An industrial site in the port of Tarragona (Spain) is contaminated with mainly chlorinated ethenes, chlorinated methane and chlorinated ethane. The contamination was caused by leakages of the sewer system in the past. High concentrations (DNAPL) of mainly tetrachloroethene (PCE) and chloroform are present as well as degradation products. The remediation goals are 37 µg/l and 6.5 mg/kg for PCE in groundwater and soil, respectively and 440 µg/l and 87 µg/l for Chloroform and Vinylchloride (VC), respectively.
Given the nature of the contamination and aquifer, anaerobic reductive dichlorination was identified as the most promising in-situ bioremediation solution. To test the feasibility of enhanced reductive dechlorination (ERD) to remediate the site both laboratory and a field tests were first conducted. Based on the successful results of these tests, a full-scale remediation is being carried out. Since the site is situated close to the sea, the groundwater is brackish to salt with an electrical conductivity (EC) up to 25,000 µS/cm and sulphate concentrations up to 1,600 mg/l.
The remediation consists of circulation of groundwater via extraction and infiltration wells and adding electron donor to stimulate the process of ERD. DNAPL was removed from the extracted water above ground. By circulating groundwater, the electron donor and dechlorinating microorganisms are distributed, and the contamination is being mobilized as well. Besides regular monitoring of geochemical parameters and the concentrations of contaminants also molecular analyses (qPCR and NGS) were performed to have a better understanding of the microorganisms and degradation pathways involved under these specific conditions.
In addition, to investigate whether the mobilization and subsequentially extraction of residual DNAPL could be enhanced, a push-pull test was performed by using a surfactant solution. In accordance with the consultant, it was decided to use a 5% surfactant solution.
Based on the promising results obtained, a field test was subsequently carried out. The surfactant solution was circulated within the groundwater circulation system together with the electron donor.
Concentrations as high as 100,000 µg/l of PCE,41,000 µg/l of VC and 340,000 µg/l of Chloroform were found on site. After about six months, degradation products had increased in concentrations significantly and after about 12 months, ethene increased significantly from below detection limit up to 26,000 µg/l.
Molecular analyses of groundwater were performed and confirmed that there is a strong increase in the number of bacteria and enzymes involved in dichlorination. In addition, the gene encoding for chloroform reductive dehalogenase (cfrA) increased dramatically from below detection limit up to 8.9E+7 copies/l.
The DNA from the well with the highest number of both vcrA and etnE was partially sequenced (ORVIdecode). The dominant genus of the bacterial community consisted of Dehalococcoides (27%) of which 70% was closely related to Dehalococcoides mccartyi. The surfactant tests revealed a clear DNAPL (PCE and chloroform) mobilization. Thus, TCE and Cis are more likely degradation products of the PCE that had become bioavailable. Concentrations of VC and ethene did not increase significantly, probably due to the sudden increase of PCE and chloroform. The results also suggested that the infiltration of the extracted water did not spread outside of the infiltration area. Molecular analysis also demonstrated an increase of viable cells, indicating that microbial activity increased after surfactant supply.
To date in between 7,500 and 21,640 kg of contaminant mass was removed. Contaminant mass removal has been calculated by molecular analyses of Geobacter and Dehalococcoides and degradation rate testing.
The biological degradation capacity was significantly increased as a result of the dosage of electron donor (Dehalo-GS) and the circulation of groundwater.
The results prove the complete reductive de-chlorination of chlorinated ethenes regardless of the high concentration levels of chloroform and brakish/saline conditions of the groundwater. After 2 years, the active remediation system at the outsider border of the source area could be switched off. The surfactant tests performed at the site demonstrated the enhanced DNAPL mobilization as it is further applied in the ongoing remediation of the source area. After 5 years theactive remediation was ended, and is currently in a monitoring phase.
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