Authors:
Oliver Trötschler | VEGAS, University of Stuttgart | Germany
Hans-Peter Koschitzky | University of Stuttgart, VEGAS | Germany
Bernd Lidola | Stadtbauamt Villingen-Schwenningen | Germany
Isabell Kleeberg | Stadtbauamt Villingen-Schwenningen | Germany
Stefan Schulze | GEOsens | Germany
On the site of a former incineration plant for liquid organic waste contaminants (CHC, BTEX) percolated in the underlaying aquifer systems. During the demolition of the plant and the excavation of the top soil about 1,600 kg of CHC were removed. The remaining source zone covers an area of approx. 2,900 m² and a depth of 37 m in the affected sandstone aquifers. The corresponding plume extended over several hectares. A detailed site investigation estimated a total mass of 10 to 50 tons of CHC in the sandstone aquifers. The concentration of CHC in the groundwater ranged from 1 mg/l in the saturated zone to up to 40 mg/l in the surface water drainage system (6 m bgs.). The content of CHC in the soil vapour was up to 4 g CHC per m³ in the source zone. The upper zone of the sandstone aquifer (15 m bgs.) and the unsaturated zone contain the majority of contaminant mass.
In 2009 VEGAS accomplished a pilot trial of steam-air injection on demand of the problem owner, the city of Villingen-Schwenningen. The heating period by steam-air injection lasted for 19 weeks. The test field extended to 2,000 m³ of fractured rock including the upper aquifer. The thermal radius of steam propagation was 5 m in the target zone between 3 – 15 m bgs. Contaminants that had penetrated into the sandstone matrix were thermally desorbed during the conductive heating of the bedrock. More than 91 % of the total extracted mass (560 kg CHC) was removed from the groundwater fluctuation and unsaturated zones via the soil vapour extraction system, less than 6% thereof via the groundwater containment (34 kg of CHC). 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 4,500 - 6,000 m³ of fractured bedrock. 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 (SVE) wells. The costs were estimated to 2.6 million EUR to treat 40,000 m³ of sandstone during four years of operation.
The steam-air driven remediation started in July 2012. The remediation is currently under full operation and the steam injection phase will last until September 2015. The steam-air mixture is subsequently injected by means of four to five dual-screened wells and 300 – 500 kW to heat up the single treatment sections. There are 10 – 12 soil vapour extraction wells surrounding the injection wells in each section.
Until the end of November 2014 (simultaneous operation in sections 5 and 6) the total CHC mass removal summed up to approximately 3,500 kg. Approximately 3,400 kg CHC were removed by SVE and 90 kg CHC by the groundwater containment, respectively. During the ongoing thermal in-situ remediation the values in the downstream plume range between 200 – 300 µg/L and a CHC mass flux of 70 g/d. The goal of the remediation is to fall short of a maximum emission of 20 g CHC daily.
During the remediation the heating strategy had to be adopted since the desorption process of the CHC from the sandstone bedrock is slower than determined during the pilot study. The time demand will be increased by 35%, the energy consumption by 25%. Since the heat and steam propagation is wider than expected two sections can be heated simultaneously while SVE is extracted from 3 – 4 sections. Therefore the SVE system was extended to ensure pneumatic control. The energy is effectively stored in the bedrock. During desorption phase the average temperature exceeds 88°C in 5,000 tons of heated sandstone. By the end of November 2014 the steam-air mixture was injected via 16 injection wells and approximately 800 m³/h of soil vapour was extracted by means of 45 wells. In total 13 m³/h of groundwater was extracted and treated.
Transport and displacement of evaporated contaminants in the fractures was observed. The monthly depths-specific monitoring of the SVE-wells indicates an almost complete removal of the contaminants from the upper sandstone and the claystone layers between 3 – 8 m bgs. CHC contents are less than 10 mg/m³. In some parts of the lower platy sandstone CHC contents of 100 – 300 mg/m³ were detected after thermal treatment. This indicates a mass removal of more than 90%.
The duration of the cooling time is expected to last for 6 -8 months while contaminants might still be evaporated from the sandstone.
The presentation will show the development of the current remediation and focus on the lessons learned using in-situ thermal treatment by steam-air injection in fractured rock, which is the first application of this technique in Germany.
The environmental agency of Baden-Württemberg (LUBW), the regional council (RP Freiburg) and the community of Villingen-Schwenningen support the application of a thermally enhanced remediation of the site by steam-air injection. The pilot project was carried out with financial support of the Helmholtz Centre for Environmental Research, UFZ, Leipzig, the community of Villingen-Schwenningen and the regional council and the State of Baden-Württemberg. The local consultant GEOsens assisted the pilot study and is the leading consultant for the remedy, the scientific supervision for the pilot and the remedy is the responsibility of VEGAS. The remediation company is Bauer Umwelt GmbH.