"Steam-Air Injection in fractured bedrock: completion of a CHC remediation at the site Biswurm (Villingen- Schwenningen, Germany)"On the site of a former incineration plant for liquid organic waste (CHC, BTEX) in Villingen-Schwenningen, Germany, a long lasting contaminant leakage caused a plume covering several hectares. The source zone extended over 2,800 m². The affected fractured sandstone aquifers were contaminated down to 37 m bgs. The upper platy sandstone comprising the groundwater fluctuation zone and the unsaturated zone contained the majority of contaminant mass.
The application of a thermally enhanced remediation using steam-air injection was selected to remove chlorinated hydrocarbons from a fractured sandstone aquifer. A pilot application was conducted in 2009 to determine the effectiveness of the technology prior to designing the full scale thermally enhanced remediation scheme .
The full scale remediation started in 2012 to treat approximately 43,000 m³ of sandstone and claystone. The site was divided into nine treatment sections. The duration of the steam-air injection phase (steam injection power of 400 kW) was determined to last 33 months. A total of 32 two-level injection wells and 37 soil vapour extraction wells (SVE) were installed on site. The total costs of the four years running remediation was estimated as 3.5 million EUR incl. tax.
The thermal enhanced remediation process is divided in two steps. First, a heating phase when the steam-air mixture propagates in the fractures and heats up the bedrock by conduction. During this phase the easily accessible contaminant mass is evaporated. Second, a thermal desorption phase where impregnated contaminants are slowly desorbed from the sandstone and claystone matrix.
In 2013 the concept of a compartment-wise treatment of the bedrock had to be adapted. Both, the effective heating time and the duration of the desorption phase were significantly longer as indicated during the pilot trial. The time demand was increased by 45%, the energy consumption by 35%. The heat propagation of up to 10 m in radius allowed the simultaneous treatment of two sections and in 2016 that of three sections. The steam and heat flux was less than 500 kW. This resulted in an effective doubling of the treatment time of a single section to seven months duration which provides the required desorption time. In addition the SVE was revised and extended by 50% to cover 4 5 treatment sections. Instead of the initially intended operation of 10 SVE wells and 4 5 two-level injection wells up to 45 SVE wells and 10 two-level steam injection wells were under operation.
Due to the steam front propagation and the groundwater containment the initially saturated zone below 11 m bgs. was drained. Thus, the target temperature could be minimized from 88 °C to 80 °C. The temperatures in the bedrock ranged between 80 88 °C.
Until the end of August 2016 in total 4,970 kg of CHC were removed from the site. 4,770 kg of CHC were removed by SVE and 200 kg CHC by the groundwater containment. Including the mass removal during the pilot study (560 kg), in total more than 5,300 kg of CHC were removed by the soil vapour extraction during the thermally enhanced remediation. The mean daily mass removal was 3.2 kg of CHC with a maximum of 20 kg per day from the sections 2 and 6. At the end of the steam-injection period less than 10 mg/m³ CHC was measured in the soil vapour, down from initial values above 2,000 mg/m³.
After 48 months of steam-air injection the remediation was successfully finished. The CHC emissions in the groundwater were below 10 g/d. The latest groundwater analysis indicate a reduction of the CHC content by 99.4% from 3,905 µg/L CHC to 18 µg/L. The remediation target was an emission of 20 g/d.
After 7 months of cooling the average temperature was below 40°C, and the emission was 6 g/d. In May 2017 the monitoring phase started for at least 12 months by operating a low-flux SVE and a groundwater treatment to further cool the aquifer and control the low emission from the warm bedrock in the unsaturated zone.
Due to the irregular fractured system the duration of the desorption of the contaminants was prolonged by 35%. The remediation procedure was to be adopted and optimized several times. In total the state of Baden-Wuerttemberg and the town Villingen-Schwenningen will have spent 4.4 Mio EUR for the remediation. This is an increase of the estimated cost of 3.5 Mio EUR in 2012 by 26%.