Investigating the impacts of water table fluctuations on natural source zone depletion rates

Abstract:

Natural source zone depletion (NSZD) can be an effective management strategy for many sites impacted by petroleum hydrocarbon contamination. However, if NSZD is going to be relied upon to attenuate contaminants, characterization of the extent of the source zone and rate of natural degradation is required. This characterization can be difficult at many sites and can require expensive and invasive approaches. A non-invasive approach to quantifying NSZD is to measure surface effluxes of carbon dioxide (CO₂) and methane (CH₄) generated from degradation. To date, the validity of this approach has focused on steady-state systems; however, in reality, unconfined groundwater systems are dynamic due to changes in water table elevation from a variety of natural processes.

In this study, the effects of water table fluctuations on the actual and perceived NSZD rates were investigated using a 400 cm long, 100 cm wide and 150 cm tall sand tank in which biodiesel was released. Over the duration of the experiment, the water table was lowered and raised while measuring soil gas composition (CO₂, CH₄ and O₂), isotopic signature (δ¹³C) and surface effluxes of CO₂ and CH₄ at high spatial and temporal resolution. Results show that lowering the water table led to both short-term (perceived) and long-term (actual) changes on NSZD rates. In the short-term, accumulated biogenic gas produced in the anaerobic saturated zone was released due to free-product resting on the water table redistributing, allowing connected gas channels to form, transporting gas towards the surface. This immediate release of gas increased CH₄ effluxes at the surface up to 344 times baseline effluxes. After this immediate release, it was found that quasi-steady state was reached, where CO₂ production was 1.4 to 1.6 times higher than baseline as a result of increased O₂ ingress to exposed free product, enhancing microbial metabolism, therefore increasing the actual rate of attenuation. In contrast, when the water table was raised, O₂ ingress decreased and quasi-steady state CO₂ and CH₄ effluxes decreased to values 0.9 and 0.4 times that of the baseline fluxes, respectively. Findings suggest that continuous monitoring has advantages over one-time or discrete temporal sampling and that isotopic analysis can be used to determine if the measured rates are actual or perceived.