At this active chemical production site in France, former activities have caused a cVOC contamination in soil and groundwater. Vertical migration of the cVOC contamination through the semi permeable first aquifer into the underlying chalk aquifer is a potential hazard.
Maximum soil contamination is found between 13,5-13,75 m-bgl with concentrations of 340 mg/kg of 1,2-DCA and 260 mg/kg of DCM. In addition, maximum groundwater contamination is detected at a depth of 8-19 m-bgl with concentrations of 435.000 µg/l of 1,2 DCA, 373.000 µg/l of DCM and 1.700 µg/l of VC. GreenSoil conducted a first a field test to investigate the feasibility of enhanced reductive dichlorination (ERD) to reduce the risk of spreading, and then implemented a full-scale system.
The main contaminants are known to be biologically degradable under anaerobic conditions. The selected remedial strategy consists out of:
a) stimulation of the biological anaerobic degradation of all VOCs by dosing Dehalo-GS (electron donor);
b) heating the recirculated groundwater. By circulation of groundwater, dechlorinating bacteria, electron donor and heated water, the contaminants will be mobilized and therefore become more bioavailable. Water is heated to about 25°C to stimulate biological activity based on microcosm tests that were done prior to the pilot test.
As the contamination was located within an ATEX-zone on an active chemical plant, specific safety measures had to be included within the work plan.
The pilot test provided important insights into the proposed bioremediation process. GreenSoil made several recommendations which were implemented in the full-scale project. These include:
· Using downhole pumps rather than surface pumps.
· Increasing well diameters.
· Improving sealing arrangements at injection wells.
· Relying on circulation of groundwater and dosing of electron donor.
Restricting the use of heating systems,particularly in deeper layers, as this can lead to scaling and clogging of the heater and requires additional power consumption for marginal gains in bacterial numbers and activity.
After positive results observed during the pilot, a full-scale remediation was installed.
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A fast and clear decrease of highconcentrations of DCM and 1,2 DCA was observed. Concentrations up to 200,000μg/l were degraded within 3 months. Increased concentrations of the degradationproducts ethene (up to 12,000 μg/l) and methane (up to about 7,000 μg/l),indicated successful biological degradation.
Concentrations of Chloromethane, a possible degradationproduct of DCM, were very low and often below detection limit, indicating thatit did not accumulate and was likely further degraded to methane or DCM wasdegraded via another degradation pathway.
Consequently, these results demonstrated the high efficiency of the biological anaerobic remediation approach with orwithout heating.
Molecular analysis also confirmed the presence and increase of the natural bacterial population that are most likely involved in the degradation of DCA and DCM.
Dehalococcoides, Dehalobacter and Dehalogenimonas were detected in high numbers up to 1,010 cells/l. Dehalococcoides increased significantly compared to amount measured before the pilot test. Most of the degradation took place before the heating of the groundwater was observed in-situ. Therefore the fast degradation is mainly attributed to the dosage of electron donor.
Although heating of groundwater was demonstrated during the pilot test, the additional effect on the degradation could not be fully assessed as complete degradation has preceded the heating process. These results demonstrated the high efficiency of the biological anaerobic remediation approach with or without heating.
Based on the positive results observed during the pilot, a full-scale remediation was installed. The full-scale system operated efficiently, and objective limits were reached within 2 years.
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