Weathering Reactions Research Articles

Development of thermodynamically-based models for simulation of hydrogeochemical processes coupled to channel flow processes in abandoned underground mines

Abstract Accurate modeling of changing geochemistry in mine water can be an important tool in post-mining site management. The Pollutant Sources and Sinks in Underground Mines (POSSUM) model and Pollutant Loadings Above Average Pyrite Influenced Geochemistry POSSUM (PLAYING POSSUM) model were developed using object-oriented programming techniques to simulate changing geochemistry in abandoned underground mines over time. The conceptual model was created to avoid significant simplifying assumptions that decrease the accuracy and defensibility of model solutions. POSSUM and PLAYING POSSUM solve for changes in flow rate and depth of flow using a finite difference hydrodynamics model then, subsequently, solve for geochemical changes at distinct points along the flow path. Geochemical changes are modeled based on a suite of 28 kinetically controlled mineral weathering reactions. Additional geochemical transformations due to reversible sorption, dissolution and precipitation of acid generating salts and mineral precipitation are also simulated using simplified expressions. Contaminant transport is simulated using a novel application of the Random-Walk method. By simulating hydrogeochemical changes with a physically and thermodynamically controlled model, the ‘state of the art’ in post-mining management can be advanced.

Experimental measurement of portlandite carbonation kinetics with supercritical CO 2

A series of experiments was performed to observe and quantify the reactivity of portlandite with supercritical CO 2. The experiments were carried out in a batch reactor under different conditions: pressure of 160 bar, temperatures of 80, 120, and 200 °C, presence or absence of liquid water. Initial reaction rates are proposed, based on two independent techniques: measurements of the advancement of reaction by X-ray diffractometry (i.e. quantification of newly formed mineral assemblage), and monitoring of volume of CO 2 injected into the batch reactor to compensate for pressure drop during the experiment (CO 2 consumption, exsolution of secondary H 2O). The scanning electron microscopy observations show two different behaviours: in the presence of liquid water, the reaction leads to the complete carbonation of the portlandite into calcite. On the other hand, in the absence of a liquid phase, partial carbonation leads to the passivation of the portlandite surface by precipitation of a coating of tiny calcite crystals, and an incomplete weathering reaction of the portlandite.

Arsenic in shallow aquifer in the eastern region of Bangladesh: insights from principal component analysis of groundwater compositions

Probable sources and mechanisms of arsenic (As) release in shallow aquifer in eastern Bangladesh are evaluated using statistical analysis of groundwater compositions. Dissolved As in 39 samples ranged from 8.05 to 341.5 microg/L with an average of 95.14 microg/L. Ninety seven percent of wells exceed the WHO limit (10 microg/L) for safe drinking water. Principal component analysis is applied to reduce 16 measured compositional variables to five significant components (principal components--PCs) that explain 86.63% of the geochemical variance. Two component loadings, namely PC 1 and PC 2 (45.31% and 23.05%) indicate the natural processes within the aquifers in which organic matter is a key reactant in the weathering reactions. Four groups of wells are defined by the PCA and each group of wells represents distinct physicochemical characteristics. Among them, group III groundwater shows higher As concentration together with high concentrations of Fe, Mn, dissolved organic carbon, PO4(3-) and HCO3(-) than groups I and II. Speciation calculations suggest that only wells of group III are saturated with respect to siderite, and all groups of samples are supersaturated with respect of rhodochrosite. The relationship of As with these parameters in the different groups of wells of the study area suggests that reductive dissolution of Fe-Mn oxyhydroxides with microbially mediated degradation of organic matter is considered to be the dominant processes to release As in groundwater.

Comparison of laboratory testing protocols to field observations of the weathering of sulfide-bearing mine tailings

A laboratory weathering study using a humidity cell procedure was conducted on two sulfide-bearing tailing samples from a metallurgical site in Ontario (Canada). The test was accompanied by microbiological studies to enumerate the major groups of sulfur-oxidizing bacteria and determine their potential role at different stages during the oxidation process. To evaluate the utility of this method, results were compared with those of previous laboratory and field studies on the same materials. The mineralogy of the laboratory samples differs only by the addition of a small amount of hydronium-bearing natrojarosite [(Na,H 3O)Fe 3(SO 4) 2(OH) 6] to one sample. The progress of sulfide oxidation and the rates of solute release were determined to evaluate the extent of mineral dissolution. These processes were influenced strongly by the capacity of the material to generate acidity, which was enhanced by the presence of hydronium-bearing natrojarosite. Acid-neutralization processes occurring during the laboratory tests were affected by reaction kinetics, consistent with field observations. In particular, the extent of carbonate-mineral dissolution appears to be different in the laboratory than in the field, where more prolonged rock–water interaction allowed more complete chemical equilibration. As a consequence, the capacity of this test procedure to predict weathering reactions in mine tailings is limited by its inability to reproduce the weathering sequence observed in the field. The results of the microbiological study showed that distinct groups of sulfur-oxidizing bacteria operate at different stages of the oxidative process, as was observed in field studies where tailings oxidation occurred under natural conditions, suggesting that microbiological tests performed for laboratory studies are reflective of field conditions.

Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing

In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing a 0.2–2 m thick zone of partially weathered rock layers (∼2.5 cm thickness each) called rindlets, which form concentric layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive Δ V of reaction builds up elastic strain energy. The rates of spheroidal fracturing and saprolite formation are therefore controlled by the rate of the weathering reaction. Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fracture-inducing reaction. This reaction occurs with an expansion in d (0 0 1) from 10.0 to 10.5 Å, forming “altered biotite”. Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of 8.2 × 10 −14 mol biotite m −2 s −1. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 μm resolution revealed that oxidized zones within individual biotite crystals are the first evidence of alteration of the otherwise unaltered corestone. Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding the rindlet–saprolite interface, hornblende dissolves to completion at a rate of 6.3 × 10 −13 mol hornblende m −2 s −1: the fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are able to describe the process by which the quartz diorite bedrock disaggregates and forms saprolite. In the corestone, biotite oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary minerals. The thickness of the resultant saprolite is maintained at steady state by a positive feedback between the denudation rate and the weathering advance rate driven by the concentration of pore water O 2 at the bedrock–saprolite interface.

Toward process-based modeling of geochemical soil formation across diverse landforms: A new mathematical framework

We present a mathematical model that integrates geochemical and geomorphic processes responsible for soils' elemental and mineralogical evolution on diverse landforms: eroding to depositional to level grounds. This new model combines a hillslope sediment mass balance and a soil geochemical mass balance. We model soils as the sum of a physically disturbed zone (PDZ) and the underlying physically undisturbed but chemically altered zone (CAZ). The model considers processes including the production of PDZ, weathering front propagation, mineral-specific dissolution, colloidal translocation, and colluvial transport. Thus this study serves to (1) review preexisting models of pedogenic processes and to (2) offer a framework to integrate the geochemical and geomorphic processes active in soils. We further discuss the implications of our model in (1) scaling chemical weathering reactions from mineral grains to pedons to soil catenas, (2) understanding the emergence of geochemical soil catenas, and (3) coupling physical and chemical weathering processes.

Geochemistry of the dissolved load of the Changjiang Basin rivers: Anthropogenic impacts and chemical weathering

This study focuses on the chemical and Sr isotopic compositions of the dissolved load of the rivers of the Changjiang Basin, one of the largest riverine systems in the world. Water samples were collected in August 2006 from the main tributaries and the main Changjiang channel. The chemical and isotopic analyses indicated that four major reservoirs (carbonates, silicates, evaporites and agriculture/urban effluents) contribute to the total dissolved solutes. The overall chemical weathering (carbonate and silicate) rate for the Changjiang is approximately 40 ton/km 2/year or 19 mm/kyr, similar to that of the Ganges–Brahmaputra system, and the basin is characterized by carbonate and silicate weathering rates ranging from 17 to 56 ton/km 2/year and from 0.7 to 7.1 ton/km 2/year, respectively. In the lower reach of the Changjiang main channel, the weathering rates are estimated to be 36 and 2.2 ton/km 2/year for carbonates and silicates, respectively. It appears that sulphuric acid may dominate chemical weathering reactions for some sub-basins. The budgets of CO 2 consumption are estimated to be 646 × 10 9 and 191 × 10 9 mol/year by carbonate and silicate weathering, respectively. The contribution of the anthropogenic inputs to the cationic TDS of the Changjiang is estimated to be 15–20% for the most downstream stations. Our study suggested that the Changjiang is strongly impacted by human activities and is very sensitive to the change of land use.

Elevated weathering rates in the Rocky Mountains during the Early Eocene Climatic Optimum

Silicate weathering reactions remove carbon dioxide from the atmosphere and store it in carbonate minerals. During the high atmospheric carbon dioxide conditions of the Early Eocene Climatic Optimum, rates of chemical weathering, physical erosion and denudation in the western USA were equivalent to the highest recorded rates in the non-glacial Quaternary. During the chemical weathering of silicate minerals, atmospheric carbon dioxide is incorporated into carbonate minerals and buried. As the rate of silicate weathering is thought to increase in response to increasing atmospheric CO2 concentrations, this represents an important negative feedback mechanism1. Quaternary records of weathering reflect a narrow range of pCO2 (180–300 p.p.m.v.)2; therefore, the extent of this feedback has been difficult to predict for increasing concentrations of atmospheric CO2. However, high CO2 levels of up to 1,125 p.p.m.v. have been suggested for the Early Eocene Climatic Optimum (52 to 50 million years ago)3,4,5. Here, we combine 40Ar/39Ar ages6 and the measured volumes of river-derived sediments and sodium-bearing evaporites to determine rates of physical erosion and chemical weathering in the Green River Basin, western United State of America, during the Early Eocene Climatic Optimum7. We find physical erosion rates of 420±79 t km2 yr−1 and chemical weathering rates of 62.5±21.9 t km2 yr−1. The calculated denudation rates of 175±30 m Myr−1 rival the highest documented non-glacial Quaternary rates for crystalline bedrock8. We suggest that elevated atmospheric CO2 levels during the Early Eocene epoch led to enhanced silicate dissolution rates9, and thus to increased production of loose rock material and higher rates of physical weathering and denudation.

Sulfuric acid as an agent of carbonate weathering constrained by δ13C DIC: Examples from Southwest China

Abstract Rock weathering by carbonic acid is thought to play an important role in the global carbon cycle because it can geologically sequestrate atmospheric CO 2. Current model of carbon cycle evolution usually assumes that carbonic acid is the major weathering agent and that other acids are not important. Here, we use carbon isotopic evidence and water chemistry of springs and rivers from the Beipanjiang River basin (Guizhou Province, Southwest China) to demonstrate that sulfuric acid is also an important agent of rock weathering. The δ 13C of dissolved inorganic carbon (DIC) in the water samples ranges from − 13.1‰ to − 2.4‰, and correlates negatively to [HCO 3 −]/([Ca 2+] + [Mg 2+]) ratios and positively to [SO 4 2−]/([Ca 2+] + [Mg 2+]) ratios. These relationships are interpreted as mixing diagrams between two reactions of carbonate weathering, using carbonic acid and sulfuric acid as a proton donor, respectively. Mixing proportions show that around 42% of the divalent cations in the spring water from Guizhou are originated from the interaction between carbonate minerals and sulfuric acid. It is shown that 40% of this sulfuric acid is derived from the atmosphere and has an anthropogenic origin. The remaining 60% are derived from the oxidative weathering of sulfide minerals in sedimentary rocks. Our results show the positive action of sulfuric acid on the chemical weathering of carbonate. Particularly, we show that sulfuric acid generated by coal combustion has increased by almost 20% the weathering rates of carbonate in Southwest China. This is a clear evidence that human activities are changing the weathering rates of rocks and demonstrates a negative feedback on the acidification of the ocean by greenhouse gases. Because of the involvement of sulfuric acid in weathering reactions, 63% of the alkalinity exported by rivers is derived from carbonate, instead of 50% when atmospheric CO 2 is the only acid involved in chemical weathering of carbonate. In the Guizhou Province, the weathering of carbonate is thus, at least transiently, a net source of CO 2 to the atmosphere. When extrapolated at global scale, sulfuric acid-induced carbonate weathering could counterbalance a significant part of the CO 2 consumed by silicate weathering. This paper highlights the competition between silicate weathering by carbonic acid and carbonate weathering by sulfuric acid for the regulation of the atmospheric CO 2 level.

Silicon control of strontium and cesium partitioning in hydroxide-weathered sediments

Cation partitioning and speciation in an aqueous soil suspension may depend on the coupling of reaction time, sorbate amount and mineral weathering reactions. These factors were varied in sediment suspension experiments to identify geochemical processes that affect migration of Sr 2+ and Cs + introduced to the subsurface by caustic high level radioactive waste (HLRW). Three glacio-fluvial and lacustrine sediments from the Hanford Site (WA, USA) were subjected to hyperalkaline (pH > 13), Na–Al–NO 3–OH solution conditions within a gradient field of (i) sorptive concentration (10 −5–10 −3 m) and (ii) reaction time (0–365 d). Strontium uptake ( q Sr) exceeded that of cesium at nearly all reaction times. Sorbent affinity for both Cs + and Sr 2+ increased with clay plus silt content at early times, but a prolonged slow uptake process was observed over the course of sediment weathering that erased the texture effect for Sr 2+; all sediments showed similar mass normalized uptake after several months of reaction time. Strontium became progressively recalcitrant to desorption after 92 d, with accumulation and aging of neoformed aluminosilicates. Formation of Cs + and Sr 2+-containing cancrinite and sodalite was observed after 183 d by SEM and synchrotron μ-XRF and μ-XRD. EXAFS data for q Sr ≈ 40 mmol kg −1 showed incorporation of Sr 2+ into both feldspathoid and SrCO 3(s) coordination environments after one year. Adsorption was predominant at early times and low sorbate amount, whereas precipitation, controlled largely by sediment Si release, became increasingly important at longer times and higher sorbate amount. Kinetics of contaminant desorption at pH 8 from one year-weathered sediments showed significant dependence on background cation (Ca 2+ versus K +) composition. Results of this study indicate that co-precipitation and ion exchange in neoformed aluminosilicates may be an important mechanism controlling Sr 2+ and Cs + mobility in siliceous sediments impacted by hyperalkaline HLRW.

Geochemical investigation of weathering processes in a forested headwater catchment: mass-balance weathering fluxes

AbstractGeochemical research on natural weathering has often been directed towards explanations of the chemical composition of surface water and ground water resulting from subsurface water-rock interactions. These interactions are often defined as the incongruent dissolution of primary silicates, such as feldspar, producing secondary weathering products, such as clay minerals and oxyhydroxides, and solute fluxes (Meunier and Velde, 1979). The chemical composition of the clay-mineral product is often ignored. However, in earlier investigations, the saprolitic weathering profile at the South Fork Brokenback Run (SFBR) watershed, Shenandoah National Park, Virginia, was characterized extensively in terms of its mineralogical and chemical composition (Piccoli, 1987; Pochatila et al., 2006; Jones et al., 2007) and its basichydrology. O’Brien et al. (1997) attempted to determine the contribution of primary mineral weathering to observed stream chemistry at SFBR. Mass-balance model results, however, could provide only a rough estimate of the weathering reactions because idealized mineral compositions were utilized in the calculations. Making use of detailed information on the mineral occurrence in the regolith, the objective of the present study was to evaluate the effects of compositional variation on mineral-solute mass-balance modelling and to generate plausible quantitative weathering reactions that support both the chemical evolution of the surface water and ground water in the catchment, as well as the mineralogical evolution of the weathering profile.

Discrepancy between mineral residence time and soil age: Implications for the interpretation of chemical weathering rates

Virtually all soil chronosequence studies have equated the degree of mineral weather- ing with the soil age, which is equal to the time since the cessation of erosion or deposition. The primary minerals from the parent material, however, enter the soil as the weathering front propagates downward and are depleted via chemical weathering. The residence time of minerals is thus a function of both the rate of conversion of parent material to soil (i.e., soil production) and the minerals' susceptibility to chemical weathering reactions. We fi nd that mineral residence times are signifi cantly shorter than the soil age. By mathematically consid- ering the interactions among soil production and chemical weathering, we demonstrate that traditional estimates of mineral-specifi c chemical weathering rates from soil chronosequences may diverge by several orders of magnitude from the actual weathering rates.

Characterization of elemental release during microbe–granite interactions at T = 28 °C

This study used batch reactors to characterize the mechanisms and rates of elemental release (Al, Ca, K, Mg, Na, F, Fe, P, Sr, and Si) during interaction of a single bacterial species ( Burkholderia fungorum) with granite at T = 28 °C for 35 days. The objective was to evaluate how actively metabolizing heterotrophic bacteria might influence granite weathering on the continents. We supplied glucose as a C source, either NH 4 or NO 3 as N sources, and either dissolved PO 4 or trace apatite in granite as P sources. Cell growth occurred under all experimental conditions. However, solution pH decreased from ∼7 to 4 in NH 4-bearing reactors, whereas pH remained near-neutral in NO 3-bearing reactors. Measurements of dissolved CO 2 and gluconate together with mass-balances for cell growth suggest that pH lowering in NH 4-bearing reactors resulted from gluconic acid release and H + extrusion during NH 4 uptake. In NO 3-bearing reactors, B. fungormum likely produced gluconic acid and consumed H + simultaneously during NO 3 utilization. Over the entire 35-day period, NH 4-bearing biotic reactors yielded the highest release rates for all elements considered. However, chemical analyses of biomass show that bacteria scavenged Na, P, and Sr during growth. Abiotic control reactors followed different reaction paths and experienced much lower elemental release rates compared to biotic reactors. Because release rates inversely correlate with pH, we conclude that proton-promoted dissolution was the dominant reaction mechanism. Solute speciation modeling indicates that formation of Al–F and Fe–F complexes in biotic reactors may have enhanced mineral solubilities and release rates by lowering Al and Fe activities. Mass-balances further reveal that Ca-bearing trace phases (calcite, fluorite, and fluorapatite) provided most of the dissolved Ca, whereas more abundant phases (plagioclase) contributed negligible amounts. Our findings imply that during the incipient stages of granite weathering, heterotrophic bacteria utilizing glucose and NH 4 only moderately elevate silicate weathering reactions that consume atmospheric CO 2. However, by enhancing the dissolution of non-silicate, Ca-bearing trace minerals, they could contribute to high Ca/Na ratios commonly observed in granitic watersheds.

Electrochemical Acceleration of Chemical Weathering as an Energetically Feasible Approach to Mitigating Anthropogenic Climate Change

We describe an approach to CO2 capture and storage from the atmosphere that involves enhancing the solubility of CO2 in the ocean by a process equivalent to the natural silicate weathering reaction. HCl is electrochemically removed from the ocean and neutralized through reaction with silicate rocks. The increase in ocean alkalinity resulting from the removal of HCI causes atmospheric CO2 to dissolve into the ocean where it will be stored primarily as HCO3- without further acidifying the ocean. On timescales of hundreds of years or longer, some of the additional alkalinity will likely lead to precipitation or enhanced preservation of CaCO3, resulting in the permanent storage of the associated carbon, and the return of an equal amount of carbon to the atmosphere. Whereas the natural silicate weathering process is effected primarily by carbonic acid, the engineered process accelerates the weathering kinetics to industrial rates by replacing this weak acid with HCI. In the thermodynamic limit--and with the appropriate silicate rocks--the overall reaction is spontaneous. A range of efficiency scenarios indicates that the process should require 100-400 kJ of work per mol of CO2 captured and stored for relevant timescales. The process can be powered from stranded energy sources too remote to be useful for the direct needs of population centers. It may also be useful on a regional scale for protection of coral reefs from further ocean acidification. Application of this technology may involve neutralizing the alkaline solution that is coproduced with HCI with CO2 from a point source or from the atmosphere prior to being returned to the ocean.

Response of silicate weathering to monsoon changes on the Chinese Loess Plateau

Abstract Eolian deposits of loess on the Chinese Loess Plateau are investigated to decipher the influence of sequentially changed monsoon climate on silicate weathering process. Detailed hydrochloric acid dissolution experiments are applied to establish a sensitive proxy for silicate weathering. Combined elemental and mineralogical studies show that the minerals which are susceptible to incipient chemical weathering can be totally dissolved in hot hydrochloric acid (80°C, 3mol/L) after 4h, while other stable minerals are nearly unaffected. Thus, the hydrochloric acid dissolvable fractions (ADF) of loess and paleosol are more sensitive than bulk samples in the weathering reaction. Since Mg is tend to be leached out from the ADF while Al is conserved during the incongruent weathering, Mg/Al ratio of the ADF (Mg/Al ADF ) could indicate the weathering intensities of the Mg-bearing minerals, mainly chlorite. The Mg/Al ADF of the modern soils on the Chinese Loess Plateau is closely correlated to the local precipitation amounts, suggests that the intensity of summer monsoon is a key factor in the weathering of Mg-silicate. The silicate weathering intensity of the loess-paleosol deposits of past 130Kyrs shows strong procession cycle of ∼ 23Kyrs coupled by glacial-interglacial variation. The ∼ 23Krys cycles are coherent to the variations of the intensity of summer monsoon, while the ∼ 100Kyrs glacial-interglacial variation may relates to the changes in winter monsoon. Phased intensification of East Asian summer monsoon, and thus enhanced silicate weathering and atmosphere CO 2 drawdown, in response to the uplift of Tibetan Plateau, may be another mechanism relating the late Cenozoic tectonics to global cooling.

Fallow management practices in Guatemala's Western Highlands: social drivers and biophysical impacts

AbstractLand pressures and environmental degradation are driving forces behind shortened fallow periods in the tropics, often resulting in reduced crop yields and increased migration from rural areas. This paper describes contemporary fallow practices in the Western Highlands of Guatemala based on interdisciplinary data collected using participatory rural appraisal and qualitative research methods in combination with a quantitative evaluation of the impacts of fallow management decisions on soil fertility. Case studies of two communities in San Marcos department illustrate contemporary and traditional land use practices. Currently, over 70 per cent of families engage in a variety of fallow management practices, with combined cropping‐fallow cycles within a field averaging 3–6 years. Despite the reduction in length of fallow cycles, new fallow practices in the study area appear to improve some aspects of soil fertility while also providing fodder and fuelwood. Calcium and magnesium concentrations in fallow soil were twice that of cropped plots, indicating that weathering reactions and atmospheric deposition during fallow periods are able to restore base cation fertility that is taken up by potato crops during cropping cycles. Soil in cropped plots, however, showed 25 per cent higher soil organic matter and five times higher nitrate concentrations than soil in fallow plots, which resulted from additions of compost and inorganic fertilizer to cropped plots. Nevertheless, the 13C/12C isotopic ratio of soil organic carbon indicated that as soil organic matter content decreases in cropped plots, the remaining carbon is increasingly degraded. Potential improvements in fallow management practices proposed by farmers and researchers are also presented. Copyright © 2007 John Wiley & Sons, Ltd.

Characterization and evaluation of the factors affecting the geochemistry of groundwater in Neyveli, Tamil Nadu, India

In order to achieve a better understanding of the nature of the factors influencing ground water composition as well as to specify them quantitatively, multivariate statistical analysis (factor analysis) were performed on the hydrochemical data of this area. R-mode factor analysis was carried out on the geochemical results of the 79-groundwater samples and the factor scores were transferred to areal maps. Fundamental chemical parameters of the groundwater have been compounded together for characterizing and interpreting a few empirical hydrogeochemical factors controlling the chemical nature of water. R-mode factor analysis reveals that the groundwater chemistry of the study area reflects the influence of anthropogenic activities, silicate weathering reactions, precipitation, dissolution and subsequent percolation into the groundwater. The data have been put into few major factors and the seasonal variation in the chemistry of water has been clearly brought out by these factors. Factor scores were transferred to contour diagrams and the factor score analysis has been used successfully to delineate the stations under study with various factors and the seasonal effect on the sample stations.

The carbonate system geochemistry of shallow groundwater-surface water systems in temperate glaciated watersheds (Michigan, USA): Significance of open-system dolomite weathering

We present here a field geochemical study of controls on carbonate weathering within rapidly circulating, shallow groundwater–surface water systems in the glaciated mid-continent region. Groundwaters and surface waters in three watersheds spanning the Upper to Lower Peninsulas of Michigan consist of Ca 2+ -Mg 2+ -HCO 3 − solutions derived from the open-system dissolution of calcite and dolomite in soils developed on mixed mineralogy glacial drift. The thermodynamic stabilities of calcite and dolomite both decrease with decreasing temperature, with dolomite more strongly affected. Thus, the low mean annual temperature of these temperate weathering environments maximizes the absolute solubility of dolomite as well as its solubility relative to calcite. Many groundwaters in the study area approach equilibrium with respect to the more soluble dolomite and are moderately supersaturated with respect to calcite. Groundwaters in each watershed have distinct and relatively narrow ranges of carbon dioxide partial pressure ( P CO 2 ) values, which increase significantly from north to south (log P CO 2 of −3.0 to −2.2 atm), suggesting that there are landscape-level differences in carbon transformation rates in soil weathering zones. Increases in weathering-zone P CO 2 values produce HCO 3 − concentrations that vary by a factor of five, but the Mg 2+ /Ca 2+ and Mg 2+ /HCO 3 − ratios of all groundwaters are similar, suggesting relatively constant weathering input ratios of calcite and dolomite. Although surface waters commonly are between 2 and 10 times supersaturated with respect to calcite, the Mg 2+ /HCO 3 − ratios of surface waters are very close to initial groundwater values, suggesting that back precipitation of calcite is not a significant process in these systems. The enhanced solubility of dolomite at low temperatures coupled with the landscape-level differences in carbon cycling suggest that temperate-zone weathering reactions in glaciated terrains are significant contributors to continent-scale fluxes of both Mg 2+ and HCO 3 − .

Major ion chemistry of the Yarlung Tsangpo–Brahmaputra river: Chemical weathering, erosion, and CO 2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya

The Yarlung Tsangpo–Brahmaputra river drains a large portion of the Himalaya and southern Tibetan plateau, including the eastern Himalayan syntaxis, one of the most tectonically active regions on the globe. We measured the solute chemistry of 161 streams and major tributaries of the Tsangpo–Brahmaputra to examine the effect of tectonic, climatic, and geologic factors on chemical weathering rates. Specifically, we quantify chemical weathering fluxes and CO 2 consumption by silicate weathering in southern Tibet and the eastern syntaxis of the Himalaya, examine the major chemical weathering reactions in the tributaries of the Tsangpo–Brahmaputra, and determine the total weathering flux from carbonate and silicate weathering processes in this region. We show that high precipitation, rapid tectonic uplift, steep channel slopes, and high stream power generate high rates of chemical weathering in the eastern syntaxis. The total dissolved solids (TDS) flux from the this area is greater than 520 tons km −2 yr −1 and the silicate cation flux more than 34 tons km −2 yr −1. In total, chemical weathering in this area consumes 15.2 × 10 5 mol CO 2 km −2 yr −1, which is twice the Brahmaputra average. These data show that 15–20% of the total CO 2 consumption by silicate weathering in the Brahmaputra catchment is derived from only 4% of the total land area of the basin. Hot springs and evaporite weathering provide significant contributions to dissolved Na + and Cl − fluxes throughout southern Tibet, comprising more than 50% of all Na + in some stream systems. Carbonate weathering generates 80–90% of all dissolved Ca 2+ and Mg 2+ cations in much of the Yarlung Tsangpo catchment.

Investigation of hydrochemical characteristics of groundwater from the Cretaceous-Eocene limestone aquifer in southern Ghana and southern Togo using hierarchical cluster analysis

The most relevant controls on the water quality within the Cretaceous-Eocene limestone aquifer of the Keta Basin, Ghana, and the coastal sedimentary basin of Togo were assessed using Q-mode hierarchical cluster analysis (HCA) and mass-balance modelling. The pattern recognition technique of HCA was employed for partitioning hydrochemical data from a total of 65 surface and borehole samples from the study area into water groups. A spatial plot of the water groups consisting of samples from the limestone aquifer shows that the vast majority of samples belonging to the same group are located in close proximity to one another, suggesting the same processes and/or flow paths in the limestone aquifer system. Geochemical reaction models of selected water groups were constructed using PHREEQC-2. The hydrochemical compositions of the water groups and the mass-balance calculations indicate that the dominant processes and reactions responsible for the hydrochemical evolution in the system are: (1) carbonate equilibria, (2) silicate weathering reactions, (3) limited mixing with saline water, and (4) ion exchange. The combined use of HCA and mass-balance modelling has shown to be a useful approach in interpreting groundwater hydrochemistry in an area where large uncertainties exist in the understanding of the groundwater flow system.