Feasibility of bioscreens for regional VOC-plume in industrial-urban area
Van de Wiele K.1, Paulus D.2, Goyens W.2, Bal N.2 , Penninckx N.²
1 OVAM, Stationsstraat 110, 2800 Mechelen, Belgium, P: +32 15 284 375, F: +32 15 284 408, kvdwiele@ovam.be
2 Tauw Belgium, Remylaan 4c bus 3, B-3018 Wijgmaal-Leuven, P: +32.16.35.28.10
Topic: Urban groundwater contamination and emerging contaminants – investigation, monitoring and management
In the industrial-urban area of Vilvorde-Machelen (situated Belgium, Flanders, ca. 170 ha, ca. 665 parcels) different historical activities caused a big regional groundwater contamination. Since sources of the contamination and their contribution to the groundwater plume were not exactly known, a region based strategy was established by the OVAM (Flemish authorities). In this integrated approach the groundwater contamination was globally mapped on a regional scale using historical and new data. Risks were evaluated. A juridical framework with guidelines was set up to create incentives for owners to deal with their local remediation challenges.
The river Zenne drains a big part of the contaminated area of Vilvorde–Machelen. The groundwater contamination with CVOC’s and benzene is therefore a potential threat for the quality of the surface water and stream bed sediments of the river Zenne. Because of the width of the groundwater plume (> 1,5 km) a ‘bioscreen’ based on enhanced natural attenuation of the contaminants was selected as a valid option to protect the river Zenne.
Two different pilot tests were conducted to investigate the feasibility of such a ‘bioscreen’. In Vilvorde a test site with low chlorinated products (1,2-DCE, VC, 1,1-DCA & MCA) and with traces of benzene was selected for a pilot test with enhanced degradation under aerobic conditions. At the Vilvorde test site a solution with calciumperoxide was injected to enhance aerobic degradation of benzene and co-metabole degradation of 1,2-DCE & VC. Direct push (top-down) using sonic drillings was used as the injection method.
In Machelen a test site with high chlorinated mother product (TCE) and low chlorinated degradation products (1,2-DCE & VC) of CVOC’s was selected for a pilot test with enhanced degradation under anaerobic conditions. The Machelen test site contained no traces of benzene. Glycerine, a re-used waste product from the soap-industry, was injected to enhance reductive dechlorination at the Machelen test site.
The effects of the injections were studied by a periodical monitoring of the concentrations of the contaminants, the degradation products and the geochemical parameters at different distances downstream of the injection points over a time span of 2 years.
From the results of the pilot test at the Vilvorde-site it’s clear that aerobic degradation of low chlorinated components is feasible. The results at the Machelen site confirmed the feasibility of enhanced anaerobic biodegradation of an highly chlorinated chloroethene contamination.
Aerobic bioremediation is a consolidated technology widely used for the removal of many organic contaminants, in particular for those of petroleum origin (TPH, DRO, GRO, BTEX, ...). This technology optimizes the ability of autochthonous bacterial strains to oxidize contaminants (electron donors) using oxygen as an electron acceptor.
The biological process can be limited by many factors such as dissolved Oxygen availability. When dissolved Oxygen is not available, bacteria will use other sources of oxygen (electron acceptor) such as nitrates (nitrification), sulfates and other oxidized species. These other sources of oxygen are thermodynamically less convenient and result in reduced rates of contaminant removal.
From a process point of view it is therefore necessary to ensure a good availability of oxygen to stimulate the most effective aerobic degradation reactions. Most of the products that release oxygen are based on CaO2 or MgO2 and should be injected into groundwater in the form of diluted suspension (slurry), which are dense and difficult to inject.
As an alternative to peroxides, Oxygel is a new concept product in gel form that can be injected “as is” at a very high concentration of available oxygen with respect of conventional diluted slurries. The easy and low-volume injections can greatly reduce the total cost (product and injection) and duration of field operations. The duration of Oxygel in the subsoil is equivalent to that of CaO2 and MgO2 based products.
Weidemeier et al. (1999) showed that over 70% of fuels natural attenuation are due to sulfates reduction by sulfate-reducing autochthonous bacteria. Further studies have shown significant success in remediation hydrocarbons exploiting the capabilities of sulfate-reducing bacteria (Reinhard et al., 1997; Anderson and Lovely, 2000; Somsamak et al., 2001; Sublette et al., 2006). For this reason, in collaboration with Carus, Redoxtech (USA) has developed OBC+ an innovative product designed to stimulate sulfate-reducing bacteria by supporting their activities with nutrients and pH buffering. As a side effect, precipitation of dissolved metals in the form of insoluble and stable metal sulfides has been observed. The use of OBC+ results in unexpected high contaminants degradation rates even in presence of residual free phase (LNAPL).
We will present the different available technologies to improve aerobic bioremediation reactions (Oxygen release, sulfate reducing bacteria stimulation) along with cost comparisons and several case studies.
Keywords
Aerobic Bioremediation, Petroleum Hydrocarbons; Fuels, Oxygen Release, Sulfate Reducing Bacteria
At a former landfill located in close proximity of the industrial area of Porto Marghera (near Venice, north-east of Italy) a complex soil and groundwater contamination was found to be present. The landfill was in operation for several decades and it was used for the disposal of highly contaminated industrial waste/sludge produced during the production cycles of the petrochemical industries located in Porto Marghera. An extensive contaminated plume is present downstream of the landfill. High concentration levels of chlorinated hydrocarbons, hydrocarbons and heavy metals have been detected in the groundwater.
A pilot field test is being conducted at the site since May 2013 to ascertain the effectiveness of the proposed remediation approach and provide information needed for the full-scale implementation. The approach consists of stimulating biological degradation by means of the installation of an anaerobic biobarrier and an aerobic biobarrier more downstream to treat the complex mixture of contaminants. In general the highly chlorinated contaminants will be reductively dechlorinated in the anaerobic biobarrier, while subsequently the lower chlorinated and non-chlorinated hydrocarbons will be degraded in the aerobic biobarrier.
In the first year of operation already a strong decrease in the concentration of chlorinated aliphatic hydrocarbons (95%) could be observed in the anaerobic biobarrier, which was even higher for the carcinogenic chlorinated aliphatic hydrocarbons (98.8 %). In the aerobic biobarrier the concentration of the sum of chlorinated aliphatic hydrocarbons further decreased with 90.7 % and 99.7 % for the carcinogenic ones. The individual BTEX compounds decreased with > 98.7 % and monochlorobenzene with 99.8 %. Additional molecular analysis also demonstrated that co-metabolic degradation processes might play a significant role in the aerobic biobarrier.
So, far the combination of anaerobic and aerobic biodegradation seems an efficient and promising technique to treat a complex mixture of contaminants at a former landfill area.
Background
Pesticide contamination of the groundwater in Denmark originates from diffuse agricultural activities as well as from point sources. Point sources are characterized by a concentrated spill of pesticide compounds that may have occurred when storing, handling/cleaning or disposing pesticides and spraying equipment. Phenoxy-acid pesticides are some of the most commonly found pesticides in Danish groundwater.
While diffuse contamination can be controlled mainly through regulation/change of agricultural practice, contamination from point sources, that can contain a large mass of pesticides in a small area, can be remediated with reasonable technical and economical means. Remediation of phenoxy-acid pesticides has normally involves containment of the contaminated plume through pump-and-treat solutions or monitored during natural attenuation. The latter has shown positive results under aerobic conditions. These experiences have motivated a currently running project with stimulation of the degradation processes through delivery of oxygen and/or pesticide-degrading microorganisms.
At a site in southern Denmark, the groundwater is contaminated with high levels of dichlorprop and 4-CPP, approximately 100-150 µg/l in the shallow sandy aquifer – well above the groundwater criteria of 0.1 g/l. The groundwater plume from the site threatens the local water supply, located 2 km downgradient. A pump and treat system currently ensures that the water supply wells are not affected. Remediation of the source zone will reduce the operational time and costs of the pump and treat system.
Aim
The project aim is to test whether the delivery of oxygen and bacteria in the source zone can stimulate aerobic biodegradation of the pesticides. Specific objectives include an evaluation of biostimulation vs. bioaugmentation as well as providing information for dimensioning a full scale clean-up method for this and similar sites. The project is cofinanced by the Danish EPA and the Region of Sealand.
Method
The pilot test consists of two test-fields. In test field 1, oxygen is delivered to the subsurface through a transect of oxygen diffusers at a very low flow. The objective of this test is to evaluate whether the creation of aerobic conditions solely can stimulate the indigenous microbial community to degrade the pesticides.
In test field 2, oxygen delivery is supplemented with bioaugmentation, i.e. the addition of specific bacteria strains that are known to degrade phenoxyacid pesticides. This test is done in collaboration with Aarhus University.
Each test field consists of three injection wells and 4 monitoring wells. Pesticide levels, oxygen levels and microbiological parameters are monitored throughout the duration of the experiment, as well as prior to pilot test start. Microbial parameters includes total CFU expression of specific degrader genes (mRNA) to indicate the degradation progress, independent of the inherent variation in the concentrations of pesticides in water samples.
A series of laboratory experiments are performed by Aarhus University in parallel to the field activities. The laboratory activities include a mapping of the microbiological diversity at the site, as well as mineralization measurements with the indigenous bacteria as well as the strain to be used for bioaugmentation in test field 2. Moreover, a study of how the degrader bacteria move through the soil has been performed in column experiments.
Results
Results from the laboratory studies show a low microbial diversity at the site (9-11 m depth). The results of the mineralization studies will be available within the next few months. Column studies of transport for employed Sphingomonas indicates that this strain can be transported several cm pr. day.
Oxygen delivery at both test fields started in September 2014. As of November 2014, oxygen levels in test field 2 are stable and augmentation with bacteria is about to be performed. It has yet not been possible to establish aerobic conditions in test field 1.
Measurements of oxygen, pesticide concentrations and microbial parameters will be performed in monthly intervals for the next six to twelve months. By the time of the conference results from at least 6 monitoring events will be available to indicate the efficiency of the biodegradation process.
Background
Leaching of pesticides from a landfill has caused a widespread pesticide contamination in the underlying large sandy aquifer. The phenoxy acids dichlorprop, MCPP and daughter compounds are documented in a broad contamination plume more than 500 meter down gradient from the landfill site. The precise location of the contaminant source is unknown and delineation and characterization of the source will require extensive research. Since 1995, the pesticide contamination has been handled by a pump and treat system (P&T). Recently, modeling has provided some evidence that the existing remediation does not effectively prevent further migration of the contaminant plume, resulting in this reassessment of the strategy.
Monitored Natural Attenuation (MNA) relies on naturally occurring processes reducing the contamination to an acceptable level. In order to document that attenuation is taking place degradation must be proven by several analysis and interpretation methods. As MNA of phenoxy acids from landfills previously has been shown effective, there are reasons to believe that this could be an attractive “green” and cost-effective remediation approach.
Aim
The aim of the project is to determine whether the green and cost effective approach of MNA is an attractive remediation technology for the pesticide contamination caused by the landfill site, as well as to test three new interpretation methods documenting in-situ natural degradation:
1 Analysis of mother and daughter compound fractions
2 Analysis of enantiomeric ratios
3 Analysis of the development in enantiomer specific isotopes
Conclusion
MNA requires solid data and must be proven by several lines of evidence. An investigation was carried out including traditional sampling in 26 filters and passive multilevel sampling in 61 points. The sampling was applied in a transect covering the pesticide plume and along a flow line giving a high vertical and horizontal discretizing. All 87 water samples were analyzed for pesticides and advanced chemical analysis was applied for three selected samples. Traditional methods such as analysis of geochemical parameters as well as spatial and temporal variations of pesticide concentrations were not found very useful in this case. Hence, a combination of three advanced interpretation methods was tested:
1 Analysis of mother and daughter compound fractions documented that dichlorprop is degraded. Actual fractions are compared with known worst-case fractions ensuring that impurities from pesticide production are not influencing the degradation assessment.
2 Analysis of enantiomer ratios gave clear indications that MCPP is degraded. Enantiomers are chiral compounds with different toxicological properties.They are known to be degraded at different rates, and thus changes in the ratios can indicate degradation.
3 Analysis of the development in enantiomer specific isotopes both indicated that dichlorprop is degraded to 4-CPP and that 4-CPP is further attenuated. Microorganisms deplete 12C-molecules faster/more easily than 13C-molecules.
The three different interpretation methods was very successful and supplemented each other providing conclusions all supporting that natural attenuation takes place. However, on the current data basis MNA may not be chosen as an independent remediation method, as indications for degradation are not strong enough in showing significant attenuation of the pollution from the landfill site. In the actual example, it is therefore decided to continue with an optimized P&T strategy, to ensure a hydraulic fixation of the pollution leaving the source area at the landfill site. A robust monitoring program for the plume, including the natural attenuation processes documented by the advanced interpretation methods, is used to establish and document stop criteria for the remediation of P&T shortening the time of operation. However, more data from traditional and advanced water analysis in a combination with further interpretation could substantiate and clarify conclusions about natural attenuation making MNA a preferable and sustainable approach for the future remediation at the landfill site.
Perspective and lessons learned
Although MNA has been an approach for handling contamination plumes over more than a decade, the implementation still challenges our understanding and handling of groundwater contamination. The method of MNA relies on advanced analysis, research and characterizations of considerable investigation cost. Moreover, authorities should accept a long timeframe for carrying out analysis and investigations. Choosing MNA as the remediation approach requires courage and conviction in order to assure population and other stakeholders that contamination is handled properly. On the other hand, MNA provides a considerable decrease in energy consumption and is as such a green and very cost effective remediation technology.