Thermally Induced Bentonite Alterations in the SKB ABM5 Hot Bentonite Experiment

Pilot sites are currently used to test the performance of bentonite barriers for sealing high-level radioactive waste repositories, but the degree of mineral stability under enhanced thermal conditions remains a topic of debate. This study focuses on the SKB ABM5 experiment, which ran for 5 years (2012 to 2017) and locally reached a maximum temperature of 250 °C. Five bentonites were investigated using XRD with Rietveld refinement, SEM-EDX and by measuring pH, CEC and EC. Samples extracted from bentonite blocks at 0.1, 1, 4 and 7 cm away from the heating pipe showed various stages of alteration related to the horizontal thermal gradient. Bentonites close to the contact with lower CEC values showed smectite alterations in the form of tetrahedral substitution of Si4+ by Al3+ and some octahedral metal substitutions, probably related to ferric/ferrous iron derived from corrosion of the heater during oxidative boiling, with pyrite dissolution and acidity occurring in some bentonite layers. This alteration was furthermore associated with higher amounts of hematite and minor calcite dissolution. However, as none of the bentonites showed any smectite loss and only displayed stronger alterations at the heater–bentonite contact, the sealants are considered to have remained largely intact.

Quantification of Carbon Cycling in a Large Aquifer Using Reactive Transport Modelling

Continental scale aquifers can store significant amounts of carbon as a result of immense water volumes, substantial concentrations of dissolved inorganic carbon (DIC) and its reactions with a matrix, thus contributing the global carbon storage and cycle. However, concentration of dissolved solutes may vary significantly over distances, which causes interpretative challenges and difficulties in process quantification. This occurs in the Guarani Aquifer System in South America, which is a subject of extensive research due to a significant strategic role in water supply. Dissolved CO2 is expected to dissociate and undergo reactions with aluminosilicate minerals, but it is unknown how much DIC may get immobilised in the aquifer. To quantify the processes, we performed reactive transport modelling which combines hydrological and geochemical information followed by global sensitivity analysis. We show that more than a half of the infiltrated CO2 may be consistently precipitated as CaCO3. The DIC concentrations across the aquifer depend primarily on the input carbon concentrations and the plagioclase hydrolysis rate, while other parameters including hydraulic conductivity, recharge rate and mineral stability are of the minor importance. We present how advanced modelling techniques may be used to interpret and quantify processes controlling water quality in continental scale groundwater systems.

Spatiotemporal variability in groundwater chemistry of a mountainous catchment with complex geologic and climate gradients in south west India

Mountainous catchments are one of the world’s important water sources that sustains a major portion of global population and a rich biodiversity. The groundwater quantity and quality of mountainous watersheds are depended generally on the geologic characteristics and climate gradients. Although many groundwater studies have been carried out in the midlands and lowlands of many river basins, not enough focus has been paid to the mountainous catchments of tropics. Here we report a case study on groundwater quality and controlling factors of a mountainous catchments of the Western Ghats mountain ranges of peninsular India - the Bhavani river basin, which is identified as a testbed for long-term monitoring of the Critical Zone process. A total of 88 water samples were collected seasonally for assessing various physico-chemical parameters, solute contents and scaling properties. The results of the study revealed that the hydrochemistry of groundwater is influenced by both silicate and carbonate weathering. Mineral stability indices computed for the groundwater reveal that about 52 % of the samples are supersaturated with carbonate minerals and often exhibit scaling due to solute overloading. Among the contributing factors that determine water quality of groundwaters, chemical weathering and anthropogenic activities play a significant role.

What's af(Fe)cting OC-Fe interactions? An experimental approach to understanding iron bound organic carbon in sediments.

<p>Drawdown of atmospheric CO<sub>2</sub> over geologic timescales is largely controlled by imbalances in the carbonate-silicate cycles and the preservation of Organic Carbon (OC) in marine sediments. Up to 85% of this OC is buried in continental shelf sediments of which ~20% is associated with reactive iron (Fe) (hydr)oxides. Association with Fe (hydr)oxides may enhance OC preservation yet despite the importance of this, little is known about which Fe (hydr)oxide phase(s) is involved in OC uptake or the binding mechanism of OC to these reactive iron minerals.  </p><p>To estimate the importance of this OC-Fe association, a citrate-dithionite-bicarbonate (CDB) extraction method is commonly used to dissolve an operationally defined ‘easily reducible iron oxide’ fraction and release the associated OC from the sediment. However, natural samples often contain a range of Fe (hydr)oxide phases extractable by CDB, and the Fe phases extracted are defined entirely on the susceptibility of their pure forms to chemical reduction. This suggests that factors affecting mineral stability, including association with OC, could lead to incomplete or excessive phase extraction, which would affect estimates of OC bound to these Fe phases.</p><p>To address these issues, we simplified the geochemical system by synthesising OC-iron (hydr)oxide composites with known Fe (hydr)oxide phases and OC moieties with differing chemical structures, added them to OC-free sediment, and then applied the CDB extraction method to determine i) the precise Fe phases extracted; ii) the impact of OC moiety on Fe release and iii) the optimal experimental conditions for the extraction.</p><p>We show that reduction of our composites by CDB results in only partial dissolution of the most easily reduced Fe phase (ferrihydrite) and a recovery of only ~20% of total Fe. We also find that the recovery is likely controlled by the functional groups present in the OC and the handling/storage/preparation of samples prior to analysis. These factors could lead to misidentification of the mineral phases extracted and an underestimation of the amount of OC associated with Fe. A change in the estimates for OC associated with Fe would have widespread implications for our understanding of the role of OC-Fe interactions in global carbon cycling.</p>