Monitored natural attenuation (MNA) has become an acceptable option for risk-based organic contaminant plume management in some cases. Under the premise that most attenuation occurs as a result of biodegradation, many, if not most of the numerous and exhaustive MNA application protocols published to date focus on the monitoring of biodegradation and redox indicator parameters. To make an overall plume NA assessment, a simple conceptual model is invariable invoked that cannot account for complex plume geometry or spatially variable attenuation processes. CORONA is a multi-national EU-funded project currently underway to refine MNA forecasting of a range of contaminant plume types by focussing on key processes that impact greatest on each plume type, and where those processes occur within the plume. Ten institutions from 7 countries are evaluating MNA issues at BTEX, chlorinated ethylene, tar acid, pesticide, and ammonium plumes. Each institution will provide technical expertise in novel site investigation and monitoring techniques, microbiology, hydrogeology, geochemistry and modeling.

At the sites under investigation by the University of Sheffield, it is hypothesised that the largest contribution of biodegradation to overall attenuation will occur near the plume fringe. Our investigations focus on determining the efficiency of dispersive and diffusive mixing between background electron acceptors and plume constituents, defining the width of the bioactive fringe, and estimating the contribution that fringe biodegradation processes make to overall plume attenuation.

Differentiating and characterising fringe-associated biodegradation from average plume biodegradation, coupled with detailed plume characterisation, will permit more robust estimates of natural attenuation. Furthermore, many plumes are not simply cigar-shaped, but have complex internal geometry as a result of aquifer heterogeneities and spatially variable source zones. Contaminants migrating as a series of plumelets have a greater total surface area compared to a uniform plume, and thus bioattenuate at a greater rate due to the significant contribution of fringe processes. Failure to apply the appropriate degradation rates to the appropriate zones within a plume may lead to inaccurate assessments of natural attenuation.