Check out our New Publishers' Select for Free Articles

Aquatic Geochemistry (v.16, #1)

Biogeochemical Zones Within a Macrotidal, Dry-Tropical Fluvial-Marine Transition Area: A Dry-Season Perspective by L. C. Radke; P. W. Ford; I. T. Webster; I. Atkinson; G. Douglas; K. Oubelkheir; J. Li; B. Robson; B. Brooke (pp. 1-29).

The Fitzroy River delivers large amounts of nutrients and fine sediments to Keppel Bay (contiguous with the Great Barrier Reef Lagoon) during intermittent flow events. This study explores sources, forms and transformations of nutrients in Keppel Bay, and develops a functional process zonation that integrates seabed geochemistry and water column nutrient characteristics which are controlled by suspended sediment. The water column and seabed properties were investigated over two dry seasons, with supplementary core incubations taken to measure carbon decomposition rates and nutrient fluxes. Keppel Bay can be divided into three zones, the: zone of maximum resuspension (ZMR); coastal transitional zone (CTZ); and blue water zone (BWZ). Mineralisation of predominantly terrestrial organic matter occurs in the ZMR where nutrient uptake by phytoplankton is light limited. The CTZ and BWZ had higher light penetration and phytoplankton growth was likely limited by N and P, respectively. The identified zones conform to the bathymetry and hydrodynamic characteristics of the bay, allowing for the development of an integrated conceptual model accounting for the benthic and pelagic biogeochemical processes. Recognition of these different zones shows that considerable variation in benthic and water column properties is possible within a small system with the bathymetric and hydrodynamic characteristics of the fluidized bed reactor.

Keywords: Resuspension; Dissolved nutrients; Seabed geochemistry; Nutrient limitation; Tide-dominated embayment; Sub-oxic fluidized bed reactor

Colloidal Control on the Distribution of Rare Earth Elements in Shallow Groundwaters by Olivier Pourret; Gérard Gruau; Aline Dia; Mélanie Davranche; Jérôme Molénat (pp. 31-59).

A 7-year monitoring period of rare earth element (REE) concentrations and REE pattern shapes was carried out in well water samples from a 450 m long transect setup in the Kervidy/Coët-Dan experimental catchment, France. The new dataset confirms systematic, topography-related REE signatures and REE concentrations variability but challenges the validity of a groundwater mixing hypothesis. Most likely, this is due to REE preferential adsorption upon mixing. However, the coupled mixing–adsorption mechanism still fails to explain the strong spatial variation in negative Ce anomaly amplitude. A third mechanism—namely, the input into the aquifer of REE-rich, Ce anomaly free, organic colloids—is required to account for this variation. Ultrafiltration results and speciation calculations made using Model VI agree with this interpretation. Indeed, the data reveal that Ce anomaly amplitude downslope decrease corresponds to REE speciation change, downhill groundwaters REE being mainly bound to organic colloids. Water table depth monitoring shows that the colloid source is located in the uppermost, organic-rich soil horizons, and that the colloid input occurs mainly when water table rises in response to rainfall events. It appears that the colloids amount that reaches groundwater increases downhill as the distance between soil organic-rich horizons and water table decreases. Topography is, therefore, the ultimate key factor that controls Ce anomaly spatial variability in these shallow groundwaters. Finally, the <0.2 μm REE fraction ultimately comes from two solid sources in these groundwaters: one located in the deep basement schist; another located in the upper, organic-rich soil horizon.

Keywords: Rare earth elements; Dissolved organic matter; Speciation modeling; Natural waters; Ultrafiltration

Temporal Variability in the Concentration and Stable Carbon Isotope Composition of Dissolved Inorganic and Organic Carbon in Two Montana, USA Rivers by Stephen R. Parker; Simon R. Poulson; M. Garrett Smith; Charmaine L. Weyer; Kenneth M. Bates (pp. 61-84).

Here we report diel (24 h) and seasonal differences in the concentration and stable carbon isotope composition of dissolved inorganic (DIC) and organic carbon (DOC) in the Clark Fork (CFR) and Big Hole (BHR) Rivers of southwestern Montana, USA. In the CFR, DIC concentration decreased during the daytime and increased at night while DOC showed an inverse temporal relationship; increasing in the daytime most likely due to release of organic photosynthates and decreasing overnight due to heterotrophic consumption. The stable isotope composition of DIC (δ¹³C-DIC) became enriched during the day and depleted over night and the δ¹³C-DOC displayed the inverse temporal pattern. Additionally, the night time molar rate of decrease in the concentration of DOC was up to two orders of magnitude smaller than the rate of increase in the concentration of DIC indicating that oxidation of DOC was responsible for only a small part of the increase in inorganic carbon. In the BHR, in two successive years (late summer 2006 & 2007), the DIC displayed little diel concentration change, however, the δ¹³C-DIC did show a more typical diel pattern characteristic of the influences of photosynthesis and respiration indicating that the isotopic composition of DIC can change while the concentration stays relatively constant. During 2006, a sharp night time increase in DOC was measured; opposite to the result observed in the CFR and may be related to the night time increase in flow and pH also observed in that year. This night time increase in DOC, flow, and pH was not observed 1 year later at approximately the same time of year. An in-stream mesocosm chamber used during 2006 showed that the night time increase in pH and DOC did not occur in water that was isolated from upstream or hyporheic contributions. This result suggests that a “pulse” of high DOC and pH water was advected to the sampling site in the BHR in 2006 and a model is proposed to explain this temporal pattern.

Keywords: Dissolved organic carbon; Carbon isotopes; Dissolved inorganic carbon; Diel

Influence of NOM on the Mobility of Metal(loid)s in Water-Saturated Porous Media by George Metreveli; Gudrun Abbt-Braun; Fritz Hartmann Frimmel (pp. 85-100).

In this work, the interaction of natural organic matter (NOM) with metal(loid)s (Cu, Pb, Zn, Pt, As) and the role of NOM on the metal(loid) transport in a water-saturated quartz sand column were investigated. For detailed information, size exclusion chromatographic (SEC) measurements and “short pulse” laboratory transport experiments with online metal(loid) and NOM detection were used. The SEC measurements showed the formation of metal–NOM complexes. Cu, Pb, Zn and Pt were predominantly bound to the high molecular mass NOM molecules. The binding capacity of the NOM for metals increased with increasing pH value and in the following order: Zn < Pb < Cu < Pt. No evidence for the formation of As–NOM complexes was found. The transport experiments showed no significant influence of NOM on the mobility of Cu, Pb and Zn. The metal–NOM complexes detected in the SEC experiments were obviously sorbed completely onto the grain surfaces in case of the quartz sand system, or they were dissociated partially during passage through the column. No influence of NOM was observed on the transport of As as well. Inorganic Zn and As species were transported through the column with increasing retardation as the pH value increased. Pt showed a high mobility at a pH of 5, and it decreased at a pH of 7 especially in the presence of NOM. The results support the known fact that a decrease in the pH value results in enhanced transport of inorganic metal(loid) species in water-saturated porous media. On the other hand, the presence of NOM can immobilise the metals through metal–NOM complex formation and the deposition of the complexes onto the stationary phase.

Keywords: NOM; Metal(loid)s; SEC; Transport experiments; Porous media

Silica Gel as a Surrogate for Biogenic Silica in Batch Dissolution Experiments at pH 9.2: Further Testing of the Shrinking Object Model and a Novel Approach to the Dissolution of a Population of Particles by Victor W. Truesdale (pp. 101-126).

Recently, the increase in dissolved concentration in the batch dissolution of various salts or sucrose has been successfully modelled with three equations, one a cubic in time. However, from three separate earlier investigations with ocean sediments and phytoplankton frustules, there is residual suspicion that biogenic silica does not follow this behaviour. This paper shows that the Shrinking Object Model applies to the dissolution of sieved silica gel particles, as well as to a sample of unsieved, freeze-dried frustules of Odentella sp. Silica gel, being readily available in quantities that can be sieved, is a useful surrogate for biogenic silica in allowing problems of experimental design to be overcome. The dissolutions covered three possible analytic integrations that arise from the model: an exponential for approach to saturation with excess solid silica; the approach to near saturation with either a slight excess or deficiency of silica; dissolution at high under-saturation. Good agreement was found between experimental results and mathematical modelling. The paper provides template calculations by which future raw results can be parameterized. Nevertheless, the reasons for non-linear kinetics reported in earlier work have not been identified, and so controversy over non-linear dissolution kinetics is enhanced. Stirring regime was found to be important with silica gel dissolution, and so biogenic silica dissolution is therefore likely to be ‘transport limited’ at low stirring rate. Accordingly, all archived and future data should be scrutinized for stirring effects before being applied to the oceanic environment. A rigorous test for determining whether a substance’s dissolution deviates from the model is recommended as a preliminary to any future dissolutions, whether in batch or with the chemo-stat. A fixed amount of frustule sample is added to a series of buffered mixtures containing increasing background silicic acid concentrations. Absence of any problem is marked by a linear plot between the increase in silicic concentration accruing over a fixed reaction period and that of the background silicic acid. A novel mathematical proof is provided to justify the test’s use. The reasons for the earlier deviations from expected behaviour of, for example, oceanic sediments, are discussed. Lastly, the paper provides a novel approach to the dissolution of a population of particles of mixed sizes which will probably find ready future application in oceanography.

Keywords: Diatoms; Frustules; Biogenic silica; Dissolution kinetics; Silica; Shrinking object model; Heterogeneous reactions; Kinetics; Poly-dispersed particles; Transition State Theory; Geo-engineering; Calcium carbonate dissolution; Poly-dispersed populations

Identifying Controls on Water Chemistry of Tropical Cloud Forest Catchments: Combining Descriptive Approaches and Multivariate Analysis by Amelie Bücker; Patricio Crespo; Hans-Georg Frede; Kellie Vaché; Felipe Cisneros; Lutz Breuer (pp. 127-149).

We investigated controls on the water chemistry of a South Ecuadorian cloud forest catchment which is partly pristine, and partly converted to extensive pasture. From April 2007 to May 2008 water samples were taken weekly to biweekly at nine different subcatchments, and were screened for differences in electric conductivity, pH, anion, as well as element composition. A principal component analysis was conducted to reduce dimensionality of the data set and define major factors explaining variation in the data. Three main factors were isolated by a subset of 10 elements (Ca²⁺, Ce, Gd, K⁺, Mg²⁺, Na⁺, Nd, Rb, Sr, Y), explaining around 90% of the data variation. Land-use was the major factor controlling and changing water chemistry of the subcatchments. A second factor was associated with the concentration of rare earth elements in water, presumably highlighting other anthropogenic influences such as gravel excavation or road construction. Around 12% of the variation was explained by the third component, which was defined by the occurrence of Rb and K and represents the influence of vegetation dynamics on element accumulation and wash-out. Comparison of base- and fast flow concentrations led to the assumption that a significant portion of soil water from around 30 cm depth contributes to storm flow, as revealed by increased rare earth element concentrations in fast flow samples. Our findings demonstrate the utility of multi-tracer principal component analysis to study tropical headwater streams, and emphasize the need for effective land management in cloud forest catchments.

Keywords: Ecuador; Tropical cloud forest; Principal component analysis; Water quality; Land-use change; Rare earth elements

Determination of Free Cd, Cu and Zn Concentrations in Lake Waters by In Situ Diffusion Followed by Column Equilibration Ion-exchange by C. Fortin; Y. Couillard; B. Vigneault; P. G. C. Campbell (pp. 151-172).

Combining in situ diffusion and column ion-exchange equilibration, we measured free metal ion concentrations (Cd, Cu and Zn) in water samples collected from the epilimnion of 14 lakes in the Rouyn-Noranda area (600 km north-west of Montreal, QC, Canada). Lakes were selected to represent a wide range of physico-chemical characteristics (hardness, pH, dissolved organic matter—DOM, degree of metal contamination), to determine the influence of these parameters on metal speciation. Total dissolved metal concentrations, as determined within the diffusion cells, varied over one to two orders of magnitude: [Cd] 0.19–2.9 nM; [Cu] 36–190 nM; [Zn] 7–2,800 nM. The proportion of total dissolved metal present as free Cd²⁺ and Zn²⁺ was relatively constant for the 14 selected lakes, despite the wide pH (4.5–8) and DOM (3–23 mg C/L) ranges, probably reflecting the inverse relationship observed between pH and DOM; this proportion did, however, vary with DOM and pH for Cu. Our experimental free metal ion concentrations were compared with those calculated with the thermodynamic models WHAM (Windermere Humic Aqueous Model VI) and ECOSAT 4.7 (incorporating the NICA-Donnan model). Measured and calculated values were in reasonable agreement for both Cd and Zn although measured values were generally slightly higher, i.e. less than one order of magnitude. For several lakes, measured free Cu concentrations were, however, much higher than the calculated values, suggesting that these models overestimate Cu complexation. The gap between measured and calculated free metal ion concentration becomes more important as the total metal concentration decreases and as pH increases.

Keywords: Metal speciation; In situ diffusion sampling; Lake water; WHAM model; NICA-Donnan model; Dissolved organic matter

Reduction of Substituted p-Benzoquinones by Fe^II Near Neutral pH by Minori Uchimiya; Alan T. Stone (pp. 173-188).

The oxidation of dihydroxyaromatics to benzoquinones by Fe^III (hydr)oxides is important in respiratory electron shuttling by microorganisms and has been extensively studied. Prior publications have noted that the Gibbs Free Energy (ΔG) for the forward reaction is sensitive to dihydroxyaromatic structure, pH, and concentrations of reactants and products. Here, we address the back reaction, benzoquinone reduction by Fe^II. Rates markedly increase with increasing pH, in accord with increases in ΔG. Ring substituents that raise the potential of the p-benzoquinone/hydroquinone half reaction raise reaction rates: –OCH₃ < –CH₃ < –C₆H₅ < –H < –Cl. p-Naphthoquinone, with a reduction potential lower than the five substituted p-benzoquinones just listed, yields the lowest reaction rates. The complexity of the reaction is reflected in lag periods and less-pronounced S-shaped time course curves. Benzoquinone reduction by Fe^II may be an important link in networks of electron transport taking place in suboxic and anoxic environments.

Keywords: Ferrous ion; Hydroquinones; Benzoquinones; Kinetics; Electron transfer

Lithium as a Silicate Weathering Proxy: Problems and Perspectives by Junyeon Yoon (pp. 189-206).

The lithium concentration of the dissolved load from the Lena River, together with major element chemistry and GIS-based area and runoff data demonstrate the importance of evaporites in controlling dissolved Li in river waters. Eighty-four percent of the Li in the dissolved load of upper Lena tributaries comes from evaporites in these drainage basins. Altogether, at least ~20% of the total Li flux of the Lena River originates from this source. This finding has important implications for using lithium as a proxy for silicate weathering. The Li flux and the ⁸⁷Sr/⁸⁶Sr ratio are compared in order to address a difference between the two silicate weathering rate proxies. The proposed controls on the dissolved δ⁷Li values in rivers (kinetic vs. equilibrium isotopic fractionation; Rayleigh-type preferential extraction of the heavy isotope) (Huh et al., Earth Planet Sci Lett 194:189–199, 2001) are evaluated using data from both the Siberian rivers and the Orinoco River. Neither of the proposed mechanisms satisfactorily explains the comprehensive data set. Instead, a ‘mineralogy-specific view’ that emphasizes the difference in the secondary mineralogy (i.e., fractionation factor) is presented as a potential rationalization in the form of the refined Rayleigh-type extraction.

Keywords: Lithium; Silicate weathering; Lena River; Evaporite; Orinoco River

Humic Acids Copper Binding Following Their Photochemical Alteration by Simulated Solar Light by Roza Vidali; Emmanouela Remoundaki; Marios Tsezos (pp. 207-218).

Humic substances exposed to solar light play the role of photosensitizers in aquatic photochemical processes, generating free radicals during UV and visible light irradiation. During irradiation, high molecular weight structures are destroyed and low molecular weight constituents are formed. Alterations of the humic acids metal binding capacity due to their photochemical alterations occur. The present work reports controlled laboratory experimental results on the binding of copper by a certified purified peat humic acid (PPHA) before and after irradiation in a laboratory scale photoreactor. A reference curve of copper binding by photochemically unaltered humic acid was experimentally determined as a function of solution pH by potentiometric titrations. The experimental data series correspond to a pH range from 3 to 8.5, necessary for the simultaneous consideration of complexation and metal species solubility contribution in the obtained results. From the experimental results, it was apparent that copper is strongly bound by humic acid even at the acidic range of pH where the percentage of copper bound reached 60 and 95% at pH values of 3.5 and 5.5, respectively. During 12 and 20 days of irradiation experiments, humic acid photoalteration was experimentally monitored by a size exclusion chromatography system (HPLC-SEC). From the potentiometric titrations of the irradiated humic acid solutions by a copper selective electrode, it was apparent that the copper binding capacity of photoaltered humic acid solutions was significantly reduced for pH values up to 6.

Keywords: Humic substances; Copper binding; Photochemical alteration; Simulated solar irradiation

Sections

Personal tools

Aquatic Geochemistry (v.16, #1)

Sections

Personal tools

Local resources

Aquatic Geochemistry (v.16, #1)