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Aquatic Geochemistry (v.14, #3)

Analytical Electron-Microscopy Fractionation of Fine and Colloidal Particulate-Phosphorus in Riverbed and Suspended Sediments by Jérôme Poulenard; Jean-Marcel Dorioz; Françoise Elsass (pp. 193-210).

The impact particulate-phosphorus (particulate-P) has on eutrophication of aquatic systems that greatly depends on its composition. As a result, analysis methods for evaluating particulate-P speciation must be capable of identifying and/or quantifying the wide range of forms particulate-P can take. In the present study, we compare the particulate P speciation of the fine and colloidal fractions of riverbed sediment and suspended matter from two rivers in the Lake Geneva basin (French Alps) as determined with chemical extractions to results of a combined Transmission Electron Microscopy and Energy Dispersive Detection (TEM–EDS) analysis of the same samples. TEM–EDS provides semi-quantitative information about the distribution of P throughout the solid fraction of a sample and on the diversity of carrier phases, which are identified by their morphology and stochiometry. EDS-detectable quantities of phosphorus were found in 15–35% of the particles in the samples analyzed. As expected, particulate-P existed in a wide variety of forms, mostly associated with Al, Fe, Ca and Si. Some types of particles, often well-crystallized phosphate minerals, had high P contents (10–30%), but the dominant carrier phases of P were diffuse matrices of particles with low P contents (<5%). These matrices had a wide range of chemical compositions and included clay minerals, crystallized and amorphous phases of Fe, as well as intermediary components. Classical chemical extractions showed major differences in P content and speciation between the upstream and downstream sediment samples. The downstream samples showed higher frequencies of particles containing P, a larger diversity of carrier phases, a higher contribution of clays as carrier phases of P, and the presence of a new fraction with Al and Fe amorphous minerals. The processes that create and select carrier phases of particulate-P are part of the general dynamics of P within a watershed.

Keywords: River sediments; Suspended sediments; Phosphorus; Particulate phosphorus; Point source pollution; TEM

Stable Carbon Isotope Biogeochemistry and Anthropogenic Impacts on Karst Ground Water, Zunyi, Southwest China by Si-Liang Li; Cong-Qiang Liu; Yun-Chao Lang; Faxiang Tao; Zhiqi Zhao; Zhihua Zhou (pp. 211-221).

Natural and anthropogenic impacts on karst ground water, Zunyi, Southwest China, are discussed using the stable isotope composition of dissolved inorganic carbon and particulate organic carbon, together with carbon species contents and water chemistry. The waters can be mainly characterized as HCO₃–Ca type, HCO₃ · SO₄–Ca type, or HCO₃ · SO₄–Ca · Mg type, according to mass balance considerations. It is found that the average δ¹³C_DIC values of ground waters are higher in winter (low-flow season) than in summer (high-flow season). Lower contents of dissolved inorganic carbon (DIC) and lower values of δ¹³C_DIC in summer than in winter, indicate that local rain events in summer and a longer residence time of water in winter play an important role in the evolution of ground water carbon in karst flow systems; therefore, soil CO₂ makes a larger contribution to the DIC in summer than in winter. The range of δ¹³C_DIC values indicate that dissolved inorganic carbon is mainly controlled by the rate of carbonate dissolution. The concentrations of dissolved organic carbon and particulate organic carbon in most ground water samples are lower than 2.0 mg C L⁻¹ and 0.5 mg C L⁻¹, respectively, but some waters have slightly higher contents of organic carbon. The waters with high organic carbon contents are generally located in the urban area where lower δ¹³C_DIC values suggest that urbanization has had an effect on the ground water biogeochemistry and might threaten the water quality.

Keywords: Stable carbon isotope; Ground water; Carbonate weathering; Anthropogenic impacts

Thermodynamic Properties of Brucite Determined by Solubility Studies and Their Significance to Nuclear Waste Isolation by Yongliang Xiong (pp. 223-238).

Solubility experiments were conducted for the dissolution reaction of brucite, Mg(OH)₂ (cr): $$ { ext{Mg}}left( {{ ext{OH}}} ight)_{ 2} left( {{ ext{cr}}} ight) + 2 { ext{H}}^{ + } = { ext{Mg}}^{{ 2+ }} + 2 { ext{H}}_{ 2} { ext{O.}}$$ Experiments were conducted from undersaturation in deionized (DI) water and 0.010–4.4 m NaCl solutions at 22.5°C. In addition, brucite solubility was measured from supersaturation in an experiment in which brucite was precipitated via dropwise addition of 0.10 m NaOH into a 0.10 m MgCl₂ solution also at 22.5°C. The attainment of the reversal in equilibrium was demonstrated in this study. The solubility constant at 22.5°C at infinite dilution calculated from the experimental results from the direction of supersaturation by using the specific interaction theory (SIT) is: $$ log K_{s}^circ = 1 7. 2pm 0. 2 { ext{ }}left( { 2sigma } ight) $$ with a corresponding value of 17.0 ± 0.2 (2σ) when extrapolated to 25°C. The dimensionless standard chemical potential (μ°/RT) of brucite derived from the solubility data in 0.010 m to 4.4 m NaCl solutions from undersaturation extrapolated to 25°C is −335.76 ± 0.45 (2σ), with the corresponding Gibbs free energy of formation of brucite, $$ Updelta _{f} G_{298.15,{ ext{brucite}}}^{circ} $$ , being −832.3 ± 1.1 (2σ) kJ mol⁻¹. In combination with the auxiliary thermodynamic data, the $$ log K_{s}^{circ} $$ is calculated to be 17.1 ± 0.2 (2σ), based on the above Gibbs free energy of formation for brucite. This study recommends an average value of 17.05 ± 0.2 in logarithmic unit as solubility constant of brucite at 25°C, according to the values from both supersaturation and undersaturation.

Keywords: Solubility studies; Specific interaction theory (SIT); Pitzer equations; Solubility constants; Hydromagnesite; Nuclear waste isolation

Hydrogeochemical and Stable Isotope Characteristics of the River Idrijca (Slovenia), the Boundary Watershed Between the Adriatic and Black Seas by Tjaša Kanduč; David Kocman; Nives Ogrinc (pp. 239-262).

The hydrogeochemical and isotope characteristics of the River Idrijca, Slovenia, where the world’s second largest mercury (Hg) mine is located, were investigated. The River Idrijca, a typical steep mountain river, has an HCO₃ ⁻–Ca²⁺–Mg²⁺ chemical composition. Its Ca²⁺/Mg²⁺ molar ratio indicates that dolomite weathering prevails in the watershed. The River Idrijca and its tributaries are oversaturated with respect to calcite and dolomite. The pCO₂ pressure is up to 13 times over atmospheric pressure and represents a source of CO₂ to the atmosphere. δ¹⁸O values in river water indicate primary control from precipitation and enrichment of the heavy oxygen isotope of infiltrating water recharging the River Idrijca from its slopes. The δ¹³C_DIC values range from −10.8 to −6.6‰ and are controlled by biogeochemical processes in terrestrial environments and in the stream: (1) exchange with atmospheric CO₂, (2) degradation of organic matter, (3) dissolution of carbonates, and (4) tributaries. The contributions of these inputs were calculated according to steady state equations and are estimated to be—11%:19%:30%:61% in the autumn and 0%:26%:39%:35% in the spring sampling seasons.

Keywords: River Idrijca; Chemical composition; Stable isotopes; Carbon; Oxygen

Increasing Iron Concentrations in UK Upland Waters by C. Neal; S. Lofts; C. D. Evans; B. Reynolds; E. Tipping; M. Neal (pp. 263-288).

Iron distributions in rainfall, streams, soils and groundwaters are described for the Upper River Severn catchment of mid-Wales. Iron is mainly supplied from within-catchment sources with highest concentrations occurring under reducing conditions. Iron concentrations have doubled over the past 20 years (~5.0 μg yr⁻¹ for the forest and ~3.7 μg yr⁻¹ for the moorland). For the forested sites, the gradients are particularly high post-1993. UK rivers/lakes monitored by the UK Acid Waters Monitoring Network show similar increases. Generally, Fe correlates with dissolved organic carbon (DOC). The greatest rates of Fe increase coincide with those for DOC. Thermodynamic modelling using WHAM/Model VI indicates that Fe(III) is mainly in microparticulate form (probably oxyhydroxides) apart from under reducing conditions. It is proposed that Fe increases for surface waters are associated with increased microparticulate Fe(III) due to stabilisation against aggregation by binding of DOM to its surface. The results relate to acidification declines and deforestation leading to land disturbance and wetter conditions within the soil. There will be greater acidification reversal following tree harvesting due to lowering of atmospheric SO_x scavenging and this may have resulted in the greater increase in Fe in the later years of the study.

Keywords: Iron; River; Acidification; Colloids/nanoparticles; DOC; WHAM

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Aquatic Geochemistry (v.14, #3)

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Aquatic Geochemistry (v.14, #3)