Quantification of methanogenesis-dominated natural attenuation processes in a carbonate aquifer with complex sources

Natural attenuation of petroleum hydrocarbons in groundwater has been extensively investigated in many industrial settings. However field-based assessments have typically focused on small scale domains. Few studies have characterised the natural attenuation processes acting on contaminant plumes associated with large and complex light non-aqueous phase liquid (LNAPL) source zones. Previous investigations of contaminant plumes have documented pollutant degradation under variable redox conditions, but overall little is know about contaminant behaviour in carbonate aquifers where methanogenesis is the dominant degradation process. The objective of this Thesis is to quantify the natural attenuation mechanisms acting on a large and complex hydrocarbon plume in a coastal carbonate sand aquifer, and develop insights into the long-term sustainability of those mechanisms. At the selected study site the natural attenuation assessment is complicated by multiple LNAPL source zones, variable groundwater recharge rates and a layered sand aquifer which sustains vertical migration to an underlying limestone unit through a separating clay aquitard. Detailed discrete-depth field installations are used to characterise the distribution of hydrocarbon compounds in the multi-component plume, and to infer natural attenuation processes and rates through model-based data interpretation. A transient three-dimensional site groundwater model is constructed, and alternative conceptual hydrogeological models are systematically studied to identify the most plausible conceptual model which generates the (i) observed plume behaviour and (ii) biogeochemical response, as a result of contaminant release, transport and degradation from known LNAPL sources. Scenario-modelling illustrates that under typical site conditions, discontinuities in the clay aquitard can sustain the necessary vertical gradients without expressing as "sinks" in site-scale water table contour maps. Quantification of natura