THE USE OF MONITORED NATURAL ATTENUATION TO MANAGE ENVIRONMENTAL RISK AT A SITE CONTAMINATED BY PETROLEUM HYDROCARBONS

The application of monitored natural attenuation (MNA) is proving to be successful at a commercial property located in the Edmonton, Alberta area. The historical release of petroleum hydrocarbons to the soil and groundwater at a former service station has resulted in the formation of a plume of impacted soil and groundwater with an approximate geometry of 65 by 50 metres. Following an extensive subsurface characterization of the site, a six-stage iterative process for developing risk management options and selecting actions to reduce environmental risk at the site was developed. The six-stage process included: problem definition; a site specific risk assessment; examining options for addressing risks; selecting the preferred options; implementing the preferred options and evaluating the action. It was concluded that the complete remediation of soil and groundwater impacts at the Site by using conventional soil excavation or in situ remediation techniques were not likely to be cost effective or technically feasible. In addition, significant uncertainty was associated with the ultimate cost and time to achieve completion of remediation. These risks were unacceptable to the property owner. Therefore, it was recognized that risk management of the soil and groundwater contaminants at the site would be required. A semi-annual monitoring program consisting of groundwater sample collection and analyses of petroleum hydrocarbon and MNA parameters has been undertaken for the past two and a half years. The monitoring data that is presented provides positive evidence that the subsurface conditions are suitable to use MNA as a means to support the natural biodegradation of groundwater impacted by petroleum hydrocarbons and to manage environmental risk for this site