Dissolved organic carbon transformations and microbial community response to variations in recharge waters in a shallow carbonate aquifer

© 2016, The Author(s). In carbonate aquifers, dissolved organic carbon from the surface drives heterotrophic metabolism, generating CO2 in the subsurface. Although this has been a proposed mechanism for enhanced dissolution at the water table, respiration rates and their controlling factors have not been widely evaluated. This study investigates the composition and concentration of dissolved organic carbon (DOC) reaching the water table from different recharge pathways on a subtropical carbonate island using a combination of DOC concentration measurements, fluorescence and absorption characterisation. In addition, direct measurements of the microbial response to the differing water types were made. Interactions of rainfall with the vegetation, via throughfall and stemflow, increase the concentration of DOC. The highest DOC concentrations are associated with stemflow, overland recharge and dissolution hole waters which interact with bark lignin and exhibit strong terrestrial-derived characteristics. The groundwater samples exhibit the lowest concentrations of DOC and are comprised of refractory humic-like organic matter. The heterotrophic response seems to be controlled by the concentration of DOC in the sample. The terrestrially sourced humic-like matter in the stemflow and dissolution hole samples was highly labile, thus increasing the amount of biologically produced CO2 to drive dissolution. Based on the calculated respiration rates, microbial activity could enhance carbonate dissolution, increasing porosity generation by a maximum of 1%kyr−1 at the top of the freshwater lens

Investigation of the Synthesis and Internal Structure of Protective Oxide Layers on High Purity Chromium with SIMS Scanning Techniques

The major problem affecting the application of chromium in high temperature processes is the ongoing spallation of the protective oxide layer formed during hot gas oxidation. This resultsin a continuous material erosion. To gain a deeper insight in the spallation and oxidation process a high purity powder metallurgically produced chromium sample was submitted to a two stage hot gas oxidation process. The formed oxide layers were investigated by 3D SIMS and Scanning SIMS. The forming of the protective oxide layer is carried by the diffusion of chromium from the bulk through the already existing oxide layer and the reaction of the diffused chromium with the oxygen from the gaseous phase. In parallel to the growing of the layer an accumulation of impurities at the interface oxide layer- bulk can be observed. The enrichment of trace elements at the interface level (for the investigated sample namely Cl and N) can be explained by the low solubility of these elements in chromium oxide and therefore their inability to diffuse through the already formed protective layer.JRC.(IAM)-Institute For Advanced Material