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Final Report Summary - STRESOIL (In situ stimulation and remediation of contaminated fractured soils)

Remediation of contaminated sites is a priority throughout Europe, due to the significant environmental impacts associated with ground pollution. Fuel soil contamination, which is common in Eastern European countries, poses the most serious environmental threats. The majority of involved countries have started assessing the involved problems; nevertheless the relevant regulatory and financial frameworks are often underdeveloped. The proposal of scientifically supported cost-effective remediation strategies is therefore a prerequisite for the successful implementation of future remediation programmes, and is highly dependent in the local soil conditions.

The soil structure of the majority of the European continent is of glacial origin and was formerly regarded as offering protection against contaminants to underlying groundwater aquifers. However, it was proven that glacier deposited sediments were often fractured and highly porous. Therefore contaminants could migrate very rapidly through the soil as long as the source was active, while sediment layers had increased storage capacity which could release contamination into underground reservoirs for a very long period after the initial spill ceased. Traditional remediation methods were usually inefficient in such cases and remediation was stimulated with the application of hydraulic fracturing process. Nevertheless, the application of combined stimulation and remediation technologies against nonaqueous phase liquids (NAPL) contamination remained fragmentarily documented.

The STRESOIL project aimed to develop scientifically supported criteria for the selection of the most efficient stimulation and remediation strategies to rehabilitate the NAPL-contaminated unsaturated zone of fractured and low permeable soils. Hydraulic fracturing was considered as a viable method for increasing the radius of influence, which improved the performance of traditional in situ remediation technologies.

The project was structured in six well connected work packages (WPs) with the following objectives:
1. Detailed geological characterisation of a selected fractured site, calculation of its permeability and establishment of a regional hydrogeological model. The extent of contamination was identified through soil sample analyses and the potential for biodegradation implementation was also investigated.
2. Application and testing of stimulation scenarios using hydraulic fracturing to highly contaminated areas within the site. Conventional techniques were also applied to allow for comparative analysis of the stimulation performance.
3. Implementation of bio-ventilation and stream injection in the stimulated areas, monitoring and construction of databases for each remediation scenario.
4. Development of innovative experimental methods and computational procedures to determine the transport properties of the porous matrix, as well as of natural and hydraulic fractures. Novel pore-space models were applied, to allow for quantification of heterogeneities, such as the permeability distribution within the soil matrix.
5. Utilisation of simulators and numerical modelling to design remediation strategies and to select operating variables, in order to assess NAPL contamination in the long term. Biodegradation was not incorporated in the simulations in favour of safety.
6. Evaluation of the NAPL removal efficiency for each area, using the concentration databases, and elaboration of cost benefit analyses of various scenarios so as to assess the method feasibility. The cost benefit analysis was based on the calculation of the quantitative soil decontamination and the costs' estimation for each case and provided a multi criteria data set, helpful for decision making during the remediation technologies' selection process.

STRESOIL indicated that innovative, cost-effective technologies combining stimulation and remediation were a realistic alternative to conventional treatment. This conclusion was of major importance given the estimated future increase in demand of in situ technologies, because of the awareness on the environmental impacts of ex situ techniques. In addition, vacuum extraction and thermal treatment of compounds appeared very promising. Finally, the knowledge on stimulating the hydraulic performance of natural fracture systems in clayey soils of glacial origin was greatly improved during STRESOIL elaboration.