The development of a decision support system to evaluate natural attenuation as a remediation strategy for petroleum contaminated sites in New Zealand

Abstract

Remediation by natural attenuation (RNA) is seen as a viable alternative to "active" remediation. The use of natural attenuation (NA) at underground storage tank sites with petroleum releases has increased significantly over the past few years. Monitored natural attenuation (MNA) is new in New Zealand, and is attracting attention as a cost-effective management option. However, little work has been done locally to isolate, specify, and quantify the controls on NA. From an understanding of the physical and chemical controls, and recommended standard industry practice, a two phased approach has been developed to evaluate NA. This is based on easily obtained field data and field tested indicators of NA. The effectiveness of NA at two leaking underground storage tank sites with ongoing long-term groundwater quality monitoring data has been evaluated. The primary indicators used to evaluate NA included plume characterisation and migration. Secondary indicators included estimation of NA rates and evaluation of changes in groundwater geochemistry as a result of intrinsic bioremediation of fuel hydrocarbons. The results show, despite some uncertainties, that NA of dissolved hydrocarbons has been occurring and is preventing the migration of the dissolved benzene, toluene, ethylbenzene and xylenes (BTEX) plume at both sites. Although several bioparameters could be used to demonstrate biodegradation, the trend of the BTEX compounds over time or distance was found to be a very good indicator for demonstrating the appropriateness and effectiveness of NA. Further research in New Zealand, in the laboratory and in the field, with various contaminants under different environmental conditions is recommended. This will further elucidate the attenuating processes and help to define the most appropriate indicator parameters to monitor the processes.