Formatted Title

The Rapid Full-Scale Remediation of Chlorinated Ethenes in Challenging Geology Applying Pneumatic Fracturing and Waste Iron Slurry

Background/Objectives

The full-scale remediation of a low-permeability aquifer contaminated with chlorinated ethenes was performed at the Duchcov site, Czechia. The project received co-funding from the EU Operational Programme Environment and, due to its rules, it had to be completed by September 2023. The timespan of the remediation process was, therefore, a mere 26 months.

A factory for the production of a wide range of products including prams, scooters and tricycles, operated in Duchcov from 1907 to 1989. The primary sources of contamination in the local quaternary aquifer, which is characterized by low permeability, comprised electroplating and degreasing facilities. The contamination consisted primarily of toxic metals and chlorinated ethenes. In the initial stage, it was difficult to determine the spatial distribution of the soil and groundwater contamination due to the presence of heaps of rubble from the demolition of buildings, which complicated the comprehensive investigation of the site. However, the limited data available indicated the existence of DNAPL chlorinated ethenes captured in low-permeability sediments, which ranged from solid claystone and plastic clays to fine sands with a content of clay. The aquifer itself had hydraulic conductivity within the range 0.2 to 2.4 m/day, which presented significant challenges in terms of the remediation process.

Approach/Activities

The initial investigation stage involved soil gas and phytoscreening sampling. This was followed by the conducting of a limited MIPHPT investigation. The results subsequently allowed for the identification of the contamination sources and the removal of the relevant obstructing rubble. Subsequent MIHPT and core sampling resulted in the complex delineation of the source zones, which enabled the excavation of the contaminated unsaturated zone and the targeted injection of the remediation agents.

The remediation technology consisted of ISCR in conjunction with ERD. This was supplemented by the use of so-called FRAC-IN technology, which is based on direct push drilling, pneumatic fracturing and the hydraulic emplacement of the remediation agents. A mixture of zerovalent iron materials ranging from waste iron chips to nanoparticles was used to promote the ISCR process and glycerol was applied to enhance the subsequent biodegradation process. A total of 106 FRAC-IN injection points were used to introduce over 90 m³ of the remediation slurry, which comprised 10 t of waste cast iron, 4.2 t of microscale and 100 kg of nanoscale ZVI, and 123 m³ of the glycerol solution with a concentration of 50 g/L.

Results/Lessons Learned

Over 350 kg of chlorinated ethenes and more than 210 kg of Cr(VI) were successfully removed with the excavated soil. The DNAPL in the source zone was eradicated, and the groundwater contamination continues to evince a decreasing trend thanks to the robust activity of the degrading bacteria and abiotic reduction. These natural processes will continue for several years, thus further reducing the contamination level without the need for additional active intervention. The remediation procedure was successful, and the owner of the property can now proceed with construction at the site.

The use of innovative site survey methods in the various stages of the remediation process facilitated the rapid identification of the sources of the contamination and their precise delineation, thus resulting in highly-effective excavation with no contamination remaining in the unsaturated zone. The three targeted FRAC-IN injection campaigns yielded excellent results and allowed for substantial savings in terms of both financial and environmental resources, as well as a reduction in the remediation time. The findings of the study underscore the effectiveness of employing innovative methods as an optimal solution for the remediation of complex sites with challenging geological conditions.