Reach-Scale Cation Exchange Controls on Major Ion Chemistry of an Antarctic Glacial Meltwater Stream

McMurdo dry valleys of Antarctica represent the largest of the ice-free areas on the Antarctic continent, containing glaciers, meltwater streams, and closed basin lakes. Previous geochemical studies of dry valley streams and lakes have addressed chemical weathering reactions of hyporheic substrate and geochemical evolution of dry valley surface waters. We examine cation transport and exchange reactions during a stream tracer experiment in a dry valley glacial meltwater stream. The injection solution was composed of dissolved Li⁺, Na⁺, K⁺, and Cl^-. Chloride behaved conservatively in this stream, but Li⁺, Na⁺, and K⁺ were reactive to varying degrees. Mass balance analysis indicates that relative to Cl^-, Li⁺ and K⁺ were taken up in downstream transport and Na⁺ was released. Simulations of conservative and reactive (first-order uptake or generation) solute transport were made with the OTIS (one-dimensional solute transport with inflow and storage) model. Among the four experimental reaches of Green Creek, solute transport simulations reveal that Li⁺ was removed from stream water in all four reaches, K⁺ was released in two reaches, taken up in one reach, and Na⁺ was released in all four reaches. Hyporheic sediments appear to be variable with uptake of Li⁺ in two reaches, uptake of K⁺ in one reach, release of K⁺ in two reaches, and uptake of Na⁺ in one reach. Mass balances of the conservative and reactive simulations show that from 1.05 to 2.19 moles of Li⁺ was adsorbed per reach, but less than 0.3 moles of K⁺ and less than 0.9 moles of Na⁺ were released per reach. This suggests that either (1) exchange of another ion which was not analyzed in this experiment or (2) that both ion exchange and sorption control inorganic solute transport. The elevated cation concentrations introduced during the experiment are typical of initial flows in each flow season, which flush accumulated dry salts from the streambed. We propose that the bed sediments (which compose the hyporheic zone) modulate the flushing of these salts during initial flows each season, due to ion exchange and sorption reactions.     

Wind erosion of saline playa sediments and its ecological effects in Ebinur Lake,Xinjiang, China

In many arid and semiarid areas, dry lake beds (saline playa) represent a tremendous source of unconsolidated salt-rich sediments that are available for aeolian transport. Severe salt-dust storms caused by the erosion of such landforms have become very harmful natural phenomena. In this study, sample analysis and field erosion monitoring of Ebinur Lake was conducted to investigate the salt content, chemical composition, and wind erosion intensity of surface salt-rich sediments. The effects of salt-dust rising from the playa on the growth and physiological health of plants were also evaluated in this study through a leaf dustfall test. The results indicate that water-soluble salts assemble densely on the dry lake bed surface. At a depth of 0–2 cm, the highest salt contents can exceed 40%, with sulfate and chloride being the main anions present and Na⁺, Ca²⁺, and Mg²⁺ being the primary cations. The annual wind erosion rate ranged from 0.48 to 5.6 cm in the northwest portion of the lake and from 0.24 to 0.96 cm in the southeast portion. Salt-dust storms caused by wind erosion of saline playa sediments seriously influenced the normal absorption of minerals by plant leaves. Under the influence of salt-dust storms, plant leaves absorb more Na⁺, but far less K⁺.     

The specific surface of Na+-humate on the basis of coion exclusion

The activity of the coion in Na⁺ -humate sol from the membrane potential and the activity of coion in the dialyzate values is calculated. The specific surface of Na⁺ -humate sol, determined by the negative absorption of coion, is close to 2000 m²g^?1. The relationships among the specific surface, the molecular weight and the size of particles of Na⁺ -humate sol are considered.     

Growth and ionic relations of fodderbeet and seabeet under saline environments

Growth and ionic relations of fodderbeet (Beta vulgaris) and seabeet (B.maritima) were studied in a greenhouse experiment using garden soil salinized with 200 mM NaCl. Both the species tolerated salinity level of 200 mM NaCl, but seabeet performed better than fodderbeet. Fresh weight of shoot increased significantly (P< 0.01) under higher salt concentration in both the species. Fodderbeet accumulated more biomass under saline conditions than seabeet. Salt treated plants accumulated significantly higher Na⁺ and CI^? content in shoot compared to untreated control plants. Chloride (CI^?) of seabeet root was significantly higher than shoot; however, chloride content of shoot was significantly lower in fodderbeet. The concentration of these ions was comparatively higher in seabeet root than fodderbeet in the later growth period. Potassium K⁺ content was low and revealed antagonistic effect with that of Na⁺ content. Calcium (Ca⁺²) ions were not significantly absorbed by shoot of both species. A significant effect of treatment by the root has been observed in both species. Magnesium (Mg⁺²) content of shoot were increased with the time. Seabeet has absorbed more Mg⁺² than fodderbeet. The fodderbeet showed a significant salt tolerance during five week growth period. Seabeet is comparatively more salt tolerant. Domesticated cultivars of fodderbeet like Majorai have considerable adaptability in the inland salt affected areas of Pakistan.     

Effects of dry density and exchangeable cations on the diffusion process of sodium ions in compacted montmorillonite

As a part of the safety assessment of the geological disposal of high-level radioactive waste, the effects of dry density and exchangeable cations on the diffusion process of Na⁺ ions in compacted bentonite were studied from the viewpoint of the activation energy for diffusion. The apparent self-diffusion coefficients of Na⁺ ions in compacted Na-montmorillonite and in a Na- and Ca-montmorillonite mixture were determined by one-dimensional, non-steady diffusion experiments at different temperatures and dry densities. A unique change in activation energy as a function of dry density was found for the Na⁺ ions in compacted Na-montmorillonite. The activation energy suddenly decreased from 18.1 to 14.1 kJ mol^− 1 as the dry density increased from 0.9 to 1.0 Mg m^− 3, whereas it increased to 24.7 kJ mol^− 1 as the dry density increased to 1.8 Mg m^− 3. Examination of the effect of exchangeable cations on the activation energies determined that the activation energies were almost constant, approximately 25 kJ mol^− 1, for the montmorillonite specimens at a dry density of 1.8 Mg m^− 3. However, three different activation energy values were obtained at a dry density of 1.0 Mg m^− 3. These findings cannot be explained by the conventional diffusion model (the pore water diffusion model), which suggests that the predominant diffusion process alternates among pore water diffusion, interlayer diffusion, and external surface diffusion.     

Impact of anthropogenic activities on the chemistry and quality of groundwater: a case study from a terrain near Zarand City, Kerman Province, SE Iran

Groundwater of an aquifer located in the vicinity of a large coal washery near Zarand City, Iran consists of two hydrochemically differing facies, which have been informally designated as groundwater (A) and groundwater (B). Groundwater (A) is native, brackish in composition and is characterized by Na⁺ > Mg²⁺ > Ca²⁺ > K⁺ and SO₄ ^2? > HCO₃ ^? > Cl^? > NO₃ ^?. Spearman’s rank correlation coefficient matrices, factor analysis data, and values of chloro-alkaline indices, C ratio and Na⁺/Cl^? molar ratio indicate that in the groundwater (A), the ionic load of Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄ ^2? and HCO₃ ^? is derived essentially from weathering of both carbonates and aluminosilicates and direct cation and reverse cation–anion exchange reactions. Groundwater (B) is the polluted variant of the groundwater (A), brackish to saline in composition, and unlike the groundwater (A), consists of HCO₃ ^? as the dominant anion. In comparison with the groundwater (A), the groundwater (B) contains higher concentrations of all ions, and its average ionic load (av. = 59.74 me/L) is 1.43 times higher than that of the groundwater (A) (av. = 41.54 me/L). Additional concentrations of Ca²⁺, Mg²⁺, K⁺, SO₄ ^2?, Cl^? and HCO₃ ^? in the groundwater (B) are provided mainly by downward infiltrating water from the coal washery tailings pond and reverse cation–anion exchange reaction between tailings pond water and exchanger of the aquifer matrix during non-conservative mixing process of groundwater (A) and tailings pond water. Certain additional concentrations of Na⁺, K⁺ and NO₃ ^? in the groundwater (B) are provided by other anthropogenic sources. Quality wise, both groundwaters are marginally suitable for cultivation of salt-tolerant crops only.     

Cadmium (Cd) concentration in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) grown in Cd-spiked soil varies with the doses and biochar feedstock

Biochar is considered a promising amendment for the reduction of metal concentration in plants; however, the effects of biochar in terms of dose and feedstock on metal uptake by plants remain widely unclear. In the current study, three individual biochars were prepared at 450 °C from different feedstocks (wheat straw, sukh chain (Pongamia pinnata), and cotton sticks). The main aim was to evaluate their ability to remediate cadmium (Cd)-spiked soil in terms of growth response and Cd uptake by wheat (Triticum aestivum) tissues. Biochars were separately applied at 0, 1, and 2% (w/w) in Cd-spiked soil and wheat was grown until maturity in pots and then morphological and physiological parameters and Cd concentrations in grains, roots, and shoots were determined. The post-harvest soil was analyzed for extractable Cd concentrations. Plants grown in Cd-spiked soil treated with biochars had higher seed germination, lengths of roots, shoots, and spikes, grains per spike and leaf relative water contents, chlorophyll contents, and dry weight of roots, shoots, and grains as compared to the untreated control. Biochar treatments significantly decreased the Cd concentrations in shoots, roots, and grains as well as total Cd uptake by grains. Soil extractable Cd concentrations were significantly decreased with biochar treatments. The application of 2.0% wheat straw biochar was the most efficient treatment in increasing grain yield and decreasing Cd in grains as well as soil extractable Cd than the other two biochars and doses applied.     

National scale evaluation of groundwater chemistry in Korea coastal aquifers: evidences of seawater intrusion

Pollution of groundwater by seawater intrusion poses a threat to sustainable agriculture in the coastal areas of Korea. Therefore, seawater intrusion monitoring stations were installed in eastern, western, and southern coastal areas and have been operated since 1998. In this study, groundwater chemistry data obtained from the seawater intrusion monitoring stations during the period from 2007 to 2009 were analyzed and evaluated. Groundwater was classified into fresh (<1,500 μS/cm), brackish (1,500–3,000 μS/cm), and saline (>3,000 μS/cm) according to EC levels. Among groundwater samples (n = 233), 56, 7, and 37% were classified as the fresh, brackish, and saline, respectively. The major dissolved components of the brackish and saline groundwaters were enriched compared with those of the fresh groundwater. The enrichment of Na⁺ and Cl⁻ was especially noticeable due to seawater intrusion. Thus, the brackish and saline groundwaters were classified as Ca–Cl and Na–Cl types, while the fresh groundwater was classified as Na–HCO₃ and Ca–HCO₃ types. The groundwater included in the Na–Cl types indicated the effects of seawater mixing. Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄ ²⁻, and Br⁻ showed good correlations with Cl⁻ of over r = 0.624. Of these components, the strong correlations of Mg²⁺, SO₄ ²⁻, and Br⁻ with Cl⁻ (r ≥ 0.823) indicated a distinct mixing between fresh groundwater and seawater. The Ca/Cl and HCO₃/Cl ratios of the groundwaters gradually decreased and approached those of seawater. The Mg/Cl, Na/Cl, K/Cl, SO₄/Cl, and Br/Cl ratios of the groundwaters gradually decreased, and were similar to or lower than those of seawater, indicating that Mg²⁺, Na⁺, K⁺, SO₄ ²⁻, and Br⁻, as well as Cl⁻ in the saline groundwater can be enriched by seawater mixing, while Ca²⁺ and HCO₃ ⁻ are mainly released by weathering processes. The influence of seawater intrusion was evaluated using threshold values of Cl⁻ and Br⁻, which were estimated as 80.5 and 0.54 mg/L, respectively. According to these criteria, 41–50% of the groundwaters were affected by seawater mixing.     

Geochemical assessment of groundwater around Macherla-Karempudi area,Guntur district,Andhra Pradesh

Groundwater in Palnad sub-basin is alkaline in nature and Na⁺-Cl⁻-HCO₃⁻ type around Macherla-Karempudi area in Guntur district, Andhra Pradesh. Total dissolved solids (TDS) show strong positive correlation with Cl⁻, Na⁺, Ca²⁺ and Mg²⁺, and positive correlation with SO₄²⁻, K⁺ and HCO₃⁻. Calcareous Narji Formation is the dominant aquifer lithology, and water-rock interaction controls the groundwater chemistry of the area. Chloro-alkaline indices (CAI) are positive at Miriyala, Adigopula, Mutukuru, Macherla and Durgi suggesting replacement of Na⁺ and K⁺ ions from water by Mg⁺⁺ and Ca⁺⁺ ions from country rock through base exchange reactions. Negative CAI values are recorded at Terala, Rayavaram and Nehrunagar, which indicate exchange of Na⁺ and K⁺ from the rock as cation-anion exchange reaction (chloro-alkaline disequilibrium). TDS range from 91 to 7100 ppm (Avg. 835 ppm) and exceed the prescribed limit of drinking water around Mutukuru, Durgi, Rayavaram, Khambampadu and Ammanizamalmadaka areas. Scanty rainfall and insufficient groundwater recharge are the prime factors responsible for high salinity in the area. Fluoride content ranges from <1 to 3.8 ppm and contaminated areas were identified around Macherla (1 sq km; 3.8ppm), Mandadi (1 sq km, 2.1ppm) and Adigopula (2 sq km, <1 to 3.7 ppm). The % Na⁺ content varies from 17 to 85 with the mean value of 57, and eighty (80) samples showed higher %Na⁺ in comparison to the prescribed limit of 60 for irrigation water. Sodium Adsorption Ratio (SAR) and % Na⁺ in relation to total salt concentration indicate that groundwater (51%) mostly falls under doubtful to poor quality for irrigation purpose. Groundwater of Adigopula village is fluoride contaminated and remedial measures are suggested to improve the water quality.     

Hydrochemical characteristics and quality assessment of regolith aquifers in Enugu metropolis, southeastern Nigeria

Twenty groundwater samples were collected from Enugu metropolis over two seasonal periods in order to characterize the groundwater and to determine its quality for domestic and irrigation purposes. The results show that groundwater of the area is strongly acidic to slightly alkaline in nature and varied from “soft water” to “moderately hard” water type. The major ionic trend is in the order Cl^? > Na⁺ > HCO₃ ^? > K⁺ > Mg²⁺ > Ca²⁺ > SO₄ ^2?and Mg²⁺ > Cl^? > Na⁺ > K⁺ > Ca²⁺ > HCO ₃ ^?  > SO₄ ^2? in abundance for dry and rainy seasons, respectively. The results also reveal that there is an increase in trend of the ionic concentrations during the dry season, which arises from weathering of the host rocks and anthropogenic activities. Two hydrochemical facies were identified, namely, Na⁺ –K⁺ –Cl^? –SO₄ ^2?and Ca²⁺ –Mg²⁺ –Cl^? –SO₄ ^2? _, with Na⁺ –K⁺ –Cl^? –SO₄ ^2? as the dominant facies for the two seasons. Groundwater quality ranges from “very poor water” to “good water” and “water unsuitable for drinking purposes” to “good water” for the dry season and rainy season investigations, respectively. The groundwater is suitable for irrigation purposes for the two seasons.     

Experimental Na/K exchange between alkali feldspar and an NaCl–KCl salt melt: chemically induced fracturing and element partitioning

The exchange of Na⁺ and K⁺ between alkali feldspar and a NaCl–KCl salt melt has been investigated experimentally. Run conditions were at ambient pressure and 850 °C as well as 1,000 °C. Cation exchange occurred by interdiffusion of Na⁺ and K⁺ on the feldspar sub-lattice, while the Si–Al framework remained unaffected. Due to the compositional dependence of the lattice parameters compositional heterogeneities resulting from Na⁺/K⁺ interdiffusion induced coherency stress and associated fracturing. Depending on the sense of chemical shift, different crack patterns developed. For the geometrically most regular case that developed when potassic alkali feldspar was shifted toward more sodium-rich compositions, a prominent set of cracks corresponding to tension cracks opened perpendicular to the direction of maximum tensile stress and did not follow any of the feldspar cleavage planes. The critical stress needed to initiate fracturing in a general direction of the feldspar lattice was estimated at ≤0.35 GPa. Fracturing provided fast pathways for penetration of salt melt or vapor into grain interiors enhancing overall cation exchange. The Na/K partitioning between feldspar and the salt melt attained equilibrium values in the exchanged portions of the grains allowing for extraction of the alkali feldspar mixing properties.     

Origin and distribution of saline groundwaters in the upper Miocene aquifer system, coastal Rhodope area, northeastern Greece

 This paper describes the origins and distribution of saline groundwaters in the coastal area of Rhodope, Greece. The aquifer system includes two aquifers within coarse-grained alluvial sediments in the coastal part of the study area. Two major water-quality groups occur in the study area, namely Ca²⁺-rich saline groundwater and Ca²⁺-poor, almost fresh groundwater. The main process controlling the groundwater chemistry is the exchange of calcium and sodium between the aquifer matrix and intruding seawater. The natural salt water in the study area is probably residual water that infiltrated the aquifer system during repeated marine transgressions in late Pleistocene time. Seawater intrusion into the coastal aquifer system occurs as a result of overpumping in two seawater wedges separated vertically by a low-permeability layer. The rate of intrusion averages 0.8 m/d and is less than expected due to a decline of the aquifer's permeability at the interface with the seawater. The application of several hydrochemical techniques (Piper and Durov diagrams; Na⁺/Cl^–, Ca²⁺/Cl^–, Mg²⁺/Cl^–, and Br^–/Cl^– molar ratios; Ca²⁺/Mg²⁺ weight ratio; and chloride concentrations), combined with field observations, may lead to a better explanation of the origin of the saline groundwater. Received, May 1997 / Revised, May 1998, December 1998 / Accepted, February 1999     

Theoretical modeling of the Al paramagnetic center and its precursors in stishovite

Previous electron paramagnetic resonance (EPR) spectroscopic study of gamma-ray-irradiated stishovite at 77 K detected an Al hole center, which was proposed to be an [O₂ ³⁻–Al³⁺] defect. First-principles quantum-mechanical calculations show that the unpaired spin is 85% localized on one of the six oxygen atoms at an AlO₆ octahedron, while the calculated ²⁷Al hyperfine constants are similar to those determined by EPR experiments. Theoretical results allow us to propose the Al center to represent an [AlO₆]⁰ defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical calculations also show that the diamagnetic precursors [AlO₆/H⁺]⁰, [AlO₆/Li⁺]⁰, and [AlO₆/Na⁺]⁰ are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred from FTIR spectra and previous theoretical calculations. The calculated [AlO₆/Li⁺]⁰ and [AlO₆/Na⁺]⁰ defects suggest that the monovalent cations such as Li⁺ and Na⁺ are potentially important in accommodating Al in stishovite in the lower mantle.     

Groundwater chemical and fecal contamination assessment of the Jerba unconfined aquifer,southeast of Tunisia

Located in the southeast of Tunisia, on the Mediterranean Sea, Jerba Island has a semiarid climate condition. The surface water scarcity has made groundwater the main source to supply the domestic, touristic, and agricultural water demand. Unconfined aquifer is a vulnerable costal aquifer system that undergoes several phenomena. This work aims at assessing the geochemical and bacteriological groundwater quality, defining groundwater pollution sources and promoting sustainable development and effective management of groundwater resources in Jerba Island. Data were collected after the wet season in 2014 from 79 wells. Electric conductivity, pH, TDS, and major and fecal tracers (total coliforms, thermotolerant coliforms, Escherichia coli, and Salmonella) were analyzed. Geochemical modeling including the relationships between geochemical tracers Na⁺ vs. Cl^?, Ca²⁺ vs. Cl^?, K⁺ vs. Cl^?, representative ionic ratios (Br^?/Cl^?, Na⁺/Cl^?, Mg²⁺/Ca²⁺), and statistical analysis were used to specify major process contributing to groundwater pollution and main factors controlling groundwater mineralization in the island. Groundwater varieties were hydrochemically classified into three types in terms of salinity values: group 1 (8.86%) to fresh water, group 2 (27.84%) to brackish water, and group 3 (63.29%) belongs to saline water. In addition, groundwater quality revealed high concentrations in chemical pollution tracers (Na⁺, Cl^?, SO₄ ^2?, and NO₃ ^?) and fecal tracers. Besides, most of the sampled wells were contaminated with nitrate (50.63%). Also, thermotolerant coliforms and E. coli were detected in all groundwater samples (96.2% of wells). Results indicated that the Jerba shallow aquifer was under serious threat from both natural and anthropogenic contamination. However, the wild discharge of domestic effluents, septic tanks, and sewage were the main origins of underground water contamination in Jerba Island. The reduction of fecal sources, through constructing normalized latrines is thus recommended.     

Multicomponent diffusion of a suite of tracers (HTO, Cl, Br, I, Na, Sr, Cs) in a single sample of Opalinus Clay

Diffusion experiments with HTO, ³⁶Cl⁻, Br⁻, I⁻, ²²Na⁺, ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ at trace concentrations in a single sample of Opalinus Clay are modeled with PHREEQC’s multicomponent diffusion module. The model is used first in a classical approach to derive accessible porosities, geometrical factors (the ratio of pore tortuosity and constrictivity) and sorption behavior of the individual tracers assuming that the clay is homogeneous. The accessible porosity for neutral species and cations is obtained from HTO, the anion exclusion volume from ³⁶Cl⁻ and Br⁻, and the cation exchange capacity from ²²Na⁺. The homogeneous model works well for tritium, the anions and ²²Na⁺. However, the ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ experiments show an early arrival of the tracer and a front-form that suggest a dual porosity structure. A model with 10% dead-end pores, containing 19% of the total exchange capacity, can satisfactorily calculate all the experimental data. The Cs⁺ diffusion model builds on a 3-site exchange model, constructed from batch sorption data. The excellent agreement of modeled and measured data contradicts earlier reports that the exchange capacity for Cs⁺ would be smaller in diffusion than in batch experiments.The geometrical factors for the anions are 1.5 times larger than for HTO, and for the cations 2-4 times smaller than for HTO. The different behavior is explained by a tripartite division of the porespace in free porewater, diffuse double layer (DDL) water, and interlayer water in montmorillonite. Differences between estimated and observed geometrical factors for cations are attributed to increased ion-pairing of the divalent cations in DDL water as a result of the low relative dielectric permittivity. Interlayer and/or surface diffusion contributes significantly to the diffusive flux of Cs⁺ but is negligible for the other solutes. The geometrical factors for anions are higher than estimated, because pore constrictions with overlapping double layers force the anions to take longer routes than HTO and the cations. Small differences among the anions can also be attributed to different ion-pairing in DDL water.     

Ion balance,acid-base regulation,and chloride cell function in the common killifish,Fundulus heteroclitus—a euryhaline estuarine teleost

The common killifish,Fundulus heteroclitus, is a euryhaline teleost common throughout estuaries of eastern North America. This symposium paper reviews the important contributions of the killifish to our present understanding of ionoregulation in seawater (SW) fish and their mechanisms of euryhalinity, and presents new data developing the killifish as a freshwater (FW) model system. Experiments on killifish have characterized (i) drinking in SW and its reduction in FW; (ii) the adaptive roles of the kidney to SW and FW conditions; (iii) the instantaneous (Phase I) and delayed (Phase II) reductions in Na⁺ outflux that occur upon transfer from SW to FW; (iv) the importance of prolactin secretion in the Phase II effect; (v) the cortisol-stimulated induction of branchial Na⁺, K⁺-ATPase that occurs upon transfer from FW to SW; (vi) the accompanying changes in morphology of the mitochondria-rich (MR) or “chloride cells” on the gills; (vii) the localization of this Na⁺, K⁺-ATPase activity to the basolateral membrane of chloride cells; and (viii) the NaCl-secretory function of these cells in SW. The opercular epithelium, which is rich in chloride cells, has been used as an in vitro model to characterize the mechanisms and control of NaCl secretion in SW fish. Much less is known about gill function in fresh water (inward NaCl transport), primarily due to the absence of a comparable freshwater model. Here we show that killifish acclimated to dilute FW ([NaCl] = 1 mmol I^?1) possess large numbers of MR cells on the opercular epithelium. When mounted in vitro with FW on the outside, the preparation develops a large inside negative transepithelial potential (TEP) that is a Na⁺ diffusion potential. By the Ussing flux ratio criterion, Na⁺ fluxes are passive, but a small active influx of Cl^? occurs, an observation that supports the involvement of MR cells in active Cl^? uptake. This FW opercular epithelium if bathed with isotonic saline on both sides does not secrete Cl^?, indicating that the MR cells indeed are of the FW type. In vivo, the fish exhibits a high rate of Na⁺ influx and outflux; Cl^? outflux is much lower, and there is no detectable Cl^? influx. Experimental variation of FW [NaCl] reveals a saturable, low affinity Na⁺ uptake mechanism, a Cl^? influx mechanism that is activated only at much higher concentrations, and no evidence of exchange diffusion. Acid-base disturbance appears to be corrected by differential regulation of the outflux components only. Hence, the FW killifish ionoregulates somewhat differently from the few other FW teleosts that have been examined, and its opercular epithelium will serve as a very useful model system.     

Ultrafiltration of fulvic and humic acids,a comparison of stirred cell and hollow fiber techniques

The ultrafiltration of solutions of well characterized samples of fulvic and humic acids or their salts, using two different techniques, show that these materials are retained by commercial ultrafiltration membranes with molecular weight cut-off values far higher than the reported molecular weights of fulvic or humic acid. The lower molecular weight fulvic acid or its Na⁺ -salt is retained even better than the higher molecular weight Na⁺-salt of humic acid. Both stirred cell and hollow fiber techniques can be employed to concentrate or desalt solutions of these humic materials, with relatively small solute losses. Very high simple electrolyte backgrounds (above 1 M) lower the performance of a given membrane considerably, moderate concentrations of simple electrolyte do not influence the ultrafiltration efficiency.     

Geochemistry and quality of groundwater of Gummanampadu sub-basin, Guntur District, Andhra Pradesh, India

Groundwater survey has been carried out in the area of Gummanampadu sub-basin located in Guntur District, Andhra Pradesh, India for assessing the factors that are responsible for changing of groundwater chemistry and consequent deterioration of groundwater quality, where the groundwater is a prime source for drinking and irrigation due to non-availability of surface water in time. The area is underlain by the Archaean Gneissic Complex, over which the Proterozoic Cumbhum rocks occur. The results of the plotting of Ca²⁺ + Mg²⁺ versus HCO₃ ^? + CO₃ ^2?, Ca²⁺ + Mg²⁺ versus total cations, Na⁺ + K⁺ versus total cations, Cl^? + SO₄ ^2? versus Na⁺ + K⁺, Na⁺ versus Cl^?, Na⁺ versus HCO₃ ^? + CO₃ ^2?, Na⁺ versus Ca²⁺ and Na⁺: Cl^? versus EC indicate that the rock–water interaction under alkaline condition is the main mechanism in activating mineral dissociation and dissolution, causing the release of Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃ ^?, CO₃ ^2?, SO₄ ^2? and F^? ions into the groundwater. The ionic relations also suggest that the higher concentrations of Na⁺ and Cl^? ions are the results of ion exchange and evaporation. The influences of anthropogenic sources are the other cause for increasing of Mg²⁺, Na⁺, Cl^?, SO₄ ^2? and NO₃ ^? ions. Further, the excess alkaline condition in water accelerates more effective dissolution of F^?-bearing minerals. Moreover, the chemical data plotted in the Piper’s, Gibbs’s and Langelier–Ludwig’s diagrams, computed for the chloro-alkaline and saturation indices, and analyzed in the principal component analysis, support the above hypothesis. The groundwater quality is, thus, characterized by Na⁺ > Ca²⁺ > Mg²⁺ > K⁺: HCO₃ ^? + CO₃ ^2? > Cl^? > SO₄ ^2? > NO₃ ^? > F^? facies. On the other hand, majority of groundwater samples are not suitable for drinking with reference to the concentrations of TDS, TH, Mg²⁺ and F^?, while those are not good for irrigation with respect to USSL’s and Wilcox’s diagrams, residual sodium carbonate, and magnesium hazard, but they are safe for irrigation with respect to permeability index. Thus, the study recommends suitable management measures to improve health conditions as well as to increase agricultural output.     

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.     

Enrichment of fluoride in groundwater under the impact of saline water intrusion at the salt lake area of Yuncheng basin,northern China

Long-term intake of high-fluoride groundwater causes endemic fluorosis. This study, for the first time, discovered that the salt lake water intrusion into neighboring shallow aquifers might result in elevation of fluoride content of the groundwater. Two cross-sections along the groundwater flow paths were selected to study the geochemical processes controlling fluoride concentration in Yuncheng basin, northern China. There are two major reasons for the observed elevation of fluoride content: one is the direct contribution of the saline water; the other is the undersaturation of the groundwater with respect to fluorite due to salt water intrusion, which appears to be more important reason. The processes of the fluorine activity reduction and the change of Na/Ca ratio in groundwater induced by the intrusion of saline water favor further dissolution of fluorine-bearing mineral, and it was modeled using PHREEQC. With the increase in Na concentration (by adding NaCl or Na₂SO₄ as Na source, calcium content kept invariable), the increase of NaF concentration was rapid at first and then became slower; and the concentrations of HF, HF₂⁻, CaF⁺, and MgF⁺ were continuously decreasing. The geochemical conditions in the study area are advantageous to the complexation of F⁻ with Na⁺ and the decline of saturation index of CaF₂, regardless of the water type (Cl–Na or SO₄–Na type water).