This work package will explore the respective roles of hydrogeological, geochemical, microbiological and environmental controls on the performance of natural attenuation at field-scale for common organic contaminants in groundwater. Using field sites within this project, one study will explore the development of non-reactive and reactive tracer mixtures to characterise the relevant properties of aquifers which influence contaminant transport and natural attenuation processes. These methods have not been widely applied in the context of natural attenuation. The research will focus on characterization which leads to new conceptual ways of modelling that account for the properties of, and interactions between, selected reactive tracers and soil/aquifer materials, and on optimized single and multiple-well tracer techniques for performance assessment of natural attenuation. Monitoring techniques will in particular include geophysics, to target specific processes, and tracers with a greater spatial coverage. Experiments will be undertaken at the laboratory-scale, using a model aquifer, and at the field-scale on a contaminated site.
Another study will examine the dynamics of microbiological community development, structure and metabolic function at the bioreactive fringe of contaminant plumes (interface between groundwater and the plume). This narrow zone (<1m) has enhanced microbial activity in plumes, due to supply of oxidants and organic substrates (pollutants) by hydrodynamic dispersion in aquifers. The research will establish the factors that lead to microbial succession and diversity in the indigenous microorganisms at the fringe of plumes, considering environmental pressures (organic contaminant type, concentration and toxicity) and molecular controls on behaviour. It will deduce how these selection pressures are expressed in the relative dominance of specific microbial populations, their interactions and biodegradation potential for organic contaminants at the plume fringe. This will be compared with the behaviour predicted for lab-scale plumes in WP6. This work package will provide an improved conceptual model of the spatial and temporal variation in organic contaminant biodegradation potential in plumes, to support management decisions on the implementation of natural attenuation in groundwater.