General information

The groundwater contaminated with chlorinated solvents is treated by enhanced anaerobic bioremediation.

In addition to standard monitoring, specific monitoring tools (DNA, hydrogen field measurements and stable compound specific isotopes) are being applied in order to redirect the remediation if necessary. 

Full report available in Dutch (see section Conclusion and Recommendations)

Research Objective
The aim of this HIP pilot is to show how specific monitoring tools are being used to determine the degradation of the chlorinated solvents, and to study the effect of the stimulation of the degradation.
Study
Past activities at a dry cleaner have caused contamination of the groundwater with chlorinated solvents in the shallow and deeper groundwater. The contaminated plume has spread underneath adjacent private properties and forms a threat for the extraction of drinking water atabout 300 m distance from the source area. The groundwater contamination is treated by enhanced anaerobic bioremediation. A mixture of lactic acidand sodium acetate is injected through three injection screens and distributed in the groundwater by extracting groundwater from several wells downstream of the injection screens. In addition to standard monitoring, specific monitoring tools (DNA, hydrogen field measurements and stable compound specific isotopes) are being applied in order to redirect the remediation if necessary.
Methods
The progress of the remediation is followed daily by measurement of the groundwater level via telemetrie.The degradation processes are followed by routinely sampling of the groundwater and analyses of thechlorinated solvent concentrations and their degradation products. The used specific monitoring tools include hydrogen analyses in the field, compound specific stable isotope analyses and molecular analysesto detect dechlorinating bacteria. Groundwater samples of the source and of the plume area are used.
Results
Hydrogen measurements are performed directly at the site and show whether the conditions are suitable fordechlorination. Hydrogen is the compound that is directly used in the dechlorination reactions and istherefore much more specific than DOC or fatty acid analyses to predict if dechlorination is taking place.Hydrogen measurements are often used at sites to demonstrate if Natural Attenuation takes place. This isnot relevant for this site, as stimulated measures had already started. However, the hydrogen measurementsshowed that that present DOC value of 20 mg/l was not sufficient to give high enough hydrogenconcentrations, and that the added electron donor gave sufficient hydrogen concentrations for thedechlorination reactions.At the site in The Hague the hydrogen concentrations showed a decrease in the plume to concentrationsbelow 1 nM, followed by an incomplete dechlorination. Higher hydrogen concentrations are detected inthe source, and correlate with a high degree of dechlorination. When lower hydrogen concentrations areseen, this is also reflected in a lower degree of dechlorination. Infiltration of additional electron donor willresult in further dechlorination. Compound specific stable isotope analyses show if a compound is degraded. For assessing NaturalAttenuation, this shows the degradation potential at a site. For stimulated bioremediation, isotopefractionation proofs that decreasing concentrations are a result of degradation, and not of other processeslike transport, dilution, etc. Fractionation of cis-dichloroethylene (CIS) is shown at the site in The Hague, especially in the sourcezone. This indicates that CIS is degraded and this is in line with an increasing dechlorination degree.Decreasing CIS concentrations are found in the plume, but isotope data do not show fractionation of CIS.This indicates that the decrease in CIS concentration is not due to degradation. Also the fluctuating VCconcentrations in the plume are not caused by microbial degradation, but due to other processes. Molecular analyses: During the first 6 months a mixture of electron donor and Dehalococcoides spp. was injected at the site,and molecular analyses were used to follow the growth and transport of the bacteria.An increase of the amount of bacteria were found, especially where fractionation of CIS and elevatedhydrogen concentrations were observed. The amount of bacteria has decreased in the plume, most likelydue to unsuitable conditions for dechlorination, as shown by the hydrogen concentrations. Stimulation ofthe dechlorination can be solved by infiltrating electron donor. If this has no effect, bacteria need to be added as well, and the successful growth of bacteria can be followed by the molecular analyses.
Conclusions and Recommendations
The following research questions have been answered: How can specific monitoring tools (hydrogen analyses, stable isotopes, molecular analyses) support thedemonstration of an effective degradation of the chlorinated solvents? Hydrogen analyses showed high concentrations in the source zone, indicating that sufficient electrondonor is present. This electron donor is needed for the dechlorination reactions. Low hydrogenconcentrations were measured in the plume, indicating a lack of electron donor. The naturally presentcarbon did not produce sufficient amounts of hydrogen (despite high DOC concentrations) and theadded electron donor in the source could not reach the plume   Various groundwater extraction systems in the area made it difficult to use concentrationmeasurements to demonstrate degradation, due to an irregular groundwater flow. Compound specificstable isotope data showed that the decrease in concentration at the site was due to degradation,especially in the source.   Dechlorinating bacteria were detected by molecular techniques in both the source and plume area.Supplying sufficient electron donor will further stimulate the growth of these bacteria anddegradation of the chlorinated solvents in the plume.    Which monitoring tools are most suitable to control a remediation? Routine analyses such as redox chemistry and concentration measurements of the contaminants areneeded to evaluate a bioremediation. In this project, the specific monitoring tools were very helpfulto understand the processes at the site. Conclusions from the three specific tools demonstrated a gooddegradation in the source and less advanced degradation in the plume, and the three tools showedcomparable results. Specific monitoring tools are mainly used for special conditions, like a stagnating remediation,

fluctuating groundwater levels, toxic effects for bacteria, etc.

Attached reports: indutch_reportpilot2
Related techniques and cases
Related cases: , , , , , , , , , , , , , , , , , , , ,