Summary and conclusions

Erfaringsopsamling for reduktiv deklorering som afværgeteknologi i moræneler

Stimulated reductive dechlorination has been used as remediation technique at several Danish locations contaminated with chlorinated solvents (primarily chlorinated ethenes) and good results have been obtained from remediation of sandy aquifers. Remediation of low permeability deposits, such as clayey till, is a more complex process as the low permeability of the sediment limits the spreading of electron donor and bacteria. In attentive report existing knowledge is gathered from Danish sites remediated using biostimulation and bioaugmentation of anaerobic reductive dechlorination in clayey till. The information is compiled to get an overview of the controlling processes in relation to transport of chlorinated compounds, electron donor and bacteria in clayey till.

Locations where the primary contamination consists of chlorinated compounds in clayey till is included in the study. 14 locations have been identified: at 3 locations full scale remediation is in progress (Gl. Kongevej 39, Vesterbrogade and Sortebrovej), pilot scale investigations have been carried out in the clayey till at 1 site (Rugårdsvej ML), full scale remediation in the underlying aquifer has taken place at 1 site (Svendborg), pilot investigations have taken place in the underlying aquifer at 3 locations (Gl. Kongevej 33, Flensborggade, Hellestedvej and Rugårdsvej S) and at 5 locations no remediation has been started (Vasbyvej, Baldersbækvej, Høje Tåstrupvej, Tværvej and Middelfartvej).

Geology and hydrogeology

The geological description of the locations is often very coarse. This carries over to descriptions of flow in and between the geological layers, where weight is placed on the horizontal flow in water-bearing layers rather than the vertical flow to these layers. At contaminated clayey till sites it is generally necessary with more detailed descriptions of clayey till types and the presence of fractures than what has been standard practice until now, since these parameters have great influence on contaminant transport and hence contaminant flux to underlying permeable layers (aquifers).

Contaminant distribution

The contaminant mass is of great importance in relation to the time perspectives of a remediation effort. Therefore, it is critical to make a good mass estimation. Often, mass calculations are coarse as they are estimated from concentrations averaged over an entire contaminated area rather than smaller sections. Furthermore, mass balances are presently associated with great uncertainties because of the expected presence of free phase contaminant as pools (mobile) or residual phase (immobile) at a number of sites (Gl. Kongevej 39, Flensborggade and Vasbyvej). To be able to improve mass estimates in the future it is necessary to divide contaminated areas into intervals/sections. Furthermore, it is necessary to establish more methods or procedures to evaluate and determine the amount of free phase present at a site so this can be incorporated in mass calculations.

Investigations prior to remediation

Prior to establishment of a full scale remediation with biostimulated and bioaugmented anaerobic reductive dechlorination, investigations are made to gain knowledge of degradation conditions and the possibility of adding electron donor and bacteria (Gl. Kongevej 39, Vesterbrogade, Sortebrovej and Rugårdsvej). It varies between sites which investigations are conducted to look into degradation conditions: analysis of degradation products, isotope fractionation, number of specific degraders (Dehalococcoides), treatability studies and pilot scale degradation tests. Upon comparison of the results of each of these types of investigation, practically the same outcome is found. In terms of time, treatability studies take significantly longer to conduct than the other tests. Therefore, it should be considered whether this can be omitted if the contamination in question consists solely of chlorinated ethenes. If a mixed contamination exists, with e.g. chlorinated ethanes or Freon compounds, it is still necessary to conduct treatability studies as these components can inhibit degradation.

To investigate the possibility of adding electron donor and bacteria, injection tests have been conducted in conjunction with pilot scale degradation tests. At Rugårdsvej ML and Sortebrovej hydraulic fracturing has been carried out, and at Vasbyvej pneumatic fracturing has been carried out. The fracturing tests were conducted to investigate if it is possible to increase fracture density and thereby decrease the distance between reaction zones in the clay matrix. The distributions achieved via hydraulic fracturing were not as expected, and the fracture density was not increased enough to overcome diffusion limitations via pneumatic fracturing.

Electron donor was injected via GeoProbe at Gl. Kongevej 39 and Vesterbrogade. At Rugårdsvej ML electron donor was injected under pressure, while gravitational injection was carried out at Sortebrovej. Injection was successful using all methods. Certain limitations in distribution are present, however, and distribution is difficult to predict and expensive to investigate.

Remediation

Full scale remediation has only been implemented at three sites: Gl. Kongevej 39, Vesterbrogade and Sortebrovej. The remediation efforts have started within the last two years, which means that they are still in the early stages of remediation. The effects of biostimulation and bioaugmentation have therefore until this point only been investigated in the water phase. Thus, knowledge of the development in the matrix exists only from pilot scale tests at Rugårdsvej ML (Jørgensen et al., 2007).

Upon implementation of full scale remediation, short and long term success criteria, remediation criteria and stop criteria must be set up. Experiences indicate that guidelines for how to set up stop criteria are needed.

At Gl. Kongevej 39 and Vesterbrogade ecological cane molasses has been used as an electron donor, and the bacteria used are a dechlorinating culture from BioClear. At Sortebrovej emulsified soybean oil (EOS^®) has been used as electron donor and KB-1^® bacteria (SIREM). The added amount of electron donor is estimated as the stoichiometric consumption multiplied by a security factor, and the number of bacteria is decided based on the recommendations of the supplier. Experiences show that it has been possible to increase the anaerobic reductive dechlorination in the water phase at all three sites. Furthermore it has been possible to achieve growth of specific bacteria at sites where, prior to remediation set-up, no Dehalococcoides were found (Gl. Kongevej 39 and Sortebrovej).

Therefore, the experiences from pilot and full scale remediations with biostimulated and bioaugmented anaerobic reductive dechlorination are promising. The development of reaction zones and the effect on anaerobic reductive dechlorination in the clay matrix should, however, be investigated further to achieve a better understanding of time perspectives for the remediation method.