Inverse Geochemical Modeling Research Articles

Geochemical Characterization of Surface Water and Groundwater in Soummam Basin, Algeria

Multivariate statistical methods and geochemical modeling were used to assess spatial variation of water quality of the Soummam basin, Algeria. The application of hierarchical cluster analysis (HCA) showed three main groups of samples. Group 1 samples are exclusively composed of surface water. Groups 2 and 3 samples consist of groundwater. Discriminant analysis assigned about 98.6% of the cases grouped by HCA. All groups are super-saturated with Ca-montmorillonite, dolomite, gibbsite, K-mica, kaolinite, and quartz, and all these groups are under-saturated with albite, anhydrite, anorthite, CO2(g), gypsum, halite, melanterite, and smithsonite. The results of analysis of variance indicate that the saturation indices of each of the mineral phases are significant except for chalcedony and quartz (p > 0.05). The results obtained by inverse geochemical modeling show the dissolution of albite, which justifies Na enrichment during the chemical evolution of groundwater. Calcite, dolomite, Ca-montmorillonite, kaolinite, illite, gibbsite, and K-mica are shown to have always precipitated.

Multivariate Statistical Analysis and Geochemical Modeling to Characterize the Surface Water of Oued Chemora Basin, Algeria

Multivariate statistical methods and inverse geochemical modeling were jointly used to deduce the genetic origin of chemical parameters of surface water in the Oued Chemora Basin. The analysis of variance indicates that variations of all parameters at the three stations are significant except for pH and T (p > 0.05). R-mode cluster analysis reveals two distinct groups at each station and led to the conclusion that the major ion chemistry in the three stations is derived from organic and artificial fertilizers due to agricultural activities in the area and water–rock interaction. Mineral saturation indices, calculated from major ions, indicate that the surface water is generally super-saturated with carbonate phases, and all the water samples are under-saturated with respect to gypsum, halite, and CO2(g). In a broad sense, the reactions responsible for the hydrochemical evolution in the Basin fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals; and (3) ion exchange.

Isotope hydrology and geochemical modeling: new insights into the recharge processes and water–rock interactions of a fissured carbonate aquifer (Gran Sasso, central Italy)

The goal of this paper was to characterize the recharge process and water–rock interactions in a regional homogeneous fractured carbonate aquifer in the Mediterranean environment (Gran Sasso, central Italy) through isotope data (δ2H, δ18O and δ13C-DIC) collected between 2006 and 2010. Samples were collected from the springs of the Gran Sasso aquifer and from the Underground Nuclear Physics Laboratories, located within the aquifer. Additionally, the hydrochemical data and the reference hydrogeological frameworks of previous studies have been used as a starting point for the geochemical modeling. The Gran Sasso aquifer, which is bounded by terrigenous and clastic units acting as aquitards, accommodates a uniquely broad regional groundwater, which feeds springs mainly at its border with high, steady discharge. In total, these springs discharge of more than 18 m3/s. At a local scale for the aquifer core and at a regional scale for the overall aquifer, δ 2H, δ18O and δ13C-DIC isotope data, the geochemical inverse modeling through PHREEQC and the δ13C-DIC fractionation modeling through NETPATH 2.0 show the following. (1) Clear processes of evaporation and related isotope enrichment may be ruled out. (2) Groundwater flow is active and extends to the overall aquifer without clear signs of layering and partitioning. (3) Groundwater flowpaths radiate from the core toward the periphery. (4) In L’Aquila Plain, Gran Sasso groundwater mixes with shallow Quaternary water at a ratio of one half. (5) The main geochemical processes are the dissolution of calcite and dolomite, and in some cases, ion exchange occurs (Mg2+ and SO42− release, Ca2+ adsorption). (6) In the less mineralized recharge groundwater, the δ13C-DIC value is influenced by the δ13C-DIC value of rainfall, while in more evolved groundwater, the final δ13C-DIC value is reached by fractionation during the flowpaths, indicating a lengthy interaction with limestone.

Spatial variation of groundwater arsenic distribution in the Chianan Plain, SW Taiwan: Role of local hydrogeological factors and geothermal sources

We present here major ion, trace element, stable and radioisotope data based on forty-six groundwater samples collected from various locations along few selected profiles across the Chianan Plain, southwestern Taiwan including the area affected by well known Blackfoot disease manifested by peripheral vascular gangrene. The objective of the study was to understand the role of local hydrogeology in terms of spatial variation of arsenic concentration in groundwater wells of the entire Chianan Plain and the foothill belt of the Central Mountain Range. An attempt has also been made to assess the contribution of nearby geothermal sources to the arsenic budget in groundwater of the Chianan Plain. Our study shows a gradual increase in all major and trace ion concentrations including total arsenic from foothill belt (arsenic: median = 4 μg/L, range = 0–667.6 μg/L, sample number n = 16) to coastal zones (arsenic: median = 42.74 μg/L, range = 0.14–348.6 μg/L, n = 15) of the plain. Inverse geochemical modeling shows that Ca may be exchanged on clays, and that the degree of the exchange increases from the foothill to the coastal zones. Inverse geochemical modeling further suggests that the oxidation of organic matter (CH2O) required in various east-west profiles across the plain to balance the total bicarbonate concentration and CO2 input from organic matters significantly increases from the foothill to the coastal zones with transfer coefficients ranging from 1.55 × 10−2 to 1.69 × 10−5 mol/L. High concentrations of tritium (mean = 1.33 ± 0.11 TU; n = 4) in foothill groundwater and low concentration of tritium in groundwater of central zone suggest gradually increasing water–rock interaction from the foothill to the coastal part. Few elevated arsenic (median = 171.8 μg/L, maximum = 667.60 μg/L, minimum = 24 μg/L; n = 6) hotspots are identified in the foothill belt. Available lithologs and aquifer test data suggest that the presence of impermeable clay around those pockets possibly inhibits vertical and lateral flushing of the aquifer and aids strong water–rock interactions subsequently leading to release of arsenic into groundwater. Using oxygen isotope and chloride mass balance method, we estimated that geothermal sources can recharge a maximum of 4% of groundwater in proximal aquifers and contribute <2% of average As concentration in the groundwater of Chianan Plain. Our preliminary observations thus show some arsenic enrichment in foothill aquifers, providing a necessity of detailed study of the aquifer systems in these understudied regions. Moreover, our research indicates that the contribution of arsenic from geothermal sources is insignificant, which stands in contrast to earlier studies suggesting that mud volcanoes and thermal springs in the Western Foothill Belt of the Central Mountain Range were potential sources of groundwater arsenic in the Chianan Plain aquifers.

An assessment of the physico-chemical parameters of Oran sebkha basin

Growing populations and increasing industrialization cause increase in living standard, which result in decrease in the quality of water and may put stresses on natural waters by impairing both the quality of the water and the hydrological budget. This research aims at determining the origin of the chemical elements of groundwater from the Oran sebkha basin. It applies the inverse geochemical modeling to derive the sources of variation in the hydrochemistry by Belkhiri et al. (doi: 10.1016/j.geoderma.2010.08.016 , 2010). Fifty-five water samples were selected from different point in Oran sebkha basin for sampling purpose in July 2011. Physico-chemical parameters such as pH and electric conductivity were measured in situ. Moreover, chloride, sulfate, alkalinity, calcium, magnesium, sodium and potassium were measured in the laboratory. Inverse geochemical models of the statistical groups were developed using PHREEQC to elucidate the chemical reactions controlling water chemistry. The inverse geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals; and (3) weathering reactions of silicate minerals by Belkhiri et al. (doi: 10.1016/j.geoderma.2010.08.016 , 2010). The high values of the physico-chemical parameters of water obtained in the present study sites indicate a variation in the physico-chemical parameters demonstrated that relatively few phases are required to derive water chemistry in the area. Range of values was found as pH (5.1–7.6), conductivity (720–15,820 μS cm−1), chloride (994–7,810 mg l−1), sulfate (6.1–112.4 mg l−1), alkalinity (421–19,962 mg l−1), calcium (80–680 mg l−1), magnesium (212.4–4,525 mg l−1), sodium (124.2–4,687.4 mg l−1) and potassium (0.9–42.5 mg l−1).

English

Growing populations and increasing industrialization causes increase in living standard, which result in decrease in the quality of water and may put stresses on natural waters by impairing both the quality of the water and the hydrological budget. This research aimed at determining the origin of the chemical elements of groundwater from the Oran Sebkha basin. It applied the inverse geochemical modeling to derive the sources of variation in the hydrochemistry. Fifty five (55) water samples were selected from different point in Oran Sebkha basin for sampling purpose in July 2011. Physico-chemical parameters such as pH and electric conductivity were measured in situ . Moreover, chloride, sulfate, alkalinity, calcium, magnesium, sodium, potassium, were measured in the laboratory. Inverse geochemical models of the statistical groups were developed using PHREEQC to elucidate the chemical reactions controlling water chemistry. The inverse geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals; and (3) weathering reactions of silicate minerals. The high values of the physico-chemical parameters of water obtained in the present study sites indicate a variation in the physico-chemical parameters and demonstrated that relatively few phases are required to derive water chemistry in the area. Range of values were found as pH (5.1-7.6), conductivity (720-15820 μS cm -1 ), chloride (994-7810 mg l -1 ), sulfate (6.1-112.4 mg l -1 ), alkalinity (421-19962 mg l -1 ), calcium (80-680 mg l -1 ), magnesium (212.4-4525 mg l -1 ), sodium (124.2-4687.4 mg l -1 ) and potassium (0.9-42.5 mg l -1 ). Key words : Physico-chemical parameters, water, Sebkha.

A Monte Carlo-Based Approach for Groundwater Chemistry Inverse Modeling

Inverse geochemical modeling of groundwater entails identifying a set of geochemical reactions which can explain observed changes in water chemistry between two samples that are spatially related in some sense, such as two points along a flow pathway. A common inversion approach is to solve a set of simultaneous mass and electron balance equations involving water-rock and oxidation-reduction reactions that are consistent with the changes in concentrations of various aqueous components. However, this mass-balance approach does not test the thermodynamic favorability of the resulting model and provides limited insight into the model uncertainties. In this context, a Monte Carlo-based forward-inverse modeling method is proposed that generates probability distributions for model parameters which best match the observed data using the Metro-polis-Hastings search strategy. The forward model is based on the well-vetted PHREEQC geochemical model. The proposed modeling approach is applied to two test applications, one involving an inverse modeling example supplied with the PHREEQC code that entails groundwater interactions with a granitic rock mineral assemblage, and the other concerning the impact of fuel hydrocarbon bioattenuation on groundwater chemistry. In both examples, the forward-inverse approach is able to approximately reproduce observed water quality changes invoking mass transfer reactions that are all thermodynamically favorable.

Groundwater geochemistry of Ain Azel area, Algeria

Hydrogeochemical data for 18 groundwater samples and 11 hydrochemical parameters were subjected to Q- and R-mode cluster analysis and inverse geochemical modeling. Q-mode cluster analysis resulted in three distinct water types (brackish water type, saline water type and highly saline water type). R-mode cluster analysis led to the conclusion that the water–rock interaction is the major source of contamination for the groundwater in the area. Geochemical modeling results show that carbonates, gypsum, halite, carbon dioxide (gas), and chlorite are dissolving, whereas Ca-montmorillonite, gibbsite, illite, K-mica, kaolinite, and quartz are mostly precipitating along different flow paths in the groundwater system of the area.

Revision of Fontes & Garnier's model for the initial 14C content of dissolved inorganic carbon used in groundwater dating

The widely applied model for groundwater dating using 14C proposed by Fontes and Garnier (F&G) (Fontes and Garnier, 1979) estimates the initial 14C content in waters from carbonate-rock aquifers affected by isotopic exchange. Usually, the model of F&G is applied in one of two ways: (1) using a single 13C fractionation factor of gaseous CO2 with respect to a solid carbonate mineral, εg/s, regardless of whether the carbon isotopic exchange is controlled by soil CO2 in the unsaturated zone, or by solid carbonate mineral in the saturated zone; or (2) using different fractionation factors if the exchange process is dominated by soil CO2 gas as opposed to solid carbonate mineral (typically calcite). An analysis of the F&G model shows an inadequate conceptualization, resulting in underestimation of the initial 14C values (14C0) for groundwater systems that have undergone isotopic exchange. The degree to which the 14C0 is underestimated increases with the extent of isotopic exchange. Examples show that in extreme cases, the error in calculated adjusted initial 14C values can be more than 20% modern carbon (pmc). A model is derived that revises the mass balance method of F&G by using a modified model conceptualization. The derivation yields a “global” model both for carbon isotopic exchange dominated by gaseous CO2 in the unsaturated zone, and for carbon isotopic exchange dominated by solid carbonate mineral in the saturated zone. However, the revised model requires different parameters for exchange dominated by gaseous CO2 as opposed to exchange dominated by solid carbonate minerals. The revised model for exchange dominated by gaseous CO2 is shown to be identical to the model of Mook (Mook, 1976). For groundwater systems where exchange occurs both in the unsaturated zone and saturated zone, the revised model can still be used; however, 14C0 will be slightly underestimated. Finally, in carbonate systems undergoing complex geochemical reactions, such as oxidation of organic carbon, radiocarbon ages are best estimated by inverse geochemical modeling techniques.

Geochemical modeling of groundwater in the El Eulma area, Algeria

Multivariate statistical and geochemical modeling techniques were used to determine the main factors and mechanisms controlling the chemistry of groundwaters in the El Eulma area. Three major water groups resulted from the Q-mode cluster analysis. The samples from the area were classified as low salinity (Group 1), moderate salinity (Group 2), and high salinity waters (Group 3). Inverse geochemical models of the statistical groups were developed using PHREEQC to elucidate the chemical reactions controlling water chemistry. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into three categories: (1) dissolution of evaporite minerals; (2) precipitation of carbonate minerals, quartz, kaolinite, and Ca-smectite; and (3) ion exchange.

Evolution of Radiocarbon in a Sandy Aquifer Across Large Temporal and Spatial Scales: Case Study from Southern Poland

We present the results of a comprehensive study aimed at tracing the evolution of carbon isotopic composition of the TDIC (total dissolved inorganic carbon) reservoir from the unsaturated zone down to the discharge area, in a sandy aquifer near Kraków, southern Poland. A multilevel well penetrating the unsaturated zone in the study area was equipped with horizontally mounted lysimeters with ceramic suction cups to collect samples of pore water and metal probes to collect soil air. Strong seasonal fluctuations were observed of soil pCO2 extending down to the water table, coupled with distinct, well-defined depth profiles of δ13CTDIC reaching approximately −10′ at 8 m depth and almost constant radiocarbon content in the TDIC pool, comparable to 14CO2 levels in the local atmosphere. Simple models (closed/open system) do not account for the observed depth variations of carbon isotopic composition of the TDIC pool. This suggests that the TDIC reservoir of pore waters is evolving under conditions gradually changing from an open towards a closed system. In order to explain 13C and 14C content of dissolved carbonates in groundwater in the recharge area of the studied aquifer, additional sources of carbon in the system are considered, such as organic matter decomposition accompanied by reduction of dissolved nitrates and sulfates. The piston-flow l4C ages of groundwater in the confined part of the studied system were calculated using 2 approaches: 1) the correction model proposed by Fontes and Garnier (1979) was used to calculate groundwater ages, utilizing the chemical and carbon isotopic data available for the sampled wells; and 2) inverse geochemical modeling was performed for selected pairs of wells using NETHPATH code. The calculated 14C ages of groundwater range from approximately 0.6 to 37.5 ka BP. Although both methods appeared to be in a broad agreement, NETHPATH calculations revealed that isotopic exchange processes between TDIC pool and solid carbonates present in relatively small amounts in the aquifer matrix play an important role in controlling the 13C and 14C signatures of the dissolved carbonate species in groundwater and should be taken into account when 14C ages are calculated.

Risk Evaluation of Uranium Mining: A New Kinetic Approach

Release of uranium and associated heavy metals is the main environmental concern regarding exploitation and processing of U-ore. Increasing uranium mining activities potentially increase the risks linked to radiation exposure. As a tool to evaluate these risks, a geochemical inverse modeling approach was developed to estimate the water- mineral interaction in the presence of uranium. Our methodology is based on the estimation of dissolution rate and reactive surface area of the different minerals participating in the reaction by reconstructing the chemical evolution of the interacting fluids. We found that the reactive surface area of parent-rock minerals changes over several orders of magnitude during the investigated reaction time. We propose that the formation of coatings on dissolving mineral surfaces significantly reduces reactivity. Our results show that negatively charged uranium complexes decrease when alkalinity and rock buffer capacity is similarly lower, indicating that the dissolved carbonate is an important parameter impacting uranium mobility.

Recharge processes and groundwater evolution of multiple aquifers, Beijing, China

A geological and hydrogeological survey was performed in the upper alluvial and proluvial fan and the northern mountain area of the Chaobai River in Beijing. Samples of local precipitation, surface water and groundwater (including spring discharges, groundwater from the fractured bedrock, and shallow and deep groundwater of the alluvium) were collected and analysed to assemble hydrochemical and isotopic data. Q-mode hierarchical cluster analysis (HCA) was used to classify the groundwater samples from multiple aquifers into objective groups. Two groups and six subgroups with distinct groundwater recharge sources, flowpath histories and geochemical evolution histories were identified. Inverse geochemical modelling was adopted to study the deep groundwater chemical evolution. Hydrochemical data and isotopic data on 2H, 3H and 18O were used to evaluate groundwater recharge, flowpath histories and geochemical evolution, and 87Sr was used to provide further insight into groundwater recharge sources and flow directions. It was found that particularly in areas of groundwater cones of depression, the deep alluvial groundwater may be partly derived from the groundwater of the underlying fractured bedrock. The phreatic, or unconfined, groundwater within the northern area of the alluvial and proluvial fan is sustained mainly by the lateral flow of groundwater from the piedmont regions and by recharge from surface water bodies such as the Miyun reservoir. The shallow alluvial groundwater within the central and southern areas of the alluvial and proluvial fan is recharged primarily by the local precipitation and agricultural irrigation. The deep alluvial groundwater within the central and southern areas of the fan is sustained chiefly by the flow of groundwater from the northern area of the fan or from the underlying fractured bedrock. Methodologies adopted in this work may be useful to future investigation of groundwater in multiple aquifer systems similar to this area.

Changes in sources and storage in a karst aquifer during a transition from drought to wet conditions

Understanding the sources and processes that control groundwater compositions and the timing and magnitude of groundwater vulnerability to potential surface-water contamination under varying meteorologic conditions is critical to informing groundwater protection policies and practices. This is especially true in karst terrains, where infiltrating surface water can rapidly affect groundwater quality. We analyzed the evolution of groundwater compositions (major ions and Sr isotopes) during the transition from extreme drought to wet conditions, and used inverse geochemical modeling (PHREEQC) to constrain controls on groundwater compositions during this evolution. Spring water and groundwater from two wells dominantly receiving diffuse and conduit flow (termed diffuse site and conduit site, respectively) in the Barton Springs segment of the Edwards aquifer (central Texas, USA) and surface water from losing streams that recharge the aquifer were sampled every 3–4 weeks during November 2008–March 2010. During this period, water compositions at the spring and conduit sites changed rapidly but there was no change at the diffuse site, illustrating the dual nature (i.e., diffuse vs. conduit) of flow in this karst system. Geochemical modeling demonstrated that, within a month of the onset of wet conditions, the majority of spring water and groundwater at the conduit site was composed of surface water, providing quantitative information on the timing and magnitude of the vulnerability of groundwater to potential surface-water contamination. The temporal pattern of increasing spring discharge and changing pattern of covariation between spring discharge and surface-water (steam) recharge indicates that that there were two modes of aquifer response—one with a small amount of storage and a second that accommodates more storage.

Using chemical analysis and modeling to enhance the understanding of soil solution of some calcareous soils

The chemistry of soil solutions can be altered by human activities, due to the intense agricultural and husbandry, leading to leaching of nutrients and subsequently elevating ground water levels. Multivariate statistical and inverse geochemical modeling techniques were used to determine the main factors controlling soil solution chemistry of calcareous soils. In this research, a total of 21 calcareous soils was characterized and assessed for soil solution using soil column. The major cations in the studied soil solutions were in the decreasing order as Ca2+ > Mg2+ > Na+ > K+. The anions were also arranged in decreasing order as HCO\( _{3}^{ - } \) > Cl\( ^{ - } \) > SO\( _{4}^{2 - } \) > NO\( _{3}^{ - } \). Concentrations of NO\( _{3}^{ - } \), P, and K+ in soil solutions were in the range of 6.8–307.5 mg l−1 (mean 63.2 mg l−1), 5.0–10.4 mg l−1 (mean 5.9 mg l−1), and 2.8–54.6 mg l−1 (mean 11.3 mg l−1), respectively. Results suggest that the concentration of P in the soil solutions could be primarily controlled by the solubility of dicalcium phosphate dihydrate and dicalcium phosphate. Interactions between soil properties and observed solubility of nutrients were described, and put into empirical multivariate formulations. Obtained equations contained electrical conductivity (EC) as a key factor in determining nutrients solubility. Inverse geochemical modeling of soil solution using PHREEQC indicates the dissolution of calcite, anhydrite, halite, CO2 (g), N2 (g), and hydroxyapatite, and precipitation of sulfur. Cation exchange between Ca2+, Mg2+, K+ and Na+ occurred with Mg2+ and K+ into the solution, and Ca2+ and Na+ out of the solution. Determination of soil solution will improve soil management in the area, and preventing groundwater deterioration.

The origin of thermal waters in the northeastern part of the Eger Rift, Czech Republic

An investigation of the thermal waters in the Ústí nad Labem area in the northeastern part of the Eger Rift has been carried out, with the principal objective of determining their origin. Waters from geothermal reservoirs in the aquifers of the Bohemian Cretaceous Basin (BCB) from depths of 240 to 616 m are exploited here. For comparison, thermal waters of the adjacent Teplice Spa area were also incorporated into the study. Results based on water chemistry and isotopes indicate mixing of groundwater from aquifers of the BCB with groundwater derived from underlying crystalline rocks of the Erzgebirge Mts. Unlike thermal waters in Děčín, which are of Ca–HCO3 type, there are two types of thermal waters in Ústí nad Labem, Na–HCO3–Cl–SO4 type with high TDS values and Na–Ca–HCO3–SO4 type with low TDS values. Carbon isotope data, speciation calculations, and inverse geochemical modeling suggest a significant input of endogenous CO2 at Ústí nad Labem in the case of high TDS groundwaters. Besides CO2 input, both silicate dissolution and cation exchange coupled with dissolution of carbonates may explain the origin of high TDS thermal waters equally well. This is a consequence of similar δ13C and 14C values in endogenous CO2 and carbonates (both sources have 14C of 0 pmc, endogenous CO2 δ13C around −3‰, carbonates in the range from −5‰ to +3‰ V-PDB). The source of Cl− seems to be relict brine formed in Tertiary lakes, which infiltrated into the deep rift zone and is being flushed out. The difference between high and low TDS groundwaters in Ústí nad Labem is caused by location of the high mineralization groundwater wells in CO2 emanation centers linked to channel-like conduits. This results in high dissolution rates of minerals and in different δ13C(DIC) and 14C(DIC) fingerprints. A combined δ34S and δ18O study of dissolved SO4 indicates multiple SO4 sources, involving SO4 from relict brines and oxidation of H2S. The study clearly demonstrates potential problems encountered at sites with multiple sources of C, where several evolutionary groundwater scenarios are possible.

Calibration of hydrogeochemical model by multi-isotope approach

Inverse geochemical modelling was used to identify the hydrogeochemical processes at Xinzhou basin, northern China. The modelling was based on results of chemical and isotopic analysis of the groundwater samples and chemical and mineralogical investigation of aquifer host rocks and sediments. Results of geochemical modelling clearly indicate the importance of mineral dissolution/precipitation on groundwater chemistry, but provide no clue about the effect of mixing and evaporation. Combining the hydrogeochemical data with strontium, hydrogen and oxygen isotope composition data helps in understanding the hydrogeochemical processes. On the graph of 87Sr/86Sr against 1/Sr plot, an evident mixing process occurred at the depth of 160 m, while water samples from wells with depths less than 100 m were significantly affected by evaporation. The oxygen isotope shift observed in the water samples from Xinzhou was caused by evaporative processes. The deuterium-excess for the groundwater samples from Xinzhou ranged from −0·4 to 12·2‰, with an average of 5·7294‰ reflecting the effect of the semi-arid climate. The multi-isotope approach can provide valuable information which can be used for groundwater model calibration and make the inverse modelling more valid.

Geochemistry of mine waters draining a low-sulfide, gold-quartz vein deposit, Bralorne, British Columbia

Abstract The Bralorne and Pioneer mines, now inactive, produced over 4 million ounces of Au from an orogenic lode Au deposit located on the eastern edge of the Coastal Mountains of SW British Columbia. Between 2007 and 2009, drainage from a recently developed exploration adit was investigated in order to better understand and anticipate potential environmental management issues associated with the development of this type of deposit in the future. Portal discharge rate and specific conductance were monitored continuously over a 14-month period during which 36 water samples were collected. Additional samples were collected from flooded workings within the adit. Concentrations of As and Sb at the portal range as high as 1738 and 316 μg/L, respectively, while those in the mine pool reach 3304 and 349 μg/L, respectively. Effluent chemistry is mildly alkaline (pH = 8.7) and is dominated by Na, Ca, Mg, HCO3 and SO4. Geochemical inverse modeling of effluent composition indicates weathering of albite (2515 kg/a), ferroan dolomite (718 kg/a), pyrite (456 kg/a), arsenopyrite (23 kg/a) and stibnite (2 kg/a). Modeled sulfide reaction coefficients, normalized by their corresponding host rock concentrations, suggest that oxidation of arsenopyrite is 25 times slower than that of pyrite whereas oxidation of stibnite is 1.5 times faster. Oxidative dissolution of arsenopyrite and stibnite releases 10.6 kg/a of As and 1.1 kg/a of Sb of which 57% and 46%, respectively, are sorbed to ferrihydrite and gibbsite on the bed of the shallow channel through which the mine pool drains to the portal. Although mass balance calculations predict the formation of sufficient ferrihydrite to sorb 100% of the As dissolved in the mine pool, this attenuation process was ineffective possibly because the precipitated sorbents settled to the bottom of the water column or because of competition for sorption sites from Ca and HCO3. The dissolved Sb/As molar ratio in portal effluent (0.082) is much greater than the Sb/As ratio of the mineralization (0.002) because of slower arsenopyrite oxidation and somewhat lesser sorption of Sb.

Water–rock interaction and geochemistry of groundwater from the Ain Azel aquifer, Algeria

Hydrochemical, multivariate statistical, and inverse geochemical modeling techniques were used to investigate the hydrochemical evolution within the Ain Azel aquifer, Algeria. Cluster analysis based on major ion contents defined 3 main chemical water types, reflecting different hydrochemical processes. The first group water, group 1, has low salinity (mean EC = 735 μS/cm). The second group waters are classified as Cl-HCO(3)-alkaline earth type. The third group is made up of water samples, the cation composition of which is dominated by Ca and Mg with anion composition varying from dominantly Cl to dominantly HCO(3) plus SO(4). The varifactors obtained from R-mode FA indicate that the parameters responsible for groundwater quality variations are mainly related to the presence and dissolution of some carbonate, silicate, and evaporite minerals in the aquifer. Inverse geochemical modeling along groundwater flow paths indicates the dominant processes are the consumption of CO(2), the dissolution of dolomite, gypsum, and halite, along with the precipitation of calcite, Ca-montmorillonite, illite, kaolinite, and quartz.

Hydrochemical Formation Mechanisms and Quality Assessment of Groundwater with Improved TOPSIS Method in Pengyang County Northwest China

Inverse geochemical modeling was used in this paper to quantitatively study the formation mechanisms of groundwater in Pengyang County, China. An improved TOPSIS method based on entropy weight was used to perform groundwater quality assessment in this area. The assessment results show that the groundwater in the study area is fit for human consumption and the high concentrations of some elements can be attributed to the strong water‐rock interactions. The inverse geochemical modeling reveals that the dominant reactions in different parts of the study area are different. In the south part of the study area, the precipitation of sodium montmorillonite, calcite and the dissolution of gypsum, fluorite, halite, albite and dolomite as well as CO2 dissolution and cation exchange are the major water‐rock interactions, while in the north part, the leading reactions are the precipitation of gypsum, dolomite, sodium montmorillonite, fluorite, the dissolution of calcite and albite and the CO2 emission and cation exchange are also important. All these reactions are influenced by the initial aquatic environment and hydrodynamic conditions of the flow path.