Miljøprojekt, 1146; Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Forundersøgelser til pilotprojekt om stimuleret reduktiv deklorering

Forundersøgelser til pilotprojekt om stimuleret reduktiv deklorering

Summary and conclusion

This report describes the results of the preliminary investigations for the application of the remediation technique "enhanced in situ reductive dechlorination". The preliminary investigations were carried out during 2004 and 2005 at 234 - 238 Rugårdsvej in Odense with the view to assessing whether the site is suitable for testing the remediation technology in a subsequent pilot project.

The investigations are linked to a secondary aquifer (the intermediate secondary aquifer) at a depth of approx. 10 - 14 m. This sand stratum is heavily contaminated with chlorinated solvents. If possible, the pilot project is to be carried out as an active remediation system, i.e. with continuous pumping and reinjection of ground water.

The overall objective of the technology project is to clarify the applicability of enhanced in situ reductive dechlorination as remediation technology in relation to Danish sites contaminated with chlorinated solvents.

A sub-objective of this report is to investigate whether the intermediate secondary aquifer at 234 - 238 Rugårdsvej can be used to test enhanced reductive dechlorination as a remediation technology in a subsequent pilot project.

The results of the investigation show that the site is well suited as a test site to test enhanced reductive dechlorination as a remediation technology:

In the source area and downstream, the typical concentration of chlorinated solvents is between 1,000 and 20,000 µg/l, which is considered optimal in relation to a pilot project.
The groundwater is reduced with negative redox potential, low contents of oxygen and nitrate, moderate to high contents of dissolved iron, moderate content of sulphate and minor content of methane. The redox conditions are assessed favourable for the enhanced reductive dechlorination, but the sulphate will probably have to be reduced, before dechlorination can be carried out.

The geochemical conditions show neutral pH, and moderate contents of sulphate, bicarbonate, calcium and other inorganic constituents. It is assessed that the geochemical conditions will create no problems.

The redox conditions and the geochemical conditions are typical for eastern Danish groundwater, which makes the location well suited with a view to applying the results to other Danish sites.
Dehalococcoides and ethene are present in the intermediate secondary aquifer, which indicates that there is a potential for reductive dechlorination in the intermediate secondary aquifer. Dehalococcoides are, however, only found in low concentrations. The investigations indicate that ethene is formed in the upper moraine clay and that no significant biodegradation of cis-DCE and VC in the intermediate secondary aquifer takes place at present. This has, however, not been fully documented.
The hydrogeological investigations and the ground-water modelling show that - as desired - an active system can be applied, i.e. with pumping and reinjection of groundwater. The test area of the pilot project is the intermediate secondary aquifer at a depth interval between 10 - 15 m below surface. The test area corresponds to an area of a width of approx. 20 m and a length of approx. 40 m (approx. 800 m²). The contents of chlorinated solvents in the test area vary between approx. 100 and 20,000 µg/l.
It is proposed to use lactate as the electron donor. Lactate is easy to handle in the field (no explosion risk); it is well documented in field investigations, and the laboratory tests show that when this fermentable organic substrate is used, there is a complete degradation to ethene when the bacteria culture is added.
The laboratory tests showed that it is necessary to add a dechlorinating bacterial culture (e.g. KB1^TM), if the objective is to achieve reasonable results in the pilot project within the expected timeframe of approx. 6 months.

It is proposed to carry out the remediation project at the site with continuous pumping from one extraction well (AV1) and reinjection of groundwater in three injection wells (I101, I102 and I103). An average pumping of 3 l/min., corresponding to approx. 4-4.5 m³/24 hours, is proposed. The extracted groundwater is mixed with lactate prior to injection. After approx. one month with donor added, the dechlorinating culture (KB1^TM) will be added. According to model calculations, the retention time will be approx. 40-50 days. During a test period of approx. 6 months, reinfiltration of approx. 3 void volumes in the test cell is expected.