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Hydrogeologists using the partition coefficient, or K\(_d\) approach, to quantify attachment (sorption and / or ion exchange) of heavy metal(s) in aquifers have expressed reservations about its oversimplification of the geochemistry involved, potentially undermining predictions of contaminant fate and therefore jeopardising effective remediation efforts. The aims of this project were to determine the validity of the K\(_d\) approach for the Permo-Triassic sandstone – a common aquifer type worldwide – and to propose a better way of quantifying attachment for metal ions. After characterising a sample of Permo-Triassic sandstone by way of a suite of batch experiments, the geochemical code PHREEQC was used to interpret the results using simulations incorporating both surface complexation theory and ion exchange. These demonstrated, by approximately matching attachment isotherm plots of Zn, that the model was a robust representation of the sandstone. This model was then adapted to simulate transport of Zn through a representative aquifer in a range of conditions to determine the potential importance of sorption in metal transport. The results confirmed the variability in the system with regard to pH influences, the fluctuating dominance of ion exchange and sorption, the presence of competing ions, and the resultant outcomes for Zn transport. It is expected that these results are similar for metals with chemistry similar to that of Zn.

The aim of this research was to provide a basis for quantifying the ability of red sandstone to oxidise dissolved organic carbon (DOC), and determine if results of experiments on synthetic minerals can be applied to geological materials. Ascorbic acid (H\(_2\)A) was used as a probe. Preliminary experiments revealed that it can reductively dissolve sandstone hematite and Mn oxide as research on synthetic minerals had previously shown. Sorption of H\(_2\)A to sandstone was similar to that seen for synthetic hematite (a two-slope linear isotherm). Anoxic batch experiments were undertaken under biotic and abiotic conditions. Release of Fe and Mn was found to be dependent on the concentration of H\(_2\)A and pH. Decrease in H\(_2\)A concentration exceeded increase in Fe and Mn concentrations corrected for sorption, and this was modelled by sorption of both ascorbate and dehydroascorbic acid. The rate of H\(_2\)A oxidation was higher in biotic experiments than in abiotic experiments, probably due to the differences in pH rather than bacterial presence. The rate of ascorbic acid oxidation by natural oxides was higher than found by previous researchers for synthetic hematite. This result was not expected, and only in part can be ascribed to the effect of Mn oxides. However, the mechanism of reductive dissolution determined by the synthetic mineral studies appear relevant also to the sandstone system. Many questions remain but the study shows that the sandstone provides a significant natural oxidative attenuation capacity and that the results of experimentation on synthetic minerals can only be applied qualitatively.

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