"Steam-Air-Injection in fractured bedrock: Experience and lessons learned from a CHC contaminated site"Summary and outlook
On the site of a former incineration plant for liquid organic waste (CHC, BTEX) in Villingen-Schwenningen, Germany, a long lasting contaminant leakage causes a plume covering several hectares. The source zone extends over 2,800 m². The affected fractured sandstone aquifers are contaminated down to 37 m bgs. The upper platy sandstone comprising the groundwater fluctuation zone and the unsaturated zone contains the majority of the contaminant mass.
The application of a thermally enhanced remediation using steam-air injection was a new approach 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 steam and heat propagation extended to 5 m in radius from the injection well. During the three months of operation approximately 560 kg of CHC were removed from about 2,900 m³ of fractured sandstone. A mass removal of more than 90% was indicated from the development of contaminant concentration in the soil vapour.
After a phase of planning and a request for tenders and commissioning in summer 2012 the full scale remediation started to treat approximately 43,000 m³ of sandstone and claystone. The site is divided into nine treatment sections. The duration of the steam-air injection phase (steam injection power of 400 kW) was calculated 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 will be 4 million EUR incl. tax.
During the remediation of the northern sections 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 than indicated during the pilot trial. The time demand was increased by 35%, the energy consumption by 25%. The heat propagation of up to 10 m in radius allowed the simultaneous treatment of two sections at one time. 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 more than 7 months duration which provided the required desorption time. In addition the SVE was revised and extended by 50% to cover 4 – 5 treatment sections simultaneously. Instead of the initially intended operation of 10 SVE wells and 4 – 5 two-level injection wells there are currently 40 SVE wells and 10 two-level steam injection wells under operation.
Until the end of May 2016 approximately 4,900 kg of CHC were removed from the site after the treatment of approximately 60 – 70% of the cubature. 4,700 kg of CHC were removed by SVE and 200 kg CHC by the groundwater containment. Including the mass removal during the pilot study, so far more than 5,400 kg of CHC were removed by the soil vapour extraction during the thermally enhanced remediation.
Despite the described difficulties/challenges, the effectiveness and applicability of the technology to remediate a CHC contaminated fractured sandstone and claystone was proven. Due to the irregular fractured system the duration of the desorption of the contaminants was prolonged by about 35% - 40 %. The remediation procedure was to be adopted, affecting both, cost and time. It is planned to finish the steam-air injection in July or August 2016 after 48 (49) months of operation.