have originated from partial dissolution of thick, laterally extensive Middle Devonian evaporite deposits to the east of the study area. The ‘light brine’ most probably originated from dilution of ‘heavy brine’ in post-Laramide times.     

Hydrogeological investigations of thermal waters in the Sfax Basin (Tunisia)

The Sfax Basin in eastern Tunisia is bounded to the east by the Mediterranean Sea. Thermal waters of the Sfax area have measured temperatures of 23–36°C, and electrical conductivities of 3,200 and 14,980 μS/cm. Most of the thermal waters are characterized as Na–Cl type although there are a few Na–SO₄–Cl waters. They issue from Miocene units which are made up sands and sandstones interbedded with clay. The Quaternary sediments cap the system. The heat source is high geothermal gradient which are determined downhole temperature measurements caused by graben tectonics of the area. The results of mineral equilibrium modeling indicate that the thermal waters of the Sfax Basin are undersaturated with respect to gypsum, anhydrite and fluorite, oversaturated with respect to kaolinite, dolomite, calcite, microcline, quartz, chalcedony, and muscovite. Assessments from various chemical geothermometers, Na–K–Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature of the Sfax area can reach up to 120°C. According to δ¹⁸O and δ²H values, all thermal and cold groundwater is of meteoric origin.     

Origin and granite alteration effects of hydrothermal fluid: isotopic evidence from fluorite veins, Co. Galway, Ireland

The Costelloe Murvey Granite is a chemically evolved, high heat production, leucocratic component of the 400 Ma old Galway Granite batholith and is host to hydrothermal fluorite-quartz-calcite veins. A previously reported clinopyroxene ⁴⁰Ar-³⁹Ar age of 231±4 Ma obtained from a pre-mineralization dolerite dyke is reinterpreted as dating this mineralization. The hydrothermal fluid extensively altered its granite wallrocks, leading to lower Sm and Nd and higher Rb concentrations in altered granite, disturbing both its Rb-Sr and Sm-Nd isotopic systems. The ⁸⁷Sr/⁸⁶Sr ratio of the hydrothermal fluid from which fluorite and calcite precipitated ranged from 0.7101 to 0.7139. These ratios are very much lower than in the Costelloe Murvey Granite at the time of mineralization, precluding the granite as a source for more than 2% of the hydrothermal Sr. The initial ¹⁴³Nd/¹⁴⁴Nd ratio varies between fluorite in different veins due to Nd derivation from local wallrocks, and between fluorite of petrographically distinct growth phases within a single hand specimen, highlighting the difficulty of Sm-Nd isochron dating of fluorite in cases where there are multiple sources of hydrothermal Nd. It is proposed that fluorite and calcite precipitated where hot, dilute fluids rising through the granite mixed with cooler, more saline fluids of basinal origin migrating through Lower Carboniferous limestone which then overlay the granite. Received: 3 August 1995 / Accepted: 11 April 1996     

Origin of fluorine in mineral waters of Bujanovac valley (Serbia, Europe)

This paper deals with mineral and thermomineral water occurrences of the Bujanovac valley in south eastern part of Serbia related to granitoides of the Bujanovac massif along both the margin and the floor of the valley. In past decades (1966–2010) numerous hydrogeological, hydrogeochemical and geophysical explorations were carried out. One of results of these explorations is the completion of test holes and exploratory—production wells. They provide groundwater for three water bottling factories: “Heba”, “Bivoda”, “Prohor” as well as the “Vrelo Bujanovac Banja Spa Centre” for rehabilitation, treatment, and prevention. All stated consumers use the same mineral water aquifer. The content of fluoride in the majority of examined mineral waters is higher than 4 mg/1, whereby they are singled out as typical fluoride waters. The content of calcium ions in them amounts 80 mg/1, and the values of the saturation index in relation to calcium fluoride (SI) range from 0.4 to 0.7 mg/1, which points to mineral waters saturated in relation to fluorite. However, in the study area, there are mineral water occurrences with the content of fluoride significantly lower than in the majority of analysed waters representing hydrochemical anomalies. These waters occur as hydrochemical anomalies in marl and sandstone wherein Secondary mineral water aquifers, originating from cracked granite of the Bujanovac massif, are formed. When mineral waters from granite (with the increased content of fluoride) reach these secondary aquifers, the content of fluoride ions lowers to about 1 mg/1, which is of great significance from the point of view of mineral water utilisation as table bottled water. In this paper, it is proved that, in addition to the presence of some minerals as basic fluorine bearers, the role of the lithological environment where the natural process of defluoridation occurs is significant, which is confirmed by the revitalization of the A-4 well. The paper deals with mineral water deposits of the Bujanovac valley, and the natural way of lowering of fluorine content in the given waters.     

A review of the source,behaviour and distribution of arsenic in natural waters

The range of As concentrations found in natural waters is large, ranging from less than 0.5 μg l⁻¹ to more than 5000 μg l⁻¹. Typical concentrations in freshwater are less than 10 μg l⁻¹ and frequently less than 1 μg l⁻¹. Rarely, much higher concentrations are found, particularly in groundwater. In such areas, more than 10% of wells may be ‘affected’ (defined as those exceeding 50 μg l⁻¹) and in the worst cases, this figure may exceed 90%. Well-known high-As groundwater areas have been found in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal (India), Bangladesh and Vietnam. The scale of the problem in terms of population exposed to high As concentrations is greatest in the Bengal Basin with more than 40 million people drinking water containing ‘excessive’ As. These large-scale ‘natural’ As groundwater problem areas tend to be found in two types of environment: firstly, inland or closed basins in arid or semi-arid areas, and secondly, strongly reducing aquifers often derived from alluvium. Both environments tend to contain geologically young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater. Arsenic-rich groundwaters are also found in geothermal areas and, on a more localised scale, in areas of mining activity and where oxidation of sulphide minerals has occurred. The As content of the aquifer materials in major problem aquifers does not appear to be exceptionally high, being normally in the range 1–20 mg kg⁻¹. There appear to be two distinct ‘triggers’ that can lead to the release of As on a large scale. The first is the development of high pH (>8.5) conditions in semi-arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates. This pH change leads either to the desorption of adsorbed As (especially As(V) species) and a range of other anion-forming elements (V, B, F, Mo, Se and U) from mineral oxides, especially Fe oxides, or it prevents them from being adsorbed. The second trigger is the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissolution of Fe and Mn oxides, also leading to As release. Iron (II) and As(III) are relatively abundant in these groundwaters and SO₄ concentrations are small (typically 1 mg l⁻¹ or less). Large concentrations of phosphate, bicarbonate, silicate and possibly organic matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of high groundwater As areas is the large degree of spatial variability in As concentrations in the groundwaters. This means that it may be difficult, or impossible, to predict reliably the likely concentration of As in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule. In most aquifers, the majority of wells are likely to be unaffected, even when, for example, they contain high concentrations of dissolved Fe.