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Aquatic Geochemistry (v.15, #3)

Redox Potential and Seasonal Porewater Biogeochemistry of Three Mountain Wetlands in Southeastern Kentucky, USA by Yvonne Thompson; Brian C. Sandefur; A. D. Karathanasis; Elisa D’Angelo (pp. 349-370).

Redox potentials (Eh) were monitored bimonthly and porewater chemistry was analyzed seasonally at three slightly-acidic, high-elevation Kentucky wetlands that differed in hydrology, parent materials, and vegetation. At all sites, Eh values were below 300 mV, which indicated that reducing conditions persisted within the upper 90 cm and fluctuated mainly within the range of iron and sulfate reduction. Significant relationships of Eh values with depth were observed only at the Martins Fork wetland, where precipitation was the primary water source. The strongest and most stable reducing conditions, observed at the Kentenia site, reflected consistently high water levels, which were sustained by ground water. The third wetland (Four Level) was distinguished by irregular Eh fluctuations coinciding with strong seasonal ground-water upwelling. Although Fe³⁺ and SO₄ ²⁻ were the primary terminal electron acceptors in all wetlands, porewater chemistry also varied significantly by season and soil depth in response to piezometric water level fluctuations. Additional factors that influenced porewater chemistry included: (1) the presence of limestone parent materials that affected porewater pH, Ca²⁺, and Mg²⁺; and (2) the prevalence of sphagnum moss or graminoid species that influenced dissolved organic carbon, CO₂, and CH₄. Results from this study indicated the diverse range and importance of multiple factors in controlling biogeochemical processes and properties in small, high-elevation Appalachian wetlands.

Keywords: Anoxic Appalachian seep; Dissolved organic carbon; Iron; Ground water; Hydrophytic community; Peepers; Porewater chemistry; Precipitation; Redox potential; Sulfate; Water levels

Characteristics and Influence of Phosphorus Accumulated in the Bed Sediments of a Stream Located in an Agricultural Watershed by Adam R. Hoffman; David E. Armstrong; Richard C. Lathrop; Michael R. Penn (pp. 371-389).

We investigated the accumulation and influence of bioavailable P (BAP) in sediments of a stream located in an agricultural area of the Lake Mendota watershed in Wisconsin, USA. During hydrologic events, the stream carried high concentrations of suspended sediment (up to 250 mg/l) and BAP (up to 2.5 mg/l). Bed sediments were highly enriched in BAP, as inventories of BAP in the top 10 cm of sediment ranged from 143 to 14,500 μg P/cm². Space variations in BAP inventories were related to site-specific hydrodynamics and geochemical factors, including iron (Fe; r ² = 0.71) and aluminum (Al; r ² = 0.54) concentrations. Most sites behaved as potential sinks for dissolved reactive phosphate during hydrologic events and potential sources during base-flow periods. Through the combination of site-specific factors and geochemical controls, Dorn Creek modifies the amount, timing, and composition of P delivered from the watershed to downstream sites and water bodies.

Keywords: Agricultural watersheds; Bioavailable phosphorus; Phosphorus; Sediment; Equilibrium phosphorus concentrations

Photoreduction of Fe(III) in the Acidic Mine Pit Lake of San Telmo (Iberian Pyrite Belt): Field and Experimental Work by M. Diez Ercilla; E. López Pamo; J. Sánchez España (pp. 391-419).

Light-induced reduction of dissolved and particulate Fe(III) has been observed to occur in the surface waters of the acidic mine pit lake of San Telmo (143,600 m², pH 2.8, Fe_total = 2.72 mM). This photochemical production of Fe(II) is directly related to the intensity of solar radiation and competes with biologically catalyzed reactions (i.e., bacterial re-oxidation of Fe(II)) and physical processes (including ionic diffusion, advection, and convection, which tend to homogenize the epilimnetic concentration of Fe(II) at every moment). Therefore, diel cycles of Fe(II) concentration are observed at the lake surface, with minimum values of 10–20 μM Fe(II) (0.35–0.70% Fe_total) at the sunrise and sunset, and maximum values of 90 μM Fe(II) (3.2% Fe_total) at midday in August 2005. Field and experimental work conducted in San Telmo and other pit lakes of the Iberian Pyrite Belt (IPB) (pH 2.3–3.1, Fe_total = 0.34–17 mM) indicate that the kinetics of the photoreductive reaction is zero-order and is independent of the Fe(III) concentration, but highly dependent on the intensity of solar radiation and temperature. Experimental work conducted with natural Fe(III) minerals (schwertmannite, goethite, and lepidocrocite) suggests that dissolved organic matter is an important factor contributing to the photochemical production of Fe(II). The wavelengths involved in the photoreduction of Fe(III) include not only the spectrum of UV-A radiation (315–400 nm), but also part of the photosynthetically active radiation (PAR, 400–700 nm). This finding is of prime importance for the understanding of the photoreduction processes in the pit lakes of the IPB, because the photo-reactive depth is not limited to the penetration depth of UV-A radiation (upper 1–10 cm of the water column depending on the TDS content), but it is approximately equal to the penetration depth of PAR (e.g., first 4–6 m of the water column in San Telmo on July 2007); thus, increasing the importance of photochemical processes in the hydro(bio)geochemistry of pit lakes.

Keywords: Photoreduction; Fe(III); Fe(II); UV-A; PAR; Pit lake; Colloids; Schwertmannite; AMD

Sucrose Dissolution Studies Leading to a Generic Shrinking Object Model for Batch Dissolution of Regular-Shaped Particles by Victor W. Truesdale (pp. 421-442).

The dissolution of sieved sucrose crystals has been studied spectrophotometrically by observing the increase in dissolved sucrose concentration with time. Equations recently derived from the shrinking sphere model for the batch dissolution of a solid in under-saturated conditions tested successfully on both single crystal-size and mixtures of two sizes of sucrose crystals. Single-sized crystals provided a straight line for the plot of the fraction of un-dissolved solid to the power one-third, versus time ( $$ f_{ ext{u}}^{ 1/ 3} $$ vs. t). The dissolution of mixtures of two crystal sizes fitted the non-linear equation tested earlier on sodium chloride in water-propanone mixtures. Together, these two sets of tests on ionic and covalent substances verify that many simple dissolutions will be easily modelled using this physical model based on shrinkage, where the chemical composition of the solids is very much of secondary importance. Consequently, there is an increased chance that the equations will describe the dissolution of biogenic silica in seawater, the problem which originally inspired this study. More than this, though, the equations are discovered to be mathematically generic; very many geometries other than the sphere satisfy the same equations, and the “shrinking object dissolution model” is thereby defined. The approach should also apply even to non-aqueous dissolutions. A prototype plot of shrinking object rate constant (obtained from numerical fitting of the model to sucrose) versus particle size is presented, and it is shown how analogous treatments for other substances will be central to collection and use of much dissolution data in the future. The study is placed in context with much earlier solid phase decomposition studies, concluding that the key characteristic of the simplest of all dissolutions is that the interface between solid and liquid should advance at a uniform linear rate. It is shown how this approach leads to equations of the same mathematical forms already discussed above.

Keywords: Dissolution; Dissolution kinetics; Shrinking object model; Sucrose; Diatom frustules; Silica cycling; Mixed particles

Uranyl Retention on Quartz—New Experimental Data and Blind Prediction Using an Existing Surface Complexation Model by Florian Huber; Johannes Lützenkirchen (pp. 443-456).

The adsorption behaviour of uranyl onto seven different samples of quartz was studied in batch experiments. Sea-sand (0.1–0.3 mm), Fil-Pro 12/20 (1–2 mm) and five Min-U-Sil samples with smaller particle sizes (5, 10, 15, 30 and 40 μm) were used. The uptake curves show “pH adsorption edges” in the range of pH 4–5. A good agreement of the new data with literature data was found when plotting surface-normalised distribution coefficients versus pH. Differences in the adsorption behaviour for pre-treated and untreated sea-sand samples were detectable resulting in a shift of the pH edge to higher pH values after treatment. A literature surface complexation model was applied for blind predictions of the experimental results. The simulations described the experimental observations quite well for the Min-U-Sil samples. For the two coarser quartz samples, the calculated over-predictions were explained by the larger-than-expected measured specific surface area and measurable amounts of associated minerals, for Fil-Pro 12/20 and sea-sand, respectively. Dissolution of the samples was studied as a function of pH. After 5 days, the measured Si concentrations were all higher than equilibrium quartz solubilities, but lower than those of amorphous silica. With increasing pH, dissolved silica increased. This strongly suggests that formation of dissolved uranyl–silicato complexes have to be considered based on measured silica concentrations.

Keywords: Surface complexation model; Adsorption; Batch experiments; Uranium (VI)

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Aquatic Geochemistry (v.15, #3)

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Aquatic Geochemistry (v.15, #3)