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Aquatic Geochemistry (v.14, #4)

The Equation of State for Caspian Sea Waters by Frank J. Millero; Abzar Mirzaliyev; Javid Safarov; Fen Huang; Mareva Chanson; Astan Shahverdiyev; Egon Hassel (pp. 289-299).

The density ρ of Caspian Sea waters was measured as a function of temperature (273.15–343.15) K at conductivity salinities of 7.8 and 11.3 using the Anton-Paar Densitometer. Measurements were also made on one of the samples (S = 11.38) diluted with water as a function of temperature (T = 273.15–338.15 K) and salinity (2.5–11.3). These latter results have been used to develop an equation of state for the Caspian Sea (σ = ±0.007 kg m⁻³) $$ left( { ho - ho ^{0} } ight),{ ext{kg m}}^{{-3}} = A + BS + CS^{2} $$ where ρ⁰ is the density of water and the parameters A, B and C are given by $$egin{aligned} A &= 0.0350 - 1.73{ ext{E-}}03left( {T - 273.15} ight) + 5.2{ ext{E-}}05left( {T - 273.15} ight)^{2}\ &quad- 4.947 { ext{E-}}07left( {T - 275.15} ight)^{3}end{aligned} $$ $$egin{aligned} B &= 0.9877 - 3.66{ ext{E-}}03left( {T - 273.15} ight) + 4.903{ ext{E-}}05left( {T - 273.15} ight)^{2}\ &quad - 2.276{ ext{E-}}07left( {T - 273.15} ight)^{3}end{aligned} $$ $$egin{aligned} C = - 1.25{ ext{E-}}04 end{aligned}$$ Measurements of the density of artificial Caspian Sea water at 298.15 K agree to ± 0.012 kg m⁻³ with the real samples. These results indicate that the composition of Caspian Sea waters must be close to earlier measurements of the major components. Model calculations based on this composition yield densities that agree with the measured values to ± 0.012 kg m⁻³. The new density measurements are higher than earlier measurements. This may be related to a higher concentration of dissolved organic carbon found in the present samples (500 μM) which is much higher than the values in ocean waters (~65 μM).

Keywords: Equation of state; Caspian Sea waters

Fluid Geochemistry as Indicator of Tectonically-Related, Deep Water Circulations in the Sardinian Rift-Campidano Graben (Italy): New Insights from Environmental Isotopes by Elisa Sacchi; Gian Maria Zuppi; Luca Pizzino; Fedora Quattrocchi; Salvatore Lombardi (pp. 301-319).

The EC funded Geochemical Seismic Zonation program (EEC GSZ Project 1996–1998) chose Sardinia as a low-seismicity site, in which the relationships between fluid geochemistry and seismo-tectonics had to be investigated and results compared with outcomes from other selected high-seismicity sites. A first article, examining the role of fault segmentation and seismic quiescence on the geochemical composition of groundwaters and gases, has already been presented (Angelone et al. 2005). This article deals with environmental isotopes which, together with selected hydrochemical data, give hints on tectonically-related fluid circulations. Four water-dominated hydrothermal systems were considered, all located along regional fault systems and discharging groundwaters belonging to the Na–HCO₃ and Na–Cl facies. In the considered systems, groundwater circulation takes place, principally, in the Palaeozoic Crystalline Basement (PCB), with the exception of the Logudoro system, where hydrological circuits develop in the Mesozoic Carbonate Platform (MCP). The high CO₂ contents, the non-attainment of fluid-rock equilibrium and the large lithological variability prevent the construction of a unique hydrogeological–geochemical conceptual model. In this case, stable isotopes provide a useful tool to describe the origin of fluids and their subterranean movements. Stable isotopes of water, integrated with hydrochemical data, indicate that fluids are derived from three main end members. The dominant component is a relatively recent local meteoric water; the second one is marine water; and the third one is a fossil freshwater, depleted in heavy isotopes with respect to modern rains. The latter end member entered the aquifer system in the past, when climatic conditions were greatly different from today. At least two circulation systems can be recognised, namely a shallow cold system and a deep hydrothermal system, as well as two distinct hydrological processes: (1) gravity-controlled descent of cold water towards greater depths and (2) convection linked to a thermal gradient, causing deep fluids to rise up from the hydrothermal reservoir towards the surface. The highly variable δ¹³C_TDIC values suggest the presence of two distinct CO₂ sources, namely, a biogenic one and a thermogenic one. The relation between the isotopic compositions of CO₂ and He indicates an increased mantle signature in uprising CO₂-rich fluids.

Keywords: Sardinian Rift; Campidano Graben; Fluid geochemistry; Environmental isotopes

Major Ion Geochemistry of Nam Co Lake and its Sources, Tibetan Plateau by Qianggong Zhang; Shichang Kang; Feiyue Wang; Chaoliu Li; Yanwei Xu (pp. 321-336).

The major cations and anions from lake water samples and its sources, including glacier snow, precipitation, stream, and swamp water in the Nam Co basin, central Tibetan Plateau, were studied. The concentrations of the major ions varied significantly in the five environmental matrices. Generally, the mean concentrations of most ions are in the order of lake water > swamp water > stream water > precipitation > snow. Rock weathering is the dominant process controlling the chemical compositions of the stream and swamp waters, with carbonate weathering being the primary source of the dissolved ions. The Nam Co lake water is characterized by high Na⁺ concentration and extremely low Ca²⁺ concentration relative to other ions, resulting from evapoconcentration and chemical precipitation within the lake. Comparison with the water chemistry of other lakes over the Tibetan Plateau indicated that Nam Co is located in a transition area between non-saline lakes and highly saline lakes. The relatively low concentration of total dissolved solids is possibly due to the abundant inflow of glacial meltwater and relatively high annual precipitation.

Keywords: Nam Co; Tibetan Plateau; Alpine lakes; Major ion geochemistry; Water; Weathering

The Chemical Speciation of Fe(III) in Freshwaters by Stephen Lofts; Edward Tipping; John Hamilton-Taylor (pp. 337-358).

Dialysis and chemical speciation modelling have been used to calculate activities of Fe³⁺ for a range of UK surface waters of varying chemistry (pH 4.3–8.0; dissolved organic carbon 1.7–40.3 mg l⁻¹) at 283 K. The resulting activities were regressed against pH to give the empirical model: $$ log ,a_{{{ ext{Fe}}^{3 + } }} = 2.93left( { pm 0.40} ight) - 2.70left( { pm 0.06} ight) cdot { ext{pH}} $$ . Predicted Fe³⁺ activities are consistent with a solid–solution equilibrium with hydrous ferric oxide, consistent with some previous studies on Fe(III) solubility in the laboratory. However, as has also sometimes been observed in the laboratory, the slope of the solubility equation is lower than the theoretical value of 3. The empirical model was used to predict concentrations of Fe in dialysates and ultrafiltrates of globally distributed surface and soil/groundwaters. The predictions were improved greatly by the incorporation of a temperature correction for $$ a_{{{ ext{Fe}}^{{ 3 + }} }} $$ , consistent with the temperature dependence of previously reported hydrous ferric oxide solubility. The empirical model, incorporating temperature effects, may be used to make generic predictions of the ratio of free and complexed Fe(III) to dissolved organic matter in freshwaters. Comparison of such ratios with observed Fe:dissolved organic matter ratios allows an assessment to be made of the amounts of Fe present as Fe(II) or colloidal Fe(III), where no separate measurements have been made.

Keywords: Iron; Speciation; Solubility; Freshwater; Dialysis; Ultrafiltration

Shrinking Sphere Kinetics for Batch Dissolution of Mixed Particles of a Single Substance at High Under-Saturation: Validation with Sodium Chloride, but with Biogenic Silica in Mind by Victor W. Truesdale (pp. 359-379).

The cubic equation recently derived for the increase in concentration of a solute with time, as the solid dissolves in batch according to the shrinking sphere model at high under-saturation, is extended to dissolutions of mixtures of differently sized particles. This problem needs to be solved if batch dissolutions are to play their part in the proposed amelioration of global warming and associated climate change by accelerated ‘re-burial’ of excess CO₂ in ocean sediment. The upgraded model was tested using sodium chloride dissolved in 50% aqueous propanone, whence the model fitted two separate runs with 500 and 212 μm, and 212 and 38 μm, diameter crystals, respectively. The key to simulating dissolution in this way lies in the dissolutions being independent of each other. It is further shown that although this condition was implicit in the recent derivation of the cubic equation, it was not recognised at the time. The work should be applicable to any batch dissolution of mixed particles at high under-saturation, and hence, may find use in many industrial and laboratory dissolutions. Simulations show how agglomerated mixtures can yield a straight line on the plot of ln(1 − C/C _T) versus time, as was reported to occur recently with sodium chloride taken ‘straight from the bottle’. It is shown that this probably explains why exponential dissolutions may have seemed appropriate to the dissolution of biogenic silica in earlier literature. This study suggests that a new round of biogenic silica dissolutions, but with sieved samples, would be worthwhile, with the likelihood that shrinking sphere behaviour might well be found to characterise the kinetics. The opportunity is taken to investigate a number of aspects of the shrinking sphere model not generally discussed before, e.g. the graph for the change in surface area with time. The limitations of using cubic salt crystals with the shrinking sphere model are discussed.

Keywords: Batch dissolution; Biogenic silica; Dissolution kinetics; Mineral dissolution; Salt dissolution; Silica cycling; Shrinking sphere model; Dissolution modelling; Atmospheric-CO₂ sequestration; Kinetics; Carbonate dissolution

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Aquatic Geochemistry (v.14, #4)

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Aquatic Geochemistry (v.14, #4)