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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Non-invasive quantification of redox conditions under variably saturated conditions.

Awarded: NOK 2.7 mill.

The purpose of this project is to understand and develop indirect (ie geophysical) methods to map zones with low oxygen levels due to the decomposition of organic contaminants in soil with variable water saturation. The work includes detailed modeling of bio-geochemical processes, batch experiments and degradation experiments in four laboratory sand tanks. The sand tanks were fitted with pipes to control the water level and 288 electrodes in each. Degradation of propylene glycol (PG) in batch experiments was studied by a combination of automatic and manual gas measurements, consumption of O2 and CO2, N2O, and N2 emissions, followed by chemical analyzes of iron, manganese and microorganisms. Degradation was also modeled with Monod kinetics combined with a chemical equilibrium model. Simulations of aerobic conditions highlighted the importance of initial biomass. Results of batch experiments can be used to calibrate and verify models for anaerobic conditions. Electrical resistivity tomography (ERT) and self-potential (SP) measurements were performed in the four tanks with two different sand types and degradation of PG with and without proximity to an electron bridge (iron pipe). Based on the geobattery concept, the electron bridge transports electrons released from the anaerobic degradation process at one end to oxygen at the opposite end of the pipe. The SP measurements showed an electron flow between the anaerobic and aerobic zones through the electron bridge, which also reduced the formation of dissolved iron and manganese compared to degradation at the site without an iron pipe. The results show that ERT can be used to map zones with active degradation in partially saturated soil. Both increased electrical conductivity of the water phase, due to the formation of dissolved iron and manganese, and reduced conductivity, most likely caused by methane formation, could be detected. The experiments were performed at NMBU in collaboration with Colorado School of Mines, USA, Lancaster University, UK and NIBIO. At Lancaster, induced polarisation (IP) was used to study PG degradation in partly saturated sand columns. The experiments provide a good background for further development of these geophysical methods for use in the field, as well as for improving theoretical models that describe the relationship between geophysical properties and the redox state in soil. The results of combined methods (ERT, SP and gas measurements) will increase understanding of the degradation processes and provide better models for data interpretation.

This project aims to quantify redox potential in variably saturated soils by the development of integrated non-invasive techniques including geophysical and gas measurements, and modelling tools. Degradation of organic chemicals in the unsaturated zone is a highly relevant remediation technique for protecting groundwater. The redox potential determines degradation pathways and how the soil system is affected on long term. Anaerobic conditions are a growing threat for the development of methanogenic condit ions of contaminated soils. The underlying theory for this project is that degradation affects the water chemistry, gas release and responses of the geo-electrical signature due to varying redox conditions. These signals can be interpreted through petroph ysical relationships relating geophysical responses to bio-geo-chemical activity. Modelling at different scales will be conducted to increase the understanding of processes in variably saturated soil systems. Aspects to be modelled include: diffusion and gas exchange over the air-water boundaries and ion transport in micro-areas around soil particles where degradation is taking place. A laboratory instrument will be designed on the basis of bio-geo-chemical and geophysical simulations to test present theo ries on geophysical conditions and biodegradation activities under unsaturated conditions. The instrument will provide opportunity to control water saturation and simultaneously measure gas releases, Self Potential (SP) and Electrical Resistivity (ER) and sampling of soil water. Improved theories will be tested at an existing well equipped field site for flow and transport studies and modelled with state of the art bio-geo-chemical models. The combination of measurements sensitive to gas release and geo-e lectrical properties combined with models describing the petrophysical relationships and theoretical bio-geo-chemical responses will provide quantitative information about the in-situ redox situation.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek