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Aquatic Geochemistry (v.17, #2)
Aluminium in Lake Cuicocha, Ecuador, an Andean Crater Lake: Filterable, Gelatinous and Microcrystal Al Occurrence by G. Gunkel; C. Beulker; B. Grupe; U. Gernet; F. Viteri (pp. 109-127).
In Lake Cuicocha watershed, a young caldera lake, soils consist of volcanic deposits with a high SiO2 and Al2O3 content; these andisols are in an early stage of development; and in drainage water from the watershed, aluminium concentrations reach 15.0 μmol L−1. Total aluminium concentrations in Lake Cuicocha water raises up to 7.2 μmol L−1, with nearly 70% occurring as filterable Al at neutral to weak alkaline conditions. Al polymerization to gelatinous aggregates of a few hundred micrometres in diameter as well as the occurrence of Al microcrystals like gibbsite as an ageing product of gelatinous Al polymers was noted in the lake water. The gelatinous Al leads to the formation of larger aggregates resulting in flocs of bacteria, algae, microorganisms and detritus.
Keywords: Aluminium; Al polymerization; Gelatinous Al; Lake water; Mircocrystals; Water quality
A Brine Evolution Model and Mineralogy of Chemical Sediments in a Volcanic Crater, Lake Kitagata, Uganda by Lichun Ma; Tim K. Lowenstein; James M. Russell (pp. 129-140).
Lake Kitagata, Uganda, is a hypersaline crater lake with Na–SO4–Cl–HCO3–CO3 chemistry, high pH and relatively small amounts of SiO2. EQL/EVP, a brine evaporation equilibrium model (Risacher and Clement 2001), was used to model the major ion chemistry of the evolving brine and the order and masses of chemically precipitated sediments. Chemical sediments in a 1.6-m-long sediment core from Lake Kitagata occur as primary chemical mud (calcite, magadiite [NaSi7O13(OH)3·3H2O], burkeite [Na6(CO3)(SO4)2]) and as diagenetic intrasediment growths (mirabilite (Na2SO4·10H2O)). Predicted mineral assemblages formed by evaporative concentration were compared with those observed in salt crusts along the shoreline and in the core from the lake center. Most simulations match closely with observed natural assemblages. The dominant inflow water, groundwater, plays a significant role in driving the chemical evolution of Lake Kitagata water and mineral precipitation sequences. Simulated evaporation of Lake Kitagata waters cannot, however, explain the large masses of magadiite found in cores and the formation of burkeite earlier in the evaporation sequence than predicted. The masses and timing of formation of magadiite and burkeite may be explained by past groundwater inflow with higher alkalinity and SiO2 concentrations than exist today.
Keywords: Crater lakes; Alkaline lakes; Brine evolution; Mineralogy; Magadiite; Lake Kitagata
Rate Equations and an Ion-pair Mechanism for Batch Dissolution of Gypsum: Repositioning the Shrinking Object Model at the Core of Hydrodynamic Modelling by Victor W. Truesdale (pp. 141-164).
Recent improvements to experiments and modelling of batch dissolution in a turbulent reactor, based upon the shrinking object model, are extended to middle loadings of gypsum, that is, in the region between low and high loadings, which lead, respectively, to high under-saturation or saturation with a great excess of solid left undissolved. Dissolved calcium sulphate concentration was monitored by change in electrical conductivity. This investigation uses an improved, ion-pair model for CaSO 4 0 to allow for the presence of calcium or sulphate added as common ions. The study demonstrates that the full dissolution curve for 5.82 mM loadings of 106-μm particles of gypsum (~1.00 g L−1) in de-ionised water barely changed in the presence of either 4.64 or 8.09 mM calcium chloride, or 4.39 mM sodium sulphate. However, this masked a doubling of dissolution rate imposed by comparable increases in ionic strength from sodium chloride. The results are consistent with the ion pair, CaSO 4 0 , being the key species in the rate-determining step of the back-reaction, and perhaps all salt dissolutions, including calcium carbonate. In this case, the rate equation is as follows: $$ {frac{{{ ext{d}}c}}{{{ ext{d}}t}}} = frac{S}{V} cdot (k_{1} - k_{2}^{prime } cdot [{ ext{CaSO}}_{ 4}^{0} ]) $$ , where k 1 and k 2′ are rate constants. The reported observations are interpreted as effects of ionic strength and common ion concentrations upon the formation equilibrium for the ion pair. This rate equation readily transforms mathematically to one involving the product of [Ca2+] and [SO4 2−] in the back-reaction. The parallel of this with the well-known PWP equation used in calcium carbonate dissolution is discussed, with the CaHCO3 + ion pair of the equation being replaced by that of CaCO 3 0 . Meanwhile, the earlier use of the product, [Ca2+]½ × [CO3 2−]½, in the back-reaction term of another dissolution rate equation for calcite is shown to be incorrect. Finally, it is argued that the shrinking object model should be repositioned as a logical derivative of the hydrodynamical approach to dissolution.
Keywords: Batch dissolution; Dissolution kinetics; Mineral dissolution; Salt dissolution; Shrinking Object model; Ion-pair formation; Gypsum dissolution; Calcite dissolution; Dissolution mechanism
Hydrochemical Groundwater Evolution in the Bunter Sandstone Sequence of the Odenwald Mountain Range, Germany: A Laboratory and Field Study by Florian Ludwig; Ingrid Stober; Kurt Bucher (pp. 165-193).
Field and laboratory investigations were performed to identify the principal mechanisms of the hydrochemical groundwater evolution among low mineralised groundwater in the Triassic Bunter sandstone aquifer of the Odenwald low mountain range, central Germany. Hydrochemical composition comprises low pH, SO4-rich shallow groundwaters issued by springs (Ca-Mg-SO4-type) grading to SO4-poor deep groundwaters with near-neutral pH (Ca-HCO3-type). Batch experiments of the original rock were run to determine primary mineral alteration reactions and the origin of dissolved ions. Principal experimental reactions comprise the decomposition of anorthite, K-feldspar, biotite and jarosite as mineral components of the original sandstone rock and the formation of clay minerals of the smectite group (Ca-montmorillonite, beidellite), and iron hydroxides as secondary minerals. Mobilisation of fluid inclusion in quartz grains contributes to Na and Cl concentrations in the leachates. The evolution of deep groundwater circulation proceeds by mineral alteration reactions calculated by the inverse modelling of both primary and secondary minerals to produce low-T mineral phases. The dissolution of K-feldspar converts Ca-montmorillonite to illite (illitisation). The formation of Na-beidellite correlates with decreasing concentration of Na in solution. Mineral reactions further proceed to the formation of kaolinite as stable mineral phase. As indicated by modelled adsorption curves, the decrease of SO4 concentrations during groundwater evolution relates to the adsorption of SO4 on iron hydroxides. The leaching of calcite indicated for individual groundwaters relates to the distribution of loess in the appropriate catchment areas.
Keywords: Hydrochemical groundwater evolution; Mineral alteration reactions; Sulfate adsorption; Illitisation; Triassic Bunter sandstone sequence; Germany