Operating windows to assess the technical feasibility of monitored natural attenuation (MNA)

Monitored Natural Attenuation (MNA) is likely to be a technically feasible remediation option if the contaminant persistence time along the flow path is less than (a) the transport time to the compliance point, and (b) the time available to achieve groundwater remediation objectives. Biodegradation is often the most important natural attenuation process, however many environmental variables affect its occurrence and rate. The ranges of values for these 'critical variables' over which the contaminant persistence time is less than the transport and remediation times, respectively, are 'operating windows' for MNA. The size of an operating window is site-specific as it depends on the source concentration, target concentration, distance to compliance point, groundwater velocity, retardation factor, and remediation time. The current research is using data obtained from MNA case studies to quantify relationships between critical variables and biodegradation rates for petroleum hydrocarbons and chlorinated solvents that can be used to delineate operating windows for MNA. Operating windows will provide a rapid indication of whether MNA is likely to be technically feasible at a given site during the screening stage of technical protocols for the design, evaluation and implementation of MNA. (authors)