Groundwater contamination by high-density and mobile-chlorinated volatile organic compounds (CVOCs) can lead to obstruction of all kinds of societal developments, such as the redevelopment of urban contaminated areas. Here, an integrated combination of technologies is presented in which the relatively new sustainable energy technology of Aquifer Thermal Energy Storage (ATES) is combined with in situ bioremediation (ISB) into RemediaTES (WKO-plus in Dutch). Laboratory and pilot studies at the municipality of Utrecht, the Netherlands, are combined to demonstrate the effectiveness of the proposed combination of technologies. From the results, it could be derived that a generally applicable area-oriented strategy that focuses on the prevention of CVOC transport to the receptor is most cost-effective in the long term. This combination of technologies, Aquifer Thermal Energy Storage (ATES) and in situ bioremediation (ISB), has the potential to become a breakthrough in the redevelopment of contaminated sites and a stimulus in the transition towards sustainable cities.
Aquifer Thermal Energy Storagearea redevelopmentbioremediationCVOC contaminationprotection drinking water sourcesbioaugmentationDehalococcoides spp.The author(s) declared that financial support was received for this work and/or its publication. The municipality of Utrecht, the Netherlands, funded this research to perform a pilot study at Reactorweg, Utrecht. The consortium partners, Bioclear Earth, NTP, and Stadswende, co-funded this research by providing labour input and covering the article publishing charges.section-at-acceptanceWater and Built EnvironmentIntroduction
In the early 1980s, many urban areas were confronted with groundwater contamination by chlorinated volatile organic compounds (CVOCs) in their subsurface, most often as a result of dry-cleaning activities and degreasing of metal surfaces, after which the residual solvents were discharged into the sewer system, and spills occurred during storage, handling, and use of solvents. Spills and discharge of solvents in leaking sewer systems led to the transport of pure CVOC products, known as dense non-aqueous phase liquids (DNAPLs), because of their higher density than water, deep in the subsurface until they reached impermeable clay (Pankow and Cherry, 1996).
In the city of Utrecht, the Netherlands, the first cases of CVOC contamination in groundwater were discovered in the 1980s. The results of the soil investigation studies showed that zones containing high concentrations of CVOCs and pure-phase DNAPL pools on top of the impermeable layers are likely to be present. Such DNAPL pools can function as secondary sources of CVOC in a clay layer approximately 50 m below ground level (bgl). This clay layer separates the first and second aquifers of groundwater. The soil investigation studies also showed that the extent of CVOC groundwater contamination increased over time, leading to the contamination of the subsurface below a large part of the city centre.
In the Netherlands, soil and groundwater policies were developed in the 1980s, focusing on multifunctional remediation based on Dutch intervention values (Verantwoording van Gegevens En Procedures Voor de 1e Tranche Interventiewaarden: Van RIVM-Rapporten Naar de Notitie Interventie-Waarden Bodemsanering | RIVM, n.d.). Soil remediation was characterised by a case-based sectoral approach, especially focusing on the environmental quality of the soil compartment. Legislation is also aimed at the polluter-pays principle. However, many of the companies that used these solvents had already ceased operations.
Since the introduction of the Dutch Soil Protection Act in 1987, numerous historic desks and soil investigation studies have been conducted at sites where activities have taken place that could have led to soil contamination. These studies showed approximately 760,000 suspected contaminated sites in the Netherlands (status per 2004), of which 430,000 sites were potentially seriously contaminated and approximately 60,000 sites needed remediation (inventory contaminated locations) (Aantal Locaties Bodemverontreiniging, Inventarisatie 2004 | Compendium Voor de Leefomgeving, n.d.). Societal costs for remediation to standards for multifunctional use would increase sharply with these high numbers of contaminated sites. Therefore, stagnation of societal processes, such as a delay in the redevelopment of such sites, became urgent. The debate on the necessity of multifunctional remediation of contaminated sites started, which resulted in a new policy known as functional remediation (Policy on soil quality and soil pollution) (Bodemkwaliteit En Bodemverontreiniging: Beleid | Compendium Voor de Leefomgeving, n.d.). In this approach, higher concentrations of contaminants are accepted at industrial locations compared with residential locations.
New legislation was developed, not aiming at complete remediation, but based on a risk-based approach. For areas such as the municipality of Utrecht, with multiple source zones, overlapping plumes, and numerous developments above and below ground level. This risk-based approach also enabled area-oriented groundwater management. This area-oriented groundwater management facilitated the implementation of a relatively new sustainable energy technology known as Aquifer Thermal Energy Storage (ATES), in which thermal energy (warm and cold water) is stored in contaminated aquifers. In this more integral approach, other environmental interests may prevail over prevention of contaminated migration due to the extraction and infiltration of groundwater in ATES systems. Migration of contaminants is no longer an obstacle, as long as the objectives and requirements of the area-oriented groundwater management framework within a specific area are met.
When soil policy developed, remediation technologies were also developed, most often based on physical removal of CVOC-contaminated groundwater. Remediation of contaminated groundwater by pump-and-treat appeared successful, as after several months, the CVOC concentrations decreased. However, after pumping was stopped, the CVOC concentrations rapidly rebounded to their initial levels. Later, it became clear that secondary sources consisting of pure product and desorption of residual contamination from the solid phase of the soil played an important role in the recovery of high CVOC concentrations in groundwater at contaminated sites.
At the end of the 1980s, environmental microbial research showed that man-made xenobiotic compounds, such as tetrachloroethylene [also known as perchloroethylene (PCE)], could be biodegraded in the absence of oxygen under anaerobic conditions (Assafanid et al., 1994; De Bruin et al., 1992; DiStefano et al., 1991; Fathepure and Boyd, 1988b, 1988a; Fathepure and Vogel, 1991; Freedman and Gossett, 1989; Kastner, 1991; Vogel et al., 1987; Vogel and McCarty, 1987).
This was the starting point for the development of a wide variety of bio-based remediation technologies (Alexander, 1994; Hopkins et al., 1993; Leeson and Alleman, 1999; McCarty, 1991; Middeldorp et al., 1998; Norris and Matthews, 1994; Sims, 1990; Young, 1990). After the first physical remediation technologies using pump-and-treat approaches, in situ treatment aiming at the biodegradation of PCE and trichloroethylene (TCE) in the soil readily gained popularity.
In the risk-based approach for mobile soil contamination, such as CVOCs, risk evaluation and bioremediation perspectives were jointly connected in the source–path–receptor approach (Grotenhuis and Rijnaarts, 2011). In Utrecht, the overall groundwater flow direction is north-west and is transporting dissolved CVOC contamination. Due to the high density of pure CVOC and the vertical hydraulic gradient from the first to the second aquifer, vertical contaminant transport to the second aquifer may occur, as the separating clay layer is most probably not continuously present. North-west of Utrecht, groundwater from the second aquifer is extracted for the production of drinking water. This pumping station can be seen as a potential receptor of the CVOC contamination, although modelling studies showed that the travel time from the city centre of Utrecht to the drinking water intake is about 200 years (CityChlor Pilot Project 7: Aquifer Thermal Energy Storage, 2013).
In addition to protecting drinking water sources from CVOC contamination, spatial planning aspects related to soil contamination must also be addressed. An example is Hoog Catharijne, a major shopping centre close to the main train station and the city centre, which needed an upgrade, as it was originally built in the 1970s. Originally, in the multifunctional remediation policy, the owners of contaminated sites were responsible for the remediation of CVOC-contaminated soil and groundwater. This caused a serious stagnation in the upgrading and modernization of the shopping mall, as remediation technologies such as excavation and pump-and-treat caused high extra costs compared to building on pristine soil. The municipality of Utrecht was further confronted with the increasing effects of climate change and the problems of urban heat islands (KNMI - Warmte-Eilandeffect van de Stad Utrecht, 2010). Therefore, the municipality aimed for a sustainable shopping centre, using Aquifer Thermal Energy Storage (ATES). For ATES, large quantities of groundwater are recirculated to store and recover heat and cold in and from the subsurface (Fleuchaus et al., 2018).
This study describes the joint development of policy and technology following the recognition of CVOC groundwater contamination in the municipality of Utrecht. Herein, the risks to drinking water production and the limitations to city development and sustainable energy systems are addressed. Furthermore, the investigation and design of pilots are described for a sustainable energy technology combined with a Nature-Based Solution (NBS) for groundwater remediation aimed at protecting drinking water sources. This combination of technologies was initially addressed as the ‘Biowashingmachine’ and, after optimisation, is known as the RemediaTES concept (in Dutch: WKO-plus). Finally, the preliminary results from a recent RemediaTES pilot study are presented.
Societal urban developments leading to the Biowashingmachine concept
In 2005, it was generally known that dry-cleaning companies and other industrial activities had led to historic groundwater contamination by CVOC (Ministerie van Volkshuisvesting, 2005). Below utility buildings, such as the Trade Fair Jaarbeurs, Central Station, and shopping mall Hoog Catharijne, which are located in a 1.5–2 km2 area in the centre of Utrecht, CVOC-contaminated groundwater was also present. All three locations needed an upgrade to be fit for the coming decades to fulfil their services (CU2030 | ‘K Zie Je in Utrecht, n.d.). Although there were no direct human risks, this groundwater contamination led to the obstruction of the redevelopment of the station area (CU2030 | ‘K Zie Je in Utrecht, n.d.). The need for redevelopment was the main incentive to start cooperation among all stakeholders involved in several case-based groundwater remediations around the station area (Commissiebrief Duurzaam Stationsgebied, 2009). In the cooperation, it was key that the municipality of Utrecht was responsible for the groundwater contamination as an area manager and that the commercial partners made budget available, thereby enabling the municipality to invest in a monitoring system for CVOC-contaminated groundwater in the subsurface below the centre of Utrecht. Furthermore, a remediation plan for the station area was initiated in which the Biowashingmachine concept was presented as a combination of ATES with CVOC bioremediation by natural attenuation (NA), as it was postulated that the bioremediation would be stimulated by mixing of groundwater and by the temperature increase in the warm well of the ATES system (CityChlor - Bio-Washing Machine in Utrecht - YouTube, n.d.; Saneringsplan Ondergrond Utrecht Gefaseerde Gebiedsgerichte Aanpak, 2009). In the remediation plan, the three cases were integrated into one area in which the spreading of CVOC by pumping of Aquifer Thermal Energy Storage installations was allowed, and in which it was expected that the continuous mixing of the contaminated groundwater would stimulate natural attenuation by the indigenous micro-organisms in the subsoil. Within the Biowashingmachine concept, groundwater monitoring was aimed at controlling the groundwater quality and avoiding the migration of contaminated groundwater beyond the boundaries of the area-based groundwater management plan.
The idea of combining CVOC bioremediation and Aquifer Thermal Energy Storage (ATES)CVOC bioremediation
By the end of the 1980s, increasing scientific evidence became available that showed that chlorinated solvents, such as PCE, were biologically degradable to harmless end products via a process called reductive dechlorination (Vogel et al., 1987). Laboratory research revealed the soil conditions required for complete degradation, such as methanogenic conditions at low redox potential, and the microorganisms involved in the degradation process were identified. This knowledge was used to develop and implement new soil remediation techniques for chlorinated solvents based on these degradation processes. The implementation of in situ soil remediation techniques was stimulated in the Netherlands in the 1990s by the Dutch research programme NOBIS (Nederlands Onderzoeks-programma Biotechnologische In-Situ Saneringen), which was later continued as the SKB programme (Alles over 20 Jaar Skb - 20 Jaar SKB, n.d.).
An often-applied optimisation of in situ bioremediation was the addition of an electron donor to alter the redox conditions in the soil to strongly anaerobic (methanogenic) conditions, which, according to laboratory research, were required for complete degradation to harmless end products (Magnuson et al., 2000; Major et al., 2002; Maymó-Gatell et al., 1999; Yang and McCarty, 1998). Unfortunately, in several cases, adding an electron donor did not result in complete dichlorination; the degradation process stalled at partially dechlorinated compounds such as cis-DCE and VC. For these sites, a new concept was developed within NOBIS, called the Total Concept Evenblij (TCE Concept, n.d.). This bioaugmentation concept involves the optimisation of the conditions for complete reductive dechlorination by adding the microorganisms required for complete degradation to harmless end products, together with an electron donor. The microorganisms involved are known as Dehalococcoides spp., a genus of bacteria that obtain energy via the oxidation of hydrogen and subsequent reductive dehalogenation of halogenated organic compounds such as PCE, ultimately yielding ethene (Fennell et al., 2004; He et al., 2003; Hendrickson et al., 2002; Löffler et al., 2000; Magnuson et al., 2000; Seshadri et al., 2005). Since 1999, the success of inoculation with Dehalococcoides spp. has been underpinned by the fact that these species are able to perform halorespiration (Hug et al., 2013; Luijten et al., 2004; Smidt et al., 2000; Smidt and De Vos, 2004; Taş et al., 2010; Van De Pas et al., 1999), a process by which bacteria obtain energy from the conversion of PCE to the harmless products ethene and ethane.
The TCE concept (Figure 1) was developed and first applied at full scale at a highly contaminated site in Hoogeveen, the Netherlands, in a contaminated volume of 400,000 m3. Anaerobic cultivation of Dehalococcoides spp. was performed at the site in four anaerobic bioreactors of 25 m3 each (van der Werf et al., 2001). Contaminated groundwater with a maximum concentration of 50,000 μg/L PCE was extracted and led through the four bioreactors, in which the groundwater was enriched with Dehalococcoides spp. The effluent of the bioreactor was then reinfiltrated into the contaminated groundwater. A mixture of acetate and lactate was added as an electron donor. Monitoring using q-PCR techniques showed an increase in the number of Dehalococcoides spp. in the groundwater from non-detect up to about 104–106 cells/mL and migration of Dehalococcoides spp. through the soil, and the complete degradation of the contamination to harmless end products (de Vries and Nipshagen, 2015). Since then, this concept has been further developed. For low-permeability soils, the biomass is first cultivated in an anaerobic bioreactor off-site, and the inoculum is transported in 1 m3 containers to a contaminated site for direct injection of electron donors and Dehalococcoides spp. containing biomass (DHC) in concentrations of 108 cells/mL (Pellegrini et al., 2019). For high-permeability soils, mobile remediation systems have been developed. An anaerobic bioreactor is mounted in a 30-foot shipping container; therefore, cultivation of DHC culture can be performed at the contaminated site to fulfil the specific demands of each contaminated site. These systems also contain the necessary equipment for the addition of a suitable electron donor to the anaerobic bioreactor and the contaminated soil and systems for the recirculation of groundwater, online measurements, and control of the remediation process.
Bioremediation of contaminated sites using the Total Concept Evenblij (TCE concept). Groundwater is continuously pumped into a bioreactor, where electron donors and nutrients are added to cultivate Dehalococcoides spp. The effluent of the bioreactor, enriched with Dehalococcoides spp., is then used to inoculate the contaminated groundwater.
Illustration of an urban bioremediation system showing contaminated groundwater beneath buildings, injection wells introducing dechlorinating bacteria, and harmless end products forming. Inset diagram details a bioreactor process including heating, nutrient dosage, filtration, and by-pass lines facilitating bacterial activity to transform contaminants.
The TCE concept was further developed into Bioreactor-Enhanced Augmentation Technology (BEAT™), a technology that has been proven in the real world in an operational environment (TRL 9). This technology consists of a biomass cultivation and injection system mounted in a shipping container, which is aimed at treating the contaminated soil by adding an electron donor and Dehalococcoides spp., after which the system is removed from the site. By inoculation with Dehalococcoides spp. and the addition of an electron donor, the contaminated soil is transformed into an in situ bioreactor. The progress of the degradation process is monitored. Several remediation contractors in the Netherlands own one or more mobile systems, and dozens of sites have been remediated with this concept (de Vries and Nipshagen, 2015). This technique is currently considered a proven technology for the remediation of CVOC-contaminated sites and has been the start of the development of new remediation techniques based on bioaugmentation.
Aquifer Thermal Energy Storage
In the 1980s, the first studies were published on Aquifer Thermal Energy Storage (ATES) in the USA, Canada, and the UK, mainly focusing on feasibility and hydrogeological and geological aspects (Adams et al., 1981; Eissenberg, 1978; Lau et al., 1986; Mercer et al., 1982; Skinner and Supkow, 1981). In the early 1990s, the first field experiments were described, focusing on high-temperature storage and the effect on soil geochemistry, as well as modelling of ATES systems (Kannberg, 1988; van der Heide and van Loon, 1992; Willemsen, 1992; Yokoyama et al., 1993). Around 2000, the application of ATES systems increased in countries such as China, the UK, Turkey, and the Netherlands (He et al., 2005; Holdsworth, 2004; Paksoy et al., 2000; Seuren, 2001).
In the Netherlands, the number of ATES systems had increased to over 1,000 in 2009 (Godschalk and Bakema, 2009). These developments raised concerns related to the protection of drinking water quality, which in the Netherlands originates from approximately 80% of groundwater (Bonte et al., 2011). Furthermore, concerns from a risk perspective were related to the impact of mixing by ATES systems of industrial contaminants in groundwater, such as CVOCs. From this risk perspective, ATES spreads chlorinated solvents over large volumes in the subsurface due to the recirculation of groundwater and is seen as a possible threat to drinking water production from groundwater (Zuurbier et al., 2013).
The combination of CVOC bioremediation and ATES
However, from an environmental technology perspective, ATES was seen as an accelerator of biodegradation of CVOC, as it is generally known that microbial degradation increases by a factor of 2 when the temperature is increased by 10 °C according to the Arrhenius law, although above 30 °C, the dehalogenation rate decreases (Friis et al., 2007). In 2009, this perspective resulted in the projects CityChlor and ‘Meer met Bodemenergie’ (CityChlor (TaCkling Urban SoIl and Groundwater ConTamination Caused by CHLORinated Solvents), n.d.; Meer Met Bodemenergie | Deltares, n.d.). In ‘Meer met Bodem Energie’, chemical, biological, and physical effects of ATES on the subsurface were investigated, and the feasibility of new combination concepts was studied, aimed at the combination of ATES with groundwater remediation.
Steps in the development of ATES–ISB towards the application of RemediaTES
In 2009, the municipality of Utrecht approved the implementation of the concept of the Biowashingmachine (Saneringsplan Ondergrond Utrecht Gefaseerde Gebiedsgerichte Aanpak, 2009). Monitoring results between 2010 and 2015 showed that much larger areas were contaminated with CVOC than just the area in which the Biowashingmachine was active. Therefore, an area-based approach was proposed that was valid in a much larger area than just the centre of Utrecht (Gebiedsplan Gebiedsgericht Grondwaterbeheer En Visie Op Duurzaam Gebruik van de Ondergrond Gemeente Utrecht, 2015). In the area-oriented approach, the spreading of dissolved contaminants is allowed within the boundaries of the area-oriented approach. Monitoring and hydrological modelling of the spreading of CVOC in the upper 50 m bgl and the overall direction of the contaminated groundwater were performed to underpin and design the area-oriented approach. Modelling data showed that the CVOC dissolved in the groundwater and slowly moved in a north-west direction, where a drinking water intake is located. Modelling around 2011 showed that arrival at this vulnerable receptor is expected in approximately 200 years. Further monitoring of the results in 2016 and 2021 showed that CVOC concentrations were only slightly lower compared with 2009, leading to the conclusion that the presence of ATES systems (the Biowashingmachine) did not sufficiently accelerate natural attenuation. However, laboratory research using samples from the subsurface of Utrecht using a stepwise approach showed that the NA biodegradation rate can be stimulated when the redox potential is lowered, and chlorinated intermediates are completely converted to ethene by inoculation with a DHC culture (Ni et al., 2014). Further laboratory studies showed that, at proper redox conditions, the biodegradation of CVOC could be stimulated in a simulation of an ATES system by a factor of 14 compared to natural attenuation rates (Ni et al., 2015). In a laboratory recirculation column, the resilience of reductive dechlorination to redox changes was studied, which showed that bioremediation could be initiated by biomass inoculation under low redox conditions (Ni et al., 2016). In this study, the DHC biomass concentration in the liquid phase was only 0.1% of the inoculated biomass, indicating that a large part of the added DHC culture is preferentially attached to the soil matrix.
A main concern related to ATES systems is the potential for chemical and biological clogging of the extraction and infiltration wells. Biological clogging might be enhanced by the addition of DHC and an electron donor and may hamper the proper functioning of ATES. However, more reduced conditions in the subsurface resulting from enhanced bioremediation might lower the chance of chemical clogging, which is normally caused by iron oxide precipitates. The role of chemical and biological clogging was studied at the laboratory scale under low redox conditions in a flow recirculating column with Fe(III)-rich soil samples from Utrecht at a depth of 35–38 m bgl, where mostly VC and cis-DCE were present, and with redox potentials ranging from −112 to −151 mV (Ni et al., 2018). Results show that within the period of enhanced biological activity, no increase in pressure drop over the columns was found. Instead, using lactate as the substrate for the biological enhancement reduced the build-up of the pressure drop. Therefore, it is likely that at the low redox conditions needed for reductive dichlorination, biological clogging of the wells of ATES systems does not occur.
In subsequent reactive transport modelling for various scenarios, it was shown that CVOC biodegradation is stimulated depending on the electron donor addition and that at a temperature of 15 °C, which is rather typical for low-temperature ATES, biodegradation increased significantly compared to NA conditions (Sommer, 2015).
The knowledge gained in scientific research, combined with full-scale soil remediation experiences of CVOC-contaminated sites with the TCE concept, led to the idea of combining the TCE concept with ATES systems to stimulate reductive dechlorination in an Aquifer Thermal Energy Storage–In Situ Bioremediation (ATES–ISB) concept. One of the main prerequisites for combining ATES with ISB is that the risk of clogging of the extraction and infiltration wells of the ATES may not increase because clogging of the ATES wells will block the functioning of such sustainable heating and cooling systems. The TCE concept for soil remediation has already provided the ‘proof of principle’ for the infiltration of a highly concentrated DHC culture in aquifers with recirculation systems.
Therefore, the concept of ATES–ISB is based on the addition of highly concentrated DHC cultures to the groundwater of ATES systems to increase the specific microbial biomass and lower the redox potential using separate and easy-to-install biomass injection wells. The pilots performed in the Netherlands and Denmark are all based on the addition of DHC cultures in separate biomass injection wells that are not part of the ATES system. In laboratory studies, it has already been demonstrated that more than 95% of the injected DHC culture attaches to the soil, creating a biologically active zone between the wells of an ATES system in which dissolved CVOCs are biodegraded to harmless compounds.
The ATES–ISB concept was studied at relatively high CVOC concentrations at a pilot site in Birkerød, Denmark, whereas at the pilot site of sports centre Nieuw Welgelegen in Utrecht, the functioning of ATES–ISB was performed at relatively low CVOC concentrations.
At the end of 2017, the first ATES–ISB pilot study was performed at Birkerød, Denmark, for a TCE-contaminated site with a source zone of initial concentrations of approximately 800–1,200 μg TCE/L (Europe Wide Use of Soil Energy ATES | Deltares, n.d.; Hoekstra et al., 2020; Pellegrini et al., 2019; Wienkenjohann et al., 2024). In the stepwise approach, an electron donor is first added to lower the redox conditions. This resulted in an increase in biodegradation of TCE to DCE, and no further degradation to VC and ethene was observed. Only after the addition of the DHC culture did the VC and ethene concentrations rapidly increase, leading to a sharp increase in the dechlorination degree from 20 to 30% to approximately 70% within 14 days after DHC inoculation (Europe Wide Use of Soil Energy ATES | Deltares, n.d.). Recently, in a comprehensive process-based modelling analysis of this pilot study, it was demonstrated that the bioaugmentation with DHC culture, combined with temperature increase and intensive mixing, is the most important driver for the enhanced CVOC biodegradation (Wienkenjohann et al., 2024). Actually, all three drivers are fulfilled in the combination of ATES and in situ bioremediation, which can therefore be postulated as ‘RemediaTES’.
A second pilot study was performed starting in July 2017 at the edge of the CVOC plume at the centre of Utrecht, at sports centre Nieuw Welgelegen in Utrecht, the Netherlands (Europe Wide Use of Soil Energy ATES | Deltares, n.d.). In an existing full-scale mono-well ATES system at 25–57 m bgl of the sports centre Nieuw Welgelegen, vinyl chloride (VC) was measured at fairly low concentrations in the ATES wells (0.3–6.6 μg/L), and no other CVOC was present. However, nearby higher concentrations of VC were measured (0.5–62 μg/L), which exceed the maximum allowable groundwater concentrations set by Dutch regulations. The aim of this pilot study was to determine whether a highly concentrated biomass culture injected near an existing and running ATES system leads to groundwater quality improvement without negatively interfering with the ATES system. Therefore, a biomass injection, well placed 5 m north of the ATES well, was placed 10 m north of the ATES well at 25–27 m bgl. The warm well screen is at 26–31 m bgl, and the cold well screen is at 49–58 m bgl. After a single biomass injection, without electron donor addition, the VC concentration in the biomass injection well decreased readily from 2.6 to 4.3 μg/L within 2 days after the injection of the DHC culture to below detection limits. The VC concentration temporarily resumed in the next summer to about 1.5 μg/L and fell back again to below the detection limit. This demonstrates that the injected DHC biomass was active for more than a year after injection. Also, temporally decreased VC concentrations were observed in the ATES warm well from about 2 μg/L to below the detection level for VC of 0.2 μg/L during summer (Grotenhuis, 2020). However, the VC concentration resumed thereafter in winter to a maximum of approximately 10 μg/L and decreased again to 2 μg/L the following summer. Also, in the ATES cold well, the VC concentration decreased in summer from 6 μg/L to below the detection limit and resumed in the winter thereafter to about 5 μg/L and decreased again to below 1.5 μg/L the next summer. These results show that a single DHC biomass injection has an effect for 12 months or longer, which was confirmed by elevated DHC bacteria analysis and vcrA measurements 12 months after DHC injection (Europe Wide Use of Soil Energy ATES | Deltares, n.d.). These findings support the laboratory observations that the DHC culture is preferentially attached to the soil matrix. Furthermore, the ATES mono well at Nieuw Welgelegen functioned normally and was not negatively affected by the DHC biomass injection.
Both pilots show that when strategically placed, CVOC-contaminated groundwater is transported through the created biologically active zones, resulting in the degradation of CVOC. In the Birkerød pilot study, it was proven that relatively high CVOC concentrations could be readily biodegraded and therefore protect downstream sources for drinking water intake from contamination. The Nieuw Welgelegen pilot showed that the load of CVOC can be managed close to target levels by providing DHC biomass injections in an existing ATES system
The above laboratory studies on ATES and in situ bioremediation (ISB), together with both pilots, demonstrate that the combination of ATES–ISB can overcome the limiting factors such as too high redox conditions and too low specific CVOC degrading biomass concentration. Therefore, in principle, ATES–ISB can solve the problems of the low CVOC biodegradation rates in the Biowashingmachine. Furthermore, it was demonstrated that the delivery of heat and cold by the ATES system can function normally in the combined ATES–ISB, and therefore, the ATES system can lead to the design of future RemediaTES projects.
To optimise the ATES–ISB to develop a robust area-oriented approach to protect the drinking water intake north-west of Utrecht, a pilot was designed at the site Reactorweg in Utrecht. Parameters for the effective treatment of high as well as low CVOC concentrations to protect the drinking water intake are studied in the Reactorweg pilot.
Pilot Reactorweg Utrecht: goal, experimental setup, and preliminary results
As stated earlier, groundwater recirculation and temperature differences in the subsurface of Utrecht, induced by ATES systems, are not sufficient to accelerate the natural attenuation processes of CVOC. Recent monitoring studies, performed as part of the area-oriented approach of the municipality of Utrecht, showed that two CVOC-contaminated plumes migrate towards the boundaries of the area-oriented approach and possibly exceed these boundaries in the near future. In such cases, technically complex and expensive measures may be necessary. To investigate whether RemediaTES can contribute to passing CVOC, passing the boundaries of the area-oriented approach, a RemediaTES pilot project was started at the Reactorweg site in Utrecht. At this site, relatively high concentrations of CVOC are present (mainly cis-DCE and VC with concentrations up to approximately 1,800 and 1,600 μg/L, respectively). In the first phase of the pilot study, the following research questions were addressed:
Can the final concentrations of cis-DCE and VC be below Dutch intervention values (20 and 5 μg/L, respectively) within a reasonable time?
Is the pilot system able to biodegrade all CVOC (mg/day) flowing through the bioactive zone?
How long can the injected DHC culture remain active under the environmental conditions of the subsurface in Reactorweg, Utrecht?
The pilot started with groundwater circulation on 29 October 2024. Within the contaminated area, a groundwater extraction and an infiltration well were installed. Between the wells, several injection and monitoring wells were installed to be able to infiltrate a concentrated DHC culture or a slow-release carbon source and to monitor the degradation process. A volume of 8 m3 of DHC culture was cultivated using a mobile TCE system and injected into the pilot area on 14 November 2024. The quality of the DHC biomass was determined by qPCR and showed numbers of 2.0 × 108 cells/mL, resulting in 3.4 × 106 DHC cells/mL in the injection well shortly after injection of the culture. In the pilot study, no electron donor was added.
The residence time of groundwater in the bioactive zone, created by the infiltration of 8 m3 of DHC culture, is estimated at 2.5 months (75 days) at natural groundwater flow velocity. By applying a pumping flow rate of approximately 4–6 m3/day, the retention time in the bioactive zone was reduced to approximately 2–4 days, which is more representative of the retention time in RemediaTES applications. By continuously pumping contaminated water from the extraction well to the infiltration well, the concentrations of cis-DCE and VC were expected to decrease rapidly under conditions of high biological activity. This provided an opportunity to determine the degradation at both higher (initial concentrations of approximately 1,000 μg/L cis-DCE and VC) and lower concentrations (those that would arise within a closed recirculation cell).
The degradation process was monitored by periodic groundwater sampling and analyses for CVOC and the degradation products ethene and ethane. In addition, redox measurements (nitrate, iron(II), sulphate, and methane) were performed, including redox potential (Eh), conductivity (EC), acidity (pH), and dissolved carbon (TOC).
In addition to these regular analyses, molecular DNA analyses were performed to investigate the development and migration of DHC within the pilot. qPCR was performed to determine the total number of bacteria (BAC total), the number of specific CVOC degraders (DHC count), and the number of bacteria that can specifically degrade VC (bacteria with the vcrA gene). These measurements were used to determine the bacterial population trend (growth or starvation) and the movement of bacteria through the soil. v-PCR analyses, which measure only active cells and exclude inactive bacteria, were also performed. This provides a more reliable picture of the actual biological activity.
Previous pilot studies have shown that the vast majority of injected bacteria adhere to the soil matrix. These adherent bacteria can increase, remain stable, or decrease in number over time. To investigate this phenomenon, mesocosms were used. Mesocosms are water-permeable tubes filled with small volumes of soil material, which were placed in the permeable section of the monitoring well through which the groundwater flows. At each sampling point, the content of the mesocosm was used to determine the total number of bacteria and the number of specific CVOC degraders attached to the spoil matrix.
Finally, Membrane Filtration Index (MFI) analyses were performed on the infiltrated groundwater periodically. MFI analyses determine the number of particles (inorganic and bacterial) in the groundwater. Combined with measurements of redox changes, microbial analyses, and dissolved organic matter, the susceptibility of wells to clogging can be determined.
Preliminary results showed a significant decrease in the concentrations of cis-DCE and VC in the bioactive zone of the DHC injection well within the first 55 days of the pilot, from 370 to 490 μg/L cis-DCE to less than 1 μg/L, and from 1,400 to 1,000 μg/L VC to below 1 μg/L (Figure 2). Although the extracted groundwater contained concentrations of cis-DCE and VC of around 120 and 580 μg/L, respectively, the concentrations in the bioactive zone around the DHC injection well remained low (order of magnitude 0.1–1 μg/L for at least 376 days). If the recirculation system were a perfect closed loop, the whole pilot system would have reached cis-DCE and VC concentrations below the Dutch intervention values of 20 and 5 μg/L, respectively, within 55 days (Figure 2, left lower panels). The pilot system acted as an open loop, extracting an extra load of cis-DCE and VC from the environment, which was infiltrated at the infiltration wells of the system. This extra load of cis-DCE and VC was biodegraded in the bioactive zone of the system to concentrations below the Dutch intervention value (Figure 2, lower-right panels on days 55 and 376). These monitoring results confirmed that in the pilot study, contaminant concentrations could be lowered below Dutch intervention values, the bioactive zone could readily reduce the CVOC load, and it could maintain low contaminant concentrations for a minimum of 321 days.
Schematic overview of cis-DCE and VC concentrations in a perfectly closed recirculation system versus the measured CVOC concentrations in the open-loop system at the pilot Reactorweg, Utrecht.
Five-panel graphic shows contaminant removal process over 125 days. At day zero, red arrows and bars indicate high DCE and VC concentrations. By day forty, left side panels turn green, showing dramatic reduction of concentrations and presence of a bio-active zone. Right side day forty panel uses yellow and green to illustrate infiltration CVOC removal and environmental extraction. By day 125, left panel remains green, displaying nearly undetectable contaminant levels, and right panel is orange, with further reduced CVOC levels and continued removal out of the system. Labels detail locations, contaminant types, and concentrations at each step.
Degradation of CVOC by DHC is confirmed by an increase in the numbers of DHC and vcrA genes, as shown by the DNA analyses of groundwater samples and soil samples from the mesocosms. During the pilot experiment, pressure increase due to clogging was not observed.
Perspectives of CVOC contamination in the city of Utrecht
Redevelopment of the city centre of Utrecht was the main driver to develop strategies to deal with the CVOC contamination in the groundwater and for the protection of drinking water sources. To achieve these goals, the area-oriented approach and, within this approach, the Biowashingmachine concept were developed around 2009. However, monitoring results showed that the assumption of the functioning of Natural Attenuation did not lead to a substantial lowering of the CVOC concentrations by this concept (Figure 3A).
Strategies for CVOC treatment in contaminated urban areas. (A) Natural attenuation. In this strategy, the biodegradation of indigenous biomass is expected to occur in the prevailing redox state. When low numbers of specific CVOC-degrading microorganisms and suboptimal redox conditions are present, the degradation rates are low, and the degradation of CVOC is not complete. Transport of contaminated groundwater beyond the area boundary occurs with time. (B) Acute Boundary Control (ABC) strategy. In this strategy, the treatment of CVOC-contaminated plumes occurs by RemediaTES systems at the border of the affected area to prevent migration across system boundaries. This strategy focuses on the control and short-term results. (C) Enduring Area Treatment (EAT) strategy. In this strategy, the treatment of the load of CVOC contamination occurs within the area-oriented approach. In the design of RemediaTES systems, the optimal design of the energy system is leading. Vulnerable receptors outside the contaminated area are protected, as there is plenty of time and space within the boundaries of the area-oriented approach for the natural attenuation of residual CVOC. The redevelopment at the ground level of a CVOC-contaminated site will have similar requirements compared to non-contaminated areas.
Diagram with three labeled panels compares remediation strategies for groundwater contamination: A shows natural attenuation with contaminant plumes, B displays acute boundary control with wells at the boundary area, and C demonstrates enduring area treatment with multiple wells distributed throughout plumes. Blue arrows indicate groundwater flow direction, circular dashed lines denote the boundary area approach, and color gradients represent contaminant concentration from high (dark red) to low (yellow). Legend identifies red and blue dots as warm and cold wells, respectively.
Previously, in situ bioremediation remediations were mainly directed at the treatment of source zones with relatively high CVOC concentrations at individual sites. Although several source zones were remediated in the past, only a limited number of sites with CVOC contamination were remediated successfully. Companies that caused soil contamination were closed down, source zones were inaccessible for remediation, and remediation techniques, such as pump and treat, at that time proved insufficient to deal with the difficulties related to the properties and fate of CVOC in the subsurface. An important disadvantage of technologies for highly contaminated groundwater is that a special and costly remediation design is necessary for each contaminated site.
In the development of the RemediaTES concept, it was observed that this technology results in substantial reductive dechlorination of CVOC in sites with high CVOC concentrations, as well as in sites with low concentrations of several μg CVOC/L. Therefore, the RemediaTES concept is rather flexible, as it can be applied in CVOC source zones as well as in low-concentration zones at the edge of a contaminant plume.
The RemediaTES concept in the case of Nieuw Welgelegen showed that an ordinary ATES system can easily be transferred into a combined ATES–In Situ Bioremediation system that focuses on load removal over time at minimal costs. As ATES systems operate for 50 years or more, the necessity of placing injection wells close to the warm well, with a maximum temperature effect on the stimulation of CVOC biodegradation, is not that stringent to improve the groundwater quality with time. In addition, the occupation of space at ground level is rather minimal, as only one or at most two parking lots in a public space are needed during the installation and monitoring of an injection well.
Discussion on application strategies for RemediaTES technology
The separate development of in situ bioremediation of CVOC in groundwater by the TCE concept, in which groundwater is recirculated over a bioreactor with specific dehalogenating bacteria, and the upcoming installation of the sustainable energy technology of Aquifer Thermal Energy Storage (ATES), in which unwanted spreading of CVOC-contaminated groundwater occurred, led to the merging of the TCE remediation system and the sustainable heating and cooling system into the RemediaTES concept. Two RemediaTES approaches can be considered: 1. Treatment of CVOC-contaminated plumes at the border of an affected area to prevent migration across system boundaries, a strategy that focuses on control and short-term results. This strategy can be framed as the Acute Boundary Control (ABC) strategy (Figure 3B); 2. Lowering the load of CVOC throughout the entire affected area is an area-oriented strategy that focuses on prevention and long-term results. This strategy can be summarised as the Enduring Area Treatment (EAT) strategy (Figure 3C).
Thus far, the RemediaTES concept was always initiated from the perspective of immediate and urgent action to reduce high CVOC concentrations to protect a nearby vulnerable drinking water intake or other vulnerable receptors beyond the system boundary (ABC strategy) (Figure 3B). As such, the concept of RemediaTES has been shown to be valuable because high CVOC concentrations can be degraded effectively in a relatively short time. However, specific information is needed regarding the exact location of the hotspot concentrations in the area, which may conflict with the optimal design for an ATES system. This approach may hamper the redevelopment of a contaminated location because the degrees of freedom available to spatial planners and developers will be limited. This may lead to unwanted delays in the redevelopment of such a RemediaTES ABC strategy or, at least, will be experienced as a cost increase in the redevelopment of a polluted site.
Therefore, treatment of the load of CVOC concentrations within the area-oriented approach is expected to be much more effective, as the design of the energy system will lead to the redevelopment of a CVOC-contaminated site instead of the treatment system under such an EAT strategy (Figure 3C). The spatial planners and developers will not be limited in their degree of freedom in the design of the redevelopment of the contaminated location. Only one or two parking lots in a public space should be available for the construction of a biomass injection well, as well as additional plots for monitoring groundwater quality. Using this setup, municipalities can reduce the societal costs of groundwater remediation. For future generations, this area will be free to use without any limitations related to CVOC-contaminated groundwater. This load-based approach will be effective, especially due to the long travelling time of the CVOC-contaminated groundwater to the borders of the area-oriented approach. Such a long travelling time ensures the bioremediation of CVOC, as long as the specific biomass is present and active. A major drawback of the load-based approach is the lack of urgency experienced by spatial planners and developers. Therefore, the environmental departments of municipalities play an important role in the successful development of RemediaTES.
We thank Bas van Andel and Albert Smits of NTP for sharing their practical experiences and constructive cooperation in the performance of the pilot experiment.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Edited by: Giuseppe Oliveto, University of Basilicata, Italy
Reviewed by: Laurie LaPat-Polasko, Arizona State University, United States
Henning Wienkenjohann, Technical University of Denmark, Denmark