In situ Biorestoration of an Oil Contaminated Subsoil

The feasibility of in-situ biorestoration of contaminated subsoils is being investigated by RIVM in co-operation with TNO. This project is aimed at optimizing the treatment of deeper layers of contaminated soil by enhancing microbial activity. The project consists of a literature study, research at laboratory-scale, detailed research of the selected experimental site and the actual clean-up of the contaminated site. In the literature study it was concluded that in-situ biological treatment offers reasonable prospects, concerning the technical aspects and the final results, but there were still questions about the efficiency of the clean-up of the soil itself. The selected site is a gasoline station, where the soil has been contaminated with gasoline, caused by a leaking tank. At least 30 m of normal gasoline have leaked and about 600 m3 of soil have been contaminated. Several monitoring wells have been placed to get an impression of the quantity and composition of the contamination. The soil consists of medium fine sand, without organic material and contains some clay-loam layers. The groundwater level is at about 2.5 m below the surface. Concentrations of the oil components in the soil(mainly between 1,000 and 5,000 mg/kg) as well as in the groundwater exceed the Dutch reference values which demand treatment of the site. In leaching experiments it was found that only very small quantities of the gasoline can be leached from the soil. Clean-up by percolation only would take at least 4 years. The leached quantity increased with the percolation rate, but was nearly independent of the gasoline concentration. The mobility could not be promoted by the addition of detergents. Numerous laboratory biodegradation experiments have been carried out in order to determine the degradation capacity of the contaminated soil, and to determine to what extent this biodegradation could be stimulated. The conlusions of these experiments are: 1. In the soil there is low microbial activity and also the rate of degradation of the oil is low and constant in time. From mass balance studies it was determined that the degradation rate is determined by the availability of the oil components, which is a function of the dissoiution rate into the water and the volatilization. Availability turns out to be one of the determining aspects for this type of restoration technique. 2. The degradation activity is most enhanced by the addition of seeding material from a landfarm, but is practically not feasible because of the limited mobility of microorganisms in soil. 3. The presence of water and buffering at a neutral pH contribute to an optimal biodegradation rate. 4. Some N and P additions are necessary. However, the C-N-P ratio had a very small influence on the degradation. A C-N-P ratio of 100-10-10 is recommended. 5. As alternative oxygen source, hydrogen peroxide seems suitable, but the use of high concentrations of nitrate led to a decreased degradation. 6. Detergents applied to increase the availability of the oil components did not stimulate the degradation. The degradation rate under stimulated conditions varied between 5 and 40 (mean 10) mg C/kg/day in comparison with an autonomous degradation rate of 3-4 mg C/kg/day. A stimulated clean-up duration of 1.5 years is expected. Column experiments are in progress now and are used to confirm the findings of the laboratory tests. They should also give answers about oxygen limitation and alternative oxygen sources. First examples of the hydrological set-up (infiltration by drains and withdrawal by wells, dimensioned by the desired degradation rate) confirm the feasibility of this project. It is expected that the actual clean-up operation will start in the fall of this year. In-situ treatment offers a very good alternative with advantages over excavation combined with various treatment techniques.