Modelling Zn(II) sorption onto clayey sediments using a multi-site ion-exchange model

In environmental studies, it is necessary to be able to predict the behaviour of contaminants in more or less complex physico-chemical contexts. The improvement of this prediction partly depends on establishing thermodynamic models that can describe the behaviour of these contaminants and, in particular, the sorption reactions on mineral surfaces. In this way, based on the mass action law, it is possible to use surface complexation models and ion exchange models. Therefore, the aim of this study is (i) to develop an ion-exchange model able to describe the sorption of transition metal onto pure clay minerals and (ii) to test the ability of this approach to predict the sorption of these elements onto natural materials containing clay minerals (i.e. soils/sediments) under various chemical conditions. This study is focused on the behaviour of Zn(II) in the presence of clayey sediments. Considering that clay minerals are cation exchangers containing multiple sorption sites, it is possible to interpret the sorption of Zn(II), as well as competitor cations, by ion-exchange equilibria with the clay minerals. This approach is applied with success to interpret the experimental data obtained previously in the Zn(II)–H⁺–Na⁺–montmorillonite system. The authors’ research team has already studied the behaviour of Na⁺, K⁺, Ca²⁺ and Mg²⁺ versus pH in terms of ion exchange onto pure montmorillonite, leading to the development of a thermodynamic database including the exchange site concentrations associated with montmorillonite and the selectivity coefficients of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Zn²⁺ versus H⁺.     

Ion exchange and dewaterability of clay sludges

Two clay gel systems, a sample of settled oil sands clay tailings and a montmorillonite gel, were effectively dewatered by conversion to calcium forms by ion exchange, followed by freezing and thawing in the case of the montmorillonite gel and by either centrifugation or freezing and thawing in the case of the oil sands slime. Maximum dewatering was shown to be associated with complete conversion to the calcium forms. The implications of caustic recovery during the process are discussed.     

Effects of poor quality irrigation waters on the nutrient leaching and groundwater quality from sandy soil

Sodium (Na⁺) in poor quality irrigation water participate in ion-exchange processes results in the displacement of base cations into solution and a raised concentration in groundwater. Knowledge of the rate of decrease of nutrients from soils resulting from poor water quality application is essential for long-term planning of crop production while minimizing the impact on groundwater quality. In this study, we examined the effect of sodium adsorption ratio (SAR) of irrigation water on nutrients leaching and groundwater quality in columns of sandy soil. Three types of irrigation waters at three NaCl–CaCl₂ solutions with the following levels of SAR (5, 15, and 30) were synthesized in laboratory. With the application of solutions, exchange occurred between solution Na⁺ and exchangeable cations (Ca²⁺, Mg²⁺, and K⁺), resulting in the displacement of these cations and anions into solution. Increasing the level of SAR from 5 to 15 and 30 resulted in increase in the average exchangeable sodium percentage (ESP) of the soil from 10.4 to 20.3, and 32.5, respectively. Adverse effect of high Na⁺ concentration in the solutions on raising ESP was less pronounced in solution having low SAR. Leaching of Ca²⁺, Mg²⁺, K⁺, and P from soil with the application of solutions represents a significant loss of valuable nutrients. This sandy soil showed the high risk for nutrients transfer into groundwater in concentrations exceeding the groundwater quality standard. Irrigation with poor quality water, which is generally more sodic and saline than regional groundwater, increases the rate of sodification and salinization of shallow groundwater.     

Hydrochemical characteristics of groundwater quality with special reference to fluoride concentration in parts of Mulugu-Venkatapur Mandals,Warangal district,Telangana

Hydrochemical studies were carried out in Mulugu-Venkatapur Mandals of Warangal district, Telangana state, India to find out the causes of high fluorides in groundwater and surface water causing a widespread incidence of fluorosis in local population. The fluoride concentration in groundwater ranges from 0.28 to 5.48 mg/l with a mean of 1.26 mg/l in pre-monsoon and 0.21 to 4.43 mg/l with a mean 1.45 mg/l in post-monsoon. About 32% and 34% of samples in pre and post-monsoon containing fluoride concentrations that exceed the permissible limit. The Modified Piper diagram reflects that, water belong to Ca⁺²-Mg⁺²-HCO₃ - to Na⁺-HCO₃ - facies. Negative chloroalkali indices in both the seasons prove that ion exchange took place between Na⁺ & K⁺ with Ca⁺² and Mg⁺² in aquatic solution in host rock. Different plots for major ions and molar ratios suggest that weathering of silicate rocks and water-rock interaction is responsible for major ion chemistry of water. High fluoride content in groundwater attributed to continuous water-rock interaction during the process of percolation with fluorite bearing country rocks under arid, low precipitation, and high evaporation conditions. The low calcium content in rocks and soils, and the presence of high content of sodium bicarbonate in soils and waters are important factors favouring high levels of fluoride in waters. The basement rocks provide abundant mineral sources of fluoride in the form of amphibole, biotite, fluorite, mica and apatite.     

Influence of hydrogeochemical processes on temporal changes in groundwater quality in a part of Nalgonda district, Andhra Pradesh, India

Geochemical processes that take place in the aquifer have played a major role in spatial and temporal variations of groundwater quality. This study was carried out with an objective of identifying the hydrogeochemical processes that controls the groundwater quality in a weathered hard rock aquifer in a part of Nalgonda district, Andhra Pradesh, India. Groundwater samples were collected from 45 wells once every 2 months from March 2008 to September 2009. Chemical parameters of groundwater such as groundwater level, EC and pH were measured insitu. The major ion concentrations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, and SO₄ ²⁻ were analyzed using ion chromatograph. CO₃ ⁻ and HCO₃ ⁻ concentration was determined by acid–base titration. The abundance of major cation concentration in groundwater is as Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ while that of anions is HCO₃ ⁻ > SO₄ ²⁻ > Cl⁻ > CO₃ ⁻. Ca–HCO₃, Na–Cl, Ca–Na–HCO₃ and Ca–Mg–Cl are the dominant groundwater types in this area. Relation between temporal variation in groundwater level and saturation index of minerals reveals the evaporation process. The ion-exchange process controls the concentration of ions such as calcium, magnesium and sodium. The ionic ratio of Ca/Mg explains the contribution of calcite and dolomite to groundwater. In general, the geochemical processes and temporal variation of groundwater in this area are influenced by evaporation processes, ion exchange and dissolution of minerals.     

Cation exchange processes and human activities in unconfined aquifers

During the 1999–2002 water years, a hydrogeological research project was carried out on the unconfined aquifer of Trifilia in the Peloponnese. Seawater intrusion due to overpumping, and intensive use of fertilizers caused the groundwater quality degradation that is a typical case for the coastal aquifers in Greece. Isopiezometric maps along with ion distribution balances, ion distribution maps and factor analysis indicate the existence of three zones of groundwater quality. In the first zone of saline water, a cation exchange process between the Ca²⁺ of sediments and the Na⁺ of groundwater contribute to the formation of the water type Na⁺-Ca²⁺-Cl^–. In the second zone, which is considered as a transition zone, dominate the Ca²⁺-Na⁺-HCO₃^–-Cl^– water type. In the third zone of Ca²⁺-HCO₃^–-SO₄^2– water type, relationships among Ca²⁺, SO₄^2–, NO₃^– and NH₄ can be attributed to the dissociation of ammonium nitrate and sulfate fertilizers on one hand, and Ca²⁺ derivation from cation exchange processes between water, rocks and clay minerals, such as smectite and illite, on the other.     

Hydrochemistry of groundwater (GW) and surface water (SW) for assessment of fluoride in Chinnaeru river basin,Nalgonda district, (AP) India

Hydrochemical studies were conducted in Chinnaeru river basin of Nalgonda district, Andhra Pradesh, India, to explore the causes of high fluorides in groundwater and surface water causing a widespread incidence of fluorosis in local population. The concentration of fluoride in groundwater ranges from 0.4 to 2.9 and 0.6 to 3.6 mg/l, stream water ranges from 0.9 to 3.5 and 1.4 to 3.2 mg/l, tank water ranges from 0.4 to 2.8 and 0.9 to 2.3 mg/l, for pre- and post-monsoon periods, respectively. The modified Piper diagram reflects that the water belongs to Ca²⁺–Mg²⁺–HCO₃ ^? to Na⁺–HCO₃ ^? facies. Negative chloroalkali indices in both the seasons prove that ion exchange between Na⁺ and K⁺ in aquatic solution took place with Ca²⁺ and Mg²⁺ of host rock. The interpretation of plots for different major ions and molar ratios suggest that weathering of silicate rocks and water–rock interaction is responsible for major ion chemistry of groundwater/surface water. High fluoride content in groundwater was attributed to continuous water–rock interaction during the process of percolation with fluorite bearing country rocks under arid, low precipitation, and high evaporation conditions. The low calcium content in rocks and soils, and the presence of high levels of sodium bicarbonate are important factors favouring high levels of fluoride in waters. The basement rocks provide abundant mineral sources of fluoride in the form of amphibole, biotite, fluorite, mica and apatite.     

Assessment of spatial distribution of contaminants and their levels in soil and water resources of Calabar,Nigeria using geophysical and geological data

The contamination levels of soils and water resources in Calabar, Nigeria have been investigated using resistivity (vertical electrical sounding and electrical resistivity tomography), geochemical analyses of soil and water resources and textural analysis. Sixty randomly sited VES sites were investigated in two seasons while ERT investigations were performed along four profiles. The geochemical investigations were spread across seasons in order to track seasonal changes in physico-chemical parameters: hydrogen ion concentration (pH), electrical conductivity, total dissolved solids, chloride ion (Cl^?), nitrate ion (\( {\text{NO}}_{ 3}^{ - } \)), bicarbonate (\( {\text{HCO}}_{ 3}^{ - } \)), sulphate ion (\( {\text{SO}}_{ 4}^{2 - } \)), calcium ion (Ca²⁺), sodium ion (Na⁺), potassium ion (K⁺) and magnesium ion (Mg²⁺). Additionally, concentrations of ammonium, aluminium and nitrite ions in soils were determined. Results show that ionic concentrations in the sand-dominated soils and water are within permissible limits and baseline standards. The resistivities follow known trends in the area. However, at the central waste disposal site, a localised thin (< 5 m), low resistivity (< 15 Ωm) anomaly suspected to be due to contamination by leachates was observed. Comparatively, the contaminated area is also characterised by marginal increase in ionic concentrations. Strong attenuation capacities of overlying and adjoining clay/lateritic sediments and optimal design of the waste dump site probably reduced the spread of contaminants. The contaminated zone need to be closely monitored so that it does not extend to the aquifers. Hence, all strategies presently being used in managing wastes in Calabar should be sustained.     

Seasonal variation in major ion chemistry of a tropical mountain river,the southern Western Ghats,Kerala, India

A comprehensive and systematic study to understand various geochemical processes as well as process drivers controlling the water quality and patterns of the hydrochemical composition of river water in Muthirapuzha River Basin, MRB (a major tributary of Periyar, the longest river in Kerala, India), was carried out during various seasons, such as monsoon, post-monsoon and pre-monsoon of 2007–2008, based on the data collected at 15 monitoring stations (i.e., 15 × 3 = 45 samples). Ca²⁺ and Mg²⁺ dominate the cations, while Cl^? followed by HCO₃ ^? dominates the anions. In general, major ion chemistry of MRB is jointly controlled by weathering of silicate and carbonate rocks, which is confirmed by relatively larger Ca²⁺ + Mg²⁺/Na⁺ + K⁺ ratios as well as Ca²⁺/Na⁺ vs. Mg²⁺/Na⁺ and Ca²⁺/Na⁺ vs. HCO₃ ^?/Na⁺ scatter plots. The relationship between Cl^? and Na⁺ implies stronger contributions of anthropogenic activities modifying the hydrochemical composition, irrespective of seasons. The water types emerged from this study are transitional waters or waters that changed their chemical character by mixing with waters of geochemically different ionic signatures. However, various ionic ratios, hydrochemical plots and graphical diagrams suggest seasonality over the hydrochemical composition, which is solely controlled by the rainfall pattern. Relatively higher pCO₂ indicates the disequilibrium existing in natural waterbodies vis-à-vis the atmosphere, which is an outcome of both the contribution of groundwater to stream discharge and anthropogenic activities. Hence, continuous monitoring of hydrochemical composition of mountain rivers is essential in the context of climate change, which has serious implications on tropical mountain fluvial-hydro systems.     

Interaction of bentonite colloids with Cs, Eu, Th and U in presence of humic acid: A flow field-flow fractionation study

The interaction of Cs(I), Eu(III), Th(IV) and U(VI) with montmorillonite colloids was investigated in natural Grimsel Test Site groundwater over a 3 years period. The asymmetric flow field-flow fractionation combined with various detectors was applied to study size variations of colloids and to monitor colloid association of trace metals. The colloids suspended directly in the low ionic strength (I), slightly alkaline granitic groundwater (I = 10⁻³ mol/L, pH 9.6) showed a gradual agglomeration with a size distribution shift from initially 10-200 nm to 50-400 nm within over 3 years. The Ca²⁺ concentration of 2.1 × 10⁻⁴ mol/L in the ground water is believed to be responsible for the slow agglomeration due to Ca²⁺ ion exchange against Li⁺ and Na⁺ at the permanently charged basal clay planes. Furthermore, the Ca²⁺ concentration lies close to the critical coagulation concentration (CCC) of 10⁻³ mol L⁻¹ for clay colloids. Slow destabilization may delimit clay colloid migration in this specific groundwater over long time scales. Eu(III) and Th(IV) are found predominantly bound to clay colloids, while U(VI) prevails as the UO₂(OH)₃⁻ complex and Cs(I) remains mainly as aquo ion under our experimental conditions. Speciation calculations qualitatively represent the experimental data. A focus was set on the reversibility of metal ion-colloid binding. Addition of humic acid as a competing ligand induces rapid metal ion dissociation from clay colloids in the case of Eu(III) even after previous aging for about 3 years. Interestingly only partial dissociation occurs in the case of Th(IV). Experiments and calculations prove that the humate complexes dominate the speciation of all metal ions under given conditions. The partial irreversibility of clay bound Th(IV) is presently not understood but might play an important role for the colloid-mediated transport of polyvalent actinides over wide distances in natural groundwater.     

Dissolution of silica from montmorillonite: effect of solution chemistry

The rate of silica removal from two montmorillonites (Chambers and Polkville) has been measured as a function of time, temperature, solution composition, and exchange ion on the clay. Silica removal rate increased with temperature from 200 to 350°C, decreased with time, and could be approximated initially by a parabolic rate law. Solution composition influenced silica removal rate by determining the exchange population of the clay; silica removal is most rapid when K-exchange ions are present. Thus increasing the concentration of K⁺ accelerated silica removal, whereas increasing the concentration of Na⁺, Ca²⁺, and Mg²⁺ inhibited silica removal. Activation energies for silica removal range from 5 to 10 kcal/mol. The largest values are associated with the largest concentrations of inhibitor ions in solution. Activation energies of this magnitude suggest that the rate-limiting step for silica removal is transport through a hydrated, expanded interlayer space. Application of experimental results to diagenesis in moderately to deeply buried sediments suggests that K⁺ uptake by montmorillonite may precede and accelerate illite formation.     

Hydrogeochemical processes and quality assessment of groundwater in Dumka and Jamtara districts, Jharkhand, India

The hydrogeochemical study of groundwater in Dumka and Jamtara districts has been carried out to assess the major ion chemistry, hydrogeochemical processes and groundwater quality for domestic and irrigation uses. Thirty groundwater samples were collected and analyzed for pH, electrical conductivity, total dissolved solids (TDS), total hardness, anions (F^?, Cl^?, NO₃ ^?, HCO₃ ^?, SO₄ ^2?) and cations (Ca²⁺, Mg²⁺, Na⁺, K⁺). The analytical results show the faintly alkaline nature of water and dominance of Mg²⁺ and Ca²⁺ in cationic and HCO₃ ^? and Cl^? in anionic abundance. The concentrations of alkaline earth metals (Ca²⁺?+?Mg²⁺) exceed the alkali metals (Na⁺?+?K⁺) and HCO₃ ^? dominates over SO₄ ^2??+?Cl^? concentrations in the majority of the groundwater samples. Ca?CMg?CHCO₃ is the dominant hydrogeochemical facies in 60?% of the groundwater samples, while 33?% samples occur as a mixed chemical character of Ca?CMg?CCl hydrogeochemical facies. The water chemistry is largely controlled by rock weathering and ion exchange processes with secondary contribution from anthropogenic sources. The inter-elemental correlations and factor and cluster analysis of hydro-geochemical database suggest combined influence of carbonate and silicate weathering on solute acquisition processes. For quality assessment, analyzed parameter values were compared with Indian and WHO water quality standards. In majority of the samples, the analyzed parameters are well within the desirable limits and water is potable for drinking purposes. Total hardness and concentrations of TDS, Cl^?, NO₃ ^? _, Ca²⁺ and Mg²⁺ exceed the desirable limits at a few sites, however, except NO₃ ^? all these values were below the highest permissible limits. The calculated parameters such as sodium adsorption ratio, percent sodium (%Na) and residual sodium carbonate revealed excellent to good quality of groundwater for agricultural purposes, except at few sites where salinity and magnesium hazard (MH) values exceeds the prescribed limits and demands special management.     

Hydrogeochemistry and quality of groundwater of coastal unconfined aquifer in Amol–Ghaemshahr plain,Mazandaran Province,Northern Iran

Groundwater of the unconfined aquifer (1,100 sq. km) of a two-tier coastal aquifer located in the Amol–Ghaemshahr plain, Mazandaran Province, Northern Iran, is classified into fresh and brackish water types. Fresh groundwater (FGW) samples (n = 36) are characterized by Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ and HCO₃ ^? > Cl^? > SO₄ ^2? > NO₃ ^?. Spearman’s rank correlation coefficient matrices, factor analysis data, values of the C-ratio (av. = 0.89) and CAI and values of the molar ratios of Ca²⁺/HCO₃ ^?, Ca²⁺/SO₄ ^2?, Mg²⁺/HCO₃ ^? and Mg²⁺/SO₄ ^2? indicate that the ionic load in the FGW is derived essentially from carbonic acid-aided weathering of carbonates and aluminosilicates, saline/sea water trapped in the aquifer sediments (now admixed with the groundwater) and ion exchange reactions. Values of the CAI and Na⁺/Cl^? molar ratio suggest that the part of the Ca²⁺ (±Mg²⁺) content in 23 FGW samples is derived from clay minerals of the aquifer matrix, and part of the Na⁺ content in 20, 12, and 3 FGW samples is derived, respectively, from alkali feldspar weathering, clay minerals of the aquifer matrix and rain water and/or halite. Brackish groundwater (BGW) samples (n = 4) contain Cl^? as the dominant anion and their average total ionic concentration (38.65 meq/L) is 1.79 times higher than that of the FGW samples (21.50 meq/L). BGW pockets were generated by non-conservative mixing of FGW with the upconed saline water from the underlying saline groundwater zone of the semi-confined aquifer along bore wells involved in excessive extraction of groundwater from the unconfined aquifer. Groundwater belongs essentially to “high salinity, low sodium” irrigation water class.     

The hydrogeochemistry of the Lake Waco drainage basin,Texas

The origin of surface water chemistry in highly impacted drainage basins must be investigated on a drainage-basin scale if the causes of the pollution are to be elucidated. This study characterizes and deciphers the surface water chemistry of a nutrient polluted river system in central Texas. Four tributaries of the Lake Waco reservoir were chemically characterized temporally and spatially in order to gain a complete understanding of the nature and origin of dissolved solids being transported into the lake. Temporal chemical variations measured at the base of each of the drainage basins are repetitive and seasonal. The most periodic and well-defined variation is exhibited by nitrate concentrations although many of the other solutes show seasonal changes as well. These temporal chemical changes are controlled by seasonal precipitation. During rainy seasons, the shallow aquifer is recharged resulting in stream discharge that is high in nitrate, calcium, and bicarbonate. When the shallow flow system is depleted in the summer, stream waters are dominated by deeper groundwater and become rich in sodium. Spatial variations in the chemistry of South Bosque surface waters were characterized using the snapshot technique. The spatial distribution of nitrate in surface waters is controlled by fertilizer application to row crops and the location of a munitions factory. The concentrations of naturally derived solutes such as Ca⁺, Na⁺, Cl^–, and SO₄^–2are controlled by underlying lithologies.     

Influential factors on the levels of cation exchange capacity in sediment at Langat river

An exploratory study was carried out at 22 sampling stations along the Langat River, Selangor in order to investigate on the vitality of cation exchange capacity (CEC) in sediment (0–5 cm). Parameters such as pH, Eh, salinity, and electrical conductivity (EC) were determined. The CEC in sediment has been calculated by the determination of Ca²⁺, Na⁺, Mg²⁺, and K⁺ using the flame atomic absorption spectrophotometer, while the organic matter content in sediment was ascertained using the loss on ignition method. The characteristic of the sediment shows that pH (3.09–7.46), salinity (0.02–10.71 ppt), EC (3.39–517 μS/cm) and Eh (?16.20–253.10 mV) were substantially high in variation. This study also revealed that exchangeable Ca²⁺ and Mg²⁺ were controlled by organic matter contents, while exchangeable Na⁺ and K⁺ were influenced by salinity. Salinity was observed to play a major part in controlling all the exchangeable cations, as it gives strong significant correlations with Na⁺, K⁺, Mg²⁺, CEC, and organic matter at p?<?0.01. The presence of seawater, clay mineralogy, and organic matter proves that it does play an important role in determining the CEC and soon relates to the pollution magnitude in the sediment.     

The importance of unsaturated zone biogeochemical processes in determining groundwater composition, southeastern Australia

Analysis of soil, soil water and groundwater in the Mount William Creek catchment, southeastern Australia, shows that Mg²⁺ and Ca²⁺ within infiltrating rainfall are rapidly depleted by plant uptake and adsorption on clay minerals. Na⁺ and K⁺ may exhibit minor enrichment at shallow depths but are quickly readsorbed, so that cation/Cl^– ratios typical of groundwater are observed in soil water within the upper 200 cm of the soil profile for all species. The concentrations of K⁺ and Ca²⁺ in soil and groundwater are more depleted than Na⁺ and Mg²⁺ due to preferential uptake by vegetation. Removal of organic matter results in a continuing, long-term export of all major cations from the soil profiles. The processes of biogeochemical fractionation within the unsaturated zone rapidly modify the cation/Cl^– ratios of infiltrating rainfall to values characteristic of seawater. These mechanisms may have reached steady state, because groundwaters with seawater ion/Cl^– ratios are thousands of years old; the exchange sites on the soil clays are probably saturated, so cations supplied in rainfall are exported in organic matter and incorporated into recharge infiltrating into the groundwater. Much of the chemical evolution of groundwater traditionally attributed to processes within the aquifer is complete by the time recharge occurs; this evolutionary model may have broad application.     

Mechanism and numerical simulation of multicomponent solute transport in sodic soils reclaimed by calcium sulfate

The mechanism for reclaiming sodic soils using calcium sulfate (CaSO₄) could provide a theoretical basis for the field application of CaSO₄ substitutes, including the by-products of flue gas desulfurization (BFGD), fly ash, and phosphorus gypsum. In this study, Ca²⁺ application experiment was conducted to analyze the dynamic changes of the cations in the reclamation of sodic soils with CaSO₄. A multicomponent solute transport model (UNSATCHEM) that considers ion adsorption exchange and dynamic changes in the soil’s hydraulic conductivity was subsequently used to simulate and predict the movement of ions. The Ca²⁺ application experiment consisted of four treatments with four CaSO₄ concentrations (0.5, 1, 1.5, and 2 g L^?1). When the Ca²⁺ concentrations in the supplied water were 14.71, 22.06, and 29.41 mmol L^?1, Ca²⁺ achieved penetration, and this process was faster when the Ca²⁺ concentration in the supplied water was higher. Ca²⁺ did not achieve penetration when the Ca²⁺ concentration was 7.35 mmol L^?1. UNSATCHEM was able to simulate the transportation mechanism of Ca²⁺ and Na⁺ in the soil solution in the Ca²⁺ application experiment, the adsorption and exchange between the Na⁺ in the soil colloid and Ca²⁺ in the soil solution, and the precipitation and dissolution of CaSO₄ with a high degree of accuracy. Sodic soil reclamation with CaSO₄ was not a short-term process. Compared with applying CaSO₄ only once, applying CaSO₄ in batches decreased the accumulation of soil salts and promoted its dissolution.     

Environmental geochemistry and quality assessment of surface and subsurface water of Mahi River basin, western India

The hydrogeochemical study of surface and subsurface water of Mahi River basin was undertaken to assess the major ion chemistry, solute acquisition processes and water quality in relation to domestic and irrigation uses. The analytical results show the mildly acidic to alkaline nature of water and dominance of Na⁺ and Ca²⁺ in cationic and HCO₃ ⁻ and Cl⁻ in anionic composition. In general, alkaline-earth elements (Ca²⁺ + Mg²⁺) exceed alkalis (Na⁺ + K⁺) and weak acids (HCO₃ ⁻) dominate over strong acids (SO₄ ²⁺ + Cl⁻) in majority of the surface and groundwater samples. Ca²⁺–Mg²⁺–HCO₃ ⁻ is the dominant hydrochemical facies both in surface and groundwater of the area. The weathering of rock-forming minerals mainly controlled the solute acquisition process with secondary contribution from marine and anthropogenic sources. The higher concentration of sodium and dissolved silica, high equivalent ratios of (Na⁺ + K⁺/TZ⁺), (Na⁺ + K⁺/Cl⁻) and low ratio of (Ca²⁺ + Mg²⁺)/(Na⁺ + K⁺) suggest that the chemical composition of the water is largely controlled by silicate weathering with limited contribution from carbonate weathering and marine and anthropogenic sources. Kaolinite is the possible mineral that is in equilibrium with the water, implying that the chemistry of river water favors kaolinite formation. Assessment of water samples for drinking purposes suggests that the majority of the water samples are suitable for drinking. At some sites concentrations of TDS, TH, F⁻, NO₃ ⁻ and Fe are exceeding the desirable limit of drinking. However, these parameters are well within the maximum permissible limit except for some cases. To assess the suitability for irrigation, parameters like SAR, RSC and %Na were calculated. In general, both surface and groundwater is of good to suitable category for irrigation uses except at some sites where high values of salinity, %Na and RSC restrict its uses.     

Amorphous silica solubilities—II. Effect of aqueous salt solutions at 25°C

The solubility of amorphous silica was measured at 25°C in ten separate sets of aqueous salt solutions—potassium chloride, potassium nitrate, sodium chloride, lithium chloride, lithium nitrate, magnesium chloride, calcium chloride, magnesium sulfate, sodium bicarbonate and sodium sulfate. The concentrations of the salts were varied from zero to saturation with both salt and amorphous silica. With increasing concentration of salt, the solubility of amorphous silica always decreased as expected from an average value of 0.00218 m in water. Nevertheless, the extent of decrease differed greatly from a 6% decrease in a solution saturated with NaHCO₃ to a 95.7% decrease in a solution saturated with CaCl₂. A striking correlation was observed: In the 1-1 and 2-1 electrolyte salt solutions at a given molality the effect on the solubility of silica depended upon the cation in the order Mg²⁺, Ca²⁺ > Li⁺ > Na⁺ > K ⁺.     

Osmotic Swelling and Osmotic Consolidation Behaviour of Compacted Expansive Clay

Compacted clay soils are used as barriers in geoenvironmental engineering applications and are likely to be exposed to salinization and desalinization cycles during life of the facility. Changes in pore fluid composition from salinization and desalinization cycles induce osmotic suction gradients between soil–water and reservoir (example, landfill/brine pond) solution. Dissipation of osmotic suction gradients may induce osmotic swelling and consolidation strains. This paper examines the osmotic swelling and consolidation behaviour of compacted clays exposed to salinization and desalinization cycles at consolidation pressure of 200 kPa in oedometer assemblies. During salinization cycle, sodium ions of reservoir fluid replaced the divalent exchangeable cations. The osmotic swelling strain developed during first desalinization cycle was 29-fold higher than matric suction induced swelling strain of the compacted specimen. Further, the diffusion controlled osmotic swelling strain was 100-fold slower than matric suction-driven swelling process. After establishment of ion-exchange equilibrium, saturated saline specimens develop reversible osmotic swelling strains on exposure to desalinization cycles. Likewise the saturated desalinated specimen develops reversible osmotic consolidation strains on exposure to cycles of salinization. Variations in compaction dry density has a bearing on the osmotic swelling and consolidation strains, while, compaction water content had no bearing on the osmotic volumetric strains.