Oprensning af klorerede opløsningsmidler ved stimuleret reduktiv deklorering

Cleaning of Chlorinated Solvents by Stimulated Dechlorination

Summary and conclusions

This report presents results of stimulated reductive dechlorination of a contamination with chlorinated solvents. The contamination originates from a former dry cleaning facility situated on Jægersborg Allé 24, Gentofte. The dry cleaning facility was terminated around 1983. During this period a new dry cleaning facility was opened on the nearby address; Trunnevangen 2. On both locations contamination has been subsequently detected. The contamination has spread to a regional secondary aquifer on both locations. The primary constituent of the dissolved part of the contamination is PCE, and the two plumes are expected to comingle. The most significant contamination originates from the former dry cleaning facility on Jægersborg Allé 24. The plume length is more than 200 meters, and the plume width is 50 meters. The source area is treated by airsparging and vacuum venting.

The test field, where the pilot test has been carried out, is situated app. 100 meter downgradient of the former dry cleaning facility, and includes a part of the plume in the secondary aquifer.

Geology and hydrogeology
In the test field, a layer of 1-4 meters of fillings or moraine till is detected below the ground surface. Underlying this layer, sand is found until 17 meters below grade. At this depth, a layer of moraine till is found. The aquifer is connected with the layer of sand. Aquifer conditions change from semi-confined to confined in the test field. Groundwater velocity is app. 190 m/year.

Contamination and redox conditions
The contamination primarily consists of PCE and to a lesser extent of TCE. The highest concentrations are seen in the upper aquifer (app. 500 µg/l), while lower concentrations are found in the lower aquifer (app. 50 µg/l). No significant levels of daughter products like cis-DCE are detected. The natural redox conditions are aerobic in the upper aquifer and weakly anaerobic in the middle- and lower aquifer.

Substrate injection
App. 1,800 kg of HRC and HRC Primer where injected around May 1, 2001. The primary constituent of the substrates is lactate (lactic acid), and both types of substrate where donated by the American company Regenesis. The substrate was injected through nine delivery points in two rows perpendicular to the ground water flow direction. The injection was performed using a Geoprobe rig, connected to a strong piston-pump, mixing vessel and control unit. Injection was performed using an "open tip", where substrate is delivered through an open hole at the end of the drilling rods, while these were withdrawn.

Monitoring
Prior to and after the injection, ground water conditions with respect to substrate, geochemistry and contamination were monitored. The monitoring programme included all 10 wells (each installed with screens in three depth intervals). Part of the monitoring included monitoring events with groundwater sampling of all screens, while other parts of the monitoring included groundwater sampling of selected screens in time series.

Results
The vertical distribution of substrate is uneven, and the main part of substrate is found in the lower part of the aquifer. Thus, subsequent attention has been on the groundwater chemistry in the lower part of the aquifer. The effect of addition of substrate on the redox conditions has been quite evident, as a rapid use of nitrate and sulfate is seen in the groundwater as well as production of iron(II) and later methane. Apparently ideal conditions for reductive dechlorination has been established in some parts of the lower aquifer, for a longer period of time. However, results show that the expected removal of PCE and production of chlorinated daughter products is not achieved. Absence or low activity of the appropriate microorganisms and length of the period where ideal redox conditions were present, may be the reason.

Economy
By stimulated reductive dechlorination, groundwater chemistry is altered from existing conditions, to conditions ideal for the processes to appear. The required change can be quite significant, depending on the initial conditions, e.g. from oxidised redox conditions to strongly reduced redox conditions. Thus, the amount of competing electron acceptors, which has to be removed, is crucial for the amount of substrate needed – not the degree of contamination. Less than 10% of the substrate is typically used for the actual metabolism of the chlorinated compounds.

The amount of substrate needed is crucial to the economic calculations. The aquifer volume that has to be treated is also of importance to the economy, as large savings can be made by treatment of large volumes. The calculated price of treatment of one cubic meter of groundwater is app. DKK 150. The price should be seen as initial construction costs, i.e. the price for establishment of a grid of wells and the development of ideal redox conditions in the aquifer. Operating costs for sustaining the ideal redox conditions will be much less.

Conclusion
The pilot test has provided detailed knowledge about the hydraulic conditions, groundwater chemistry and contamination in the test field. Establishment of a test field, where the effect of addition of HRC and HRC Primer can be investigated, has been successful.

Conclusions of greatest significance are:

Unsuccessful vertical distribution of substrate. The uneven distribution is primarily due to the injection method in relation to the geology.
In areas with substrate, strongly reduced redox conditions are seen. In several areas redox conditions are presumably ideal for reductive dechlorination.
No sign of reductive dechlorination has been observed, as no decline in PCE concentration or formation of daughter products (e.g. cis-DCE) is seen.
There can be several explanations why reductive dechlorination did not take place, but most likely the cause is the lack of activity or the appropriate types of indigenous microorganisms.

Perspectives
The pilot test shows that it is possible to create reduced redox conditions in the groundwater by the addition of substrate. A wide variety of substrates are expected to be able to do this. Initiating reductive dechlorination requires the presence of the appropriate microorganisms and the appropriate redox conditions over a longer period of time. The potential for stimulated reductive dechlorination, as a remedial technique, is assessed to be substantial. It is an in-situ technique with a minimum energy requirement, and the technology takes advantage of processes, that are naturally occurring in groundwater.

Comments
Regenesis, that donated the HRC, has summarised general comments to the report. These comments are reproduced in appendix M, along with some general economic estimates of remediation based on HRC in comparison to other remediation techniques.