Applied Geochemistry (v.53, #C)

Partitioning the impact of environmental factors on lake concentrations and catchment budgets for base cations in forested ecosystems by Fougère Augustin; Daniel Houle; Christian Gagnon; Suzanne Couture; François Courchesne (1-12).
Seventy-two forested lake catchments were studied in Quebec (Canada) to examine the influence of climate, atmospheric deposition and catchment characteristics on base cation (BC) concentrations in lake waters (BCC) and base cation budgets at the catchment scale (BCQ). The catchments are located along a bioclimatic gradient in a vast (180 000 km2) study area underlained by the Canadian Shield. Multivariate statistical approaches are used to simultaneously assess the effects of multiple environmental factors on cation fluxes. Mean annual BCC were 132, 40, 24 and 7 μmolc  l−1 for Ca, Mg, Na and K, respectively. Mean annual BCQ estimates showed exports of 0.826, 0.251, 0.135 and 0.043 kmolc  ha−1  an−1 for Ca, Mg, Na and K, respectively. There were strong similarities in the spatial variation of BCC and BCQ, and also in their links with environmental factors. We hypothesized that the spatial variability of both, BCC and BCQ, were strongly influenced by the spatial variability in the rates of mineral weathering reactions. Variance partitioning indicated that climate-related effects accounted for 51.6% and 52.7% of the variation in BCQ and BCC, respectively. Nonetheless, lake/catchment morphometry and variables linked to solutes sources (lithology, atmospheric deposition and soil properties) were also included in some models. Overall, BCC and BCQ were positively affected by temperature, precipitation as rain and sulphate depositions, and negatively influenced by precipitation as snow and the number of frost days. Multivariate models explaining up to 69% of variation in BCC and BCQ were developed. This study shows the strong impact of climatic drivers on base cation budgets and, thus, on mineral weathering at the regional scale on the Canadian Shield.

Kinetic modeling of interactions between iron, clay and water: Comparison with data from batch experiments by Viet V. Ngo; Alain Clément; Nicolas Michau; Bertrand Fritz (13-26).
It has been proposed that a carbon steel overpack is used as part of the engineered barrier system for the geological disposal of radioactive wastes developed by Andra. The direct contact of the iron with the geological environment creates potential physical and chemical changes in the near field environment of the repository. Therefore, a thorough understanding of the mineralogical/chemical evolution caused by the interactions of iron with clay is necessary to the assessment of the performance of the geological disposal. Geochemical models have been developed (using the code KINDIS) to simulate batch experiments on iron–claystone interactions. The experiments included iron power and Callovo-Oxfordian (COx) claystone that were reacted at temperature of 90 °C for 90 days. The overall objective of this modeling work aims at an enhanced mechanistic understanding of clay–iron interactions observed in experimental studies and possible implications for engineered barrier performance.The experimental observations were successfully reproduced by the model regarding geochemical evolution and mineralogical transformations. For example, the stability of pH around 7 and total dissolved carbon in the aqueous solution, which are controlled by saturation state of carbonates in the system, are predicted accurately. In addition, the model predicts that during the interactions between iron and clays greenalite, chukanovite, and saponite form as the main secondary minerals. Moreover, the destabilization of some important primary minerals in the claystone such as quartz, illite, and smectite are also indicated by the numerical simulations. The consistency of the predictions with the experimental observations can be shown in activity diagrams of these secondary minerals, which represent the relation of H4SiO4 activity and CO2 partial pressure or Ca2+ activity. Another important result is that both the model and experimental data indicated that magnetite is not formed in the experiments.The analysis of three sensitivity cases made clear that the uncertainty in corrosion and dissolution rates for iron, quartz, and illite plays an important role on the predicted evolution of pH in the aqueous solution and the formation of secondary minerals. Through this modeling work, the controlling mechanism of the interactions of iron, clay, and water at the specific conditions is fairly well understood. However, the robustness of the geochemical code KINDIS should also be tested against other experiments with different experimental conditions.

The multi-layered Eocene aquifer is a regional scale sedimentary aquifer system occupying ∼120,000 km2 within the Adour-Garonne district (France). Local authorities have recently identified the aquifer as being at risk from extensive irrigation abstractions, threatening the sustainability of this key resource. Because large water abstractions for human activities can significantly influence the natural functioning of such aquifer systems, e.g., with leakage between aquifer layers, which can lead to water quality degradation, the characterization of such large systems constitutes a key point to protect and prevent further deterioration of aquatic ecosystems. This study provides further insight on this large aquifer through a geochemical approach, which addresses the limited number of groundwater wells where sampling is possible. For that purpose, a geochemical analysis combining two isotope systems (δ34SSO4, δ18OSO4 and 87Sr/86Sr) has been applied. The Eocene sedimentary aquifer system (detrital to carbonate deposits) is made up of four aquifer layers, Eocene Infra-Molassic sand, Early Eocene, Middle Eocene and Late Eocene, and has a mineralized area north of the Aquitaine Basin, where groundwater shows strong mineralization and anomalous levels of critical substances (SO4, F, etc.), increasing the difficulty of resource exploitation. The extreme heterogeneity of the geochemical composition of the groundwater between the aquifers and within a single aquifer is discussed in terms of the lithological control induced by the lateral variation of facies and interconnections between aquifer layers. Geochemical tools, especially the δ34S and δ18O from dissolved sulfates and the 87Sr/86Sr ratio, suggest that evaporite dissolution (both sulfate and halite) is the main process controlling the high salinity levels observed in the groundwater, explaining the spatial variations observed at the aquifer system scale. Isotopic tools also provide new information regarding the interconnections between aquifer layers, supporting the hypothesis that the Eocene aquifer system integrates groundwater from the Oligocene–Miocene aquifer through leakage effects. These new insights will likely help decision-makers adjust their choices when managing quality problems, in particular in the “mineralized area of the Entre-Deux-Mers,” where targeted groundwater wells used for drinking water display anomalous levels of critical substances.

Major, trace and platinum-group element geochemistry of the Upper Triassic nonmarine hot shales in the Ordos basin, Central China by Xinwei Qiu; Chiyang Liu; Guangzhou Mao; Yu Deng; Feifei Wang; Jianqiang Wang (42-52).
The Upper Triassic Chang 7 non-marine hot shales have been proven to contribute to Mesozoic oil reservoirs in the Ordos basin, Central China. These shales have been studied extensively in sedimentology, petrology and organic geochemistry. However, the factors controlling the development of the shales are still ambiguously constrained. This paper presents the major, trace, and platinum-group elements (PGEs) data to address these issues. The Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), C-values and clay mineral compositions indicate that the Chang 7 hot shales were developed in a humid paleoclimate. The indicators of redox conditions (Eu/Eu, Ce/Ce, Ceanom, δU, U/Th, V/(V + Ni) and V/Cr) suggest that the Chang 7 hot shales were developed under anoxic environment. Uranium enrichment (average 46.10 ppm) is an important distinguished signature of the Chang 7 hot shales, which was mainly related to synchronous volcanic ash sediments and/or possibly hydrothermal fluids. The PGE concentrations of the Chang 7 hot shales were reported for the first time here, and can be subdivided into two groups. The total abundances of the Group One samples are relatively lower and derived mainly from detrital minerals, while the Group Two samples are relatively higher, and are also likely linked to the synchronous volcanic ash sediments and/or possibly hydrothermal fluids. Combined with the regional geological setting, we tentatively propose that the tectonics is the major control on the development of the Chang 7 hot shales.

Elemental composition of peat profiles in western Siberia: Effect of the micro-landscape, latitude position and permafrost coverage by V.A. Stepanova; O.S. Pokrovsky; J. Viers; N.P. Mironycheva-Tokareva; N.P. Kosykh; E.K. Vishnyakova (53-70).
Display OmittedThe concentrations of major and trace elements in the organic layer of peat soils across a 1800-km latitude profile of western Siberia were measured within various dominating ecosystems to evaluate the effect of landscape, latitude position and permafrost coverage on the peat chemical composition. In this study, peat core samples were collected every 10 cm along the entire length of the column, down to 3–4 m until the mineral horizon was reached. The peat samples were analyzed for major and trace elements using an ICP–MS technique following full acid digestion in a microwave oven. Depending on their concentration pattern along the peat column, several groups of elements were distinguished according to their general physico-chemical properties, mobility in soils, affinity to organic matter and plant biomass. Within similar ecosystems across various climate zones, there was a relatively weak variation in the TE concentration in the upper organic layer (green and brown parts of sphagnum mosses) with the latitude position. Within the intrinsic variability of the TE concentration over the peat column, the effects of climate, latitude position, and landscape location were not significantly pronounced. In different landscapes of the middle taiga, the peat columns collected in the fen zone, the low and mature forest, the ridge and the hollow did not demonstrate a statistically significant difference in most major and trace element concentrations over the full depth of the peat column. In live (green) parts and dead (brown) parts of sphagnum mosses from this climate zone, the concentrations of Mn, P, Ca and Cu decreased significantly with increasing moss net primary production (NPP) at various habitats, whereas the other elements exhibited no link with the NPP trends. The Al- and mineral horizon-normalized peat concentration profiles, allowing removal of the occasional contamination by the underlying mineral substrate and atmospheric dust, demonstrated a homogeneous distribution of TEs along the peat column among various climate zones in the non-permafrost regions but significantly non-conservative behavior in the discontinuous permafrost site. The peat deposits in the northern part of western Siberia potentially have very high release of metals to the surface waters and the riverine systems, depending on the persistence of the ongoing permafrost thaw and the increase in the thickness of the active layer.

Nitrates in springs and rivers of East Ukraine: Distribution, contamination and fluxes by Valeriy Yakovlev; Yuliya Vystavna; Dmytro Diadin; Yuriy Vergeles (71-78).
Spatial and temporal variation of nitrate concentrations has been studied in 4 rivers and 21 springs of the transboundary (Russia/Ukraine) Seversky Donets watershed in eastern regions of Ukraine – Kharkiv, Donetsk and Lugansk oblasts. Samples have been taken from 13 sites on the Lopan, Udy, Oskol and Seversky Donets rivers together with springs on left and right river banks between August 2013 and May 2014 and analysed on major ions and nitrates. Water temperature, pH, electrical conductivity, redox potential have been measured on site. The results showed high spatial and temporal variability of nitrate concentration in both surface and groundwater. The hydrogeological settings, seasonal trends and human impact were major factors influencing nitrates mobility and accumulation in contaminated springs, which contributed to surface water pollution. Mean nitrate concentrations were 26.7 mg L 1 (C.V. = 92%) in springs and 6.9 mg L 1 (C.V. = 114%) in rivers. The nitrate fluxes from springs to rivers were estimated at ca. 3 t km 2 annually. About 1/5 of spring water samples were characterized with higher nitrate concentrations than limits recommended by WHO and National Drinking Water Standards (Ukraine). Springs have been classified according to nitrate concentration and enrichment as high, moderate and low contaminated.

Reversibility of uranium and thorium binding on a modified bauxite refinery residue: The effects of aging temperature by M.W. Clark; T.E. Payne; J.J. Harrison; M.J. Comarmond; R.N. Collins; A.J. Reichelt-Brushett (79-90).
Display OmittedThe effects of aging on the pH-dependence and reversibility of uranium and thorium binding by a modified bauxite refinery residue (MBRR) were studied in laboratory uptake/leaching experiments. Natural uranium and thorium isotopes (predominantly 238UVI and 232ThIV) of 0.4, and 0.2 mM were loaded for an 8-day period at the natural equilibrium pH of the MBRR (approximately 8.5) and were allowed to age for 6 months in humid sealed containers at 4, 23, and 65 °C. After aging, anthropogenic 232UVI and 229ThIV were added as exchange isotopes and the pH decreased stepwise from 8 to 3 over several hours in pH dependent experiments. The relative concentrations of the isotope pairs (i.e. 238UVI and 232UVI; 232ThIV and 229ThIV) indicated that irreversibility of UVI and ThIV binding by the MBRR increased with aging and was dependent on the aging temperature and surface actinide loading. Linear combination fitting of the EXAFS data for the uranium loaded materials indicated that at low uranium concentrations sorption to minerals (particularly hematite) was a prime uranium removal mechanism, but at higher concentrations precipitation dominated. The data suggest that increased irreversibility and incorporation of UVI during aging is associated with re-crystallisation of precipitates.