Induced polarization for exploring organic pollutant transport and bioreMediAtion GEoprocesses from nano to field scale – IMAGE

With currently 7000 contaminated sites in France alone, soils, sediments and groundwater are increasingly endangered by anthropogenic contamination, especially industrial pollutants from accidental spills. Petroleum hydrocarbons, which are toxic and carcinogenic for humans, are of great environmental concern. Light non-aqueous phase liquids (LNAPLs), in particular, are the main source of contamination to soil and groundwater where they cause damage to ecosystems. In the context of the ecological transition, there is an urgent need, in France and worldwide, for efficient and sustainable remediation techniques to clean up sites polluted by LNAPLs. The technologies commonly used in this context are expensive, may be harmful for the environment, and can lead to incomplete decomposition of contaminants. On the contrary, in situ bioremediation is an eco-friendly and cost-saving method which consists in using microorganisms to degrade the organic pollutants, for example by changing the environmental conditions in order to stimulate bacterial growth. Today, the monitoring of in situ LNAPL biodegradation mostly relies on direct samplings which are intrusive, time-consuming, and restricted to the ground surface and to costly monitoring wells at discrete locations. In contrast, non-invasive geo-electrical methods can provide measurements which are integrative, easily repeated in time, at a high spatial resolution between boreholes and at a significantly lower cost. IP (induced polarization), in particular, has shown its ability to monitor hydrocarbon biodegradation from the measured resistivity and chargeability changes. However, the quantitative link between the bio-physico-chemical processes at play during biodegradation and the measured IP response is still not clearly understood due to lack of knowledge on the behaviors of interfacial processes at the pore scale. In addition, the measured IP magnitude is low, which limits its probing capability. The main objectives of IMAGE are thus to (i) explore, at different structural scales, from nm to m, the IP signature of each relevant process occurring during petroleum hydrocarbon biodegradation and (ii) improve the measurement accuracy and interpretation. We will characterize and monitor the transport and fate of BTEX, in particular toluene, and their biodegradation by bacteria in the smear zone at the top of the water table and in the dissolved hydrocarbon plume. This requires investigating the frequency-dependent electrical properties of these pollutants in porous media, and their control by biogeochemical reactive transport. To this aim, pore scale micro/millifluidic experiments combined with spectral IP measurements will allow characterizing the processes at play from optical and geochemical measurements. Combining these measurements with pore scale numerical simulations, we will characterize the link between the bio-physico-chemical processes and the IP signal. Laboratory biogeochemical and IP measurements at larger scales (columns and cuboid tanks) will allow tackling 3D porous media, and a highly intrumented plurimetric pilot in which experiments will be conducted in controlled field conditions will be used to close the gap between the laboratory and field conditions. In order to efficiently invert the IP measurements performed in these metric-to-plurimetric scale setups, suitable petrophysical relationships relating the complex conductivity to the bio-physico-chemical processes involved in LNAPL biodegradation will be developed. Furthermore, a prototype measuring low IP signals, as well as signal processing tools and inversion procedures, will be developed to accurately measure, image and interpret macro-scale IP measurements. This challenging research program will be tackled jointly by experimentalists and theorists from the fields of geophysics, hydrogeology and environmental fluid mechanics, and will thus gather one French EPIC, three academic partners and on private company.