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Institut für Wasser- und Umweltsystemmodellierung - IWS



"Steam-Air Injection in fractured Bedrock: Results and Lessons Learned of a CHC-Remediation at the Site Biswurm (Villingen-Schwenningen, Germany)"

In situ thermally enhanced remediation methods (steam-air injection and thermal wells), developed and verified in several field applications by the Research Facility for Subsurface Remediation, VEGAS, are proposed for source zone remediation of NAPL in the saturated and the unsaturated zone. Contaminants can be effectively removed within several months if hydrogeological conditions are suitable.

A novel application of steam-air injection was the removal of chlorinated hydrocarbons from a fractured sandstone aquifer. In 2009 VEGAS successfully completed a pilot trial of steam-air injection for the problem owner, the city of Villingen-Schwenningen in south-west Germany [6]. The test field extended over 2,000 m³ of fractured rock including the upper of two different aquifers. The thermal radius of steam propagation was 5 m in the target zone between 3 – 15 m bgs. The heating period by steam-air injection lasted for 19 weeks. Contaminants that had penetrated into the sandstone matrix were thermally desorbed during the conductive heating of the bedrock due to steam-air was flowing through the fractures. More than 91 % of the total extracted mass (560 kg chlorinated hydrocarbons, CHC) was removed from the groundwater fluctuation and unsaturated zones via the soil vapour extraction system, less than 6% thereof (34 kg of CHC) via the groundwater containment. The CHC values in the soil vapour and the groundwater were decreased by 95% and 85%, respectively. Based on the results of the pilot, the steam-air enhanced remediation of the groundwater fluctuation zone and of the unsaturated zone (2,900 m², 15 m thickness) for the entire site was designed. The site was divided into nine treatment sections in size of 400 – 600 m², meaning 4,500 – 6,000 m³ of fractured bedrock each. The duration of the steam-air injection phase (steam injection power of 400 kW) was calculated to last 33 months requiring 31 two-level injection wells and 34 soil vapour extraction wells (SVE). The costs were estimated to 2.6 million EUR to treat ca. 43,000 m³ of sandstone during four years of operation.