On-site biostimulation evaluation of a remediated perchloroethene-contaminated acquifer located in Jablunkov, Czech Republic

Chlorinated ethenes solvents (CEs) are one of the most common contaminants identified in groundwater ecosystems. Long used as a degreaser in industry, perchloroethene (PCE) and trichloroethene (TCE) are particularly persistent in the environment as recalcitrant to oxic degradation, and classified as unhealthy and dangerous for the environment. Under anaerobic conditions, CEs can be degraded naturally in the environment by organohalide respiring bacteria (OHRB) through dehalogenase enzymes reducing them into ethene, a non-hazardous end product. However, toxicity of the intermediate daughter compounds of the degradation (TCE, DCE and VC) is higher than PCE. As this natural attenuation is a slow process, intermediate compounds accumulate in the environment and biostimulation is often used to enhance the complete degradation of CEs. Substrates are added to the contaminated aquifer and used by fermentative bacteria releasing by-product such as H2 that is essential for the dehalorespiration of CEs. Biostimulation has been studied in a PCE contaminated-aquifer located in Jablunkov (Czech Republic). Cheese whey (CW) has been injected into the groundwater in November 2012 and June 2013. The objective of the current project is to evaluate the efficiency of the CW applications by analysing microbiologic and physico-chemical parameters describing the aquifer at six sampling dates from September 2012 to December 2013 within six wells allocated over the aquifer. Global bacterial community structures were analysed in laboratory using Polymerase Chain Reaction (PCR) and T-RFLP (Terminal Restriction Fragment Length Polymorphism). OHRB were analysed as well by specific PCR targeting 16S rRNA and dehalogenase genes of bacteria known to degrade completely the CEs. Then, contaminants and ethene concentrations as well as others Terminal Electron Acceptors implicated in the respiratory processes (Terminal Electron Accepting Processes – TEAPs) were measured to give information about the redox conditions within the aquifer. Finally, others hydrogeological parameters such as pH and temperature were measured. All data were analysed using Multiple Factor Analysis (MFA) in the R software. Interpretations provided a global model indicating the characteristics and functioning of the aquifer. The successive CW applied on-site have a gradual impact on the environmental conditions and on the structures of bacterial communities. After the two CW applications, the aquifer has partly changed to conditions favourable for complete dechlorination. It was the case for three wells over six. Two CW injections have been necessary to allow the production of ethene for two wells whereas only one was needed for the other one. Biostimulation were efficient but not sufficient to completely degrade the CEs on the whole aquifer. In one well, the accumulation of DCE were more important than the production of ethene. In two other wells, ethene was not produced but the statistical analysis showed that they tend toward complete dechlorination conditions. New CW applications would be necessary to completely remediate this aquifer.