Aquifer Thermal Energy Storage (ATES), considered as an energy saving system and sustainable energy technology, it is growing exponentially in the Netherlands. The principle of ATES system is to cold or heat when available and retrieve it or use when needed. The NL government wishes to stimulate this growth to diminish energy use and reduce emissions. However, because many urban city centres deal with contaminated soil and groundwater, more and more ATES ambitions are confronted with the presence of contaminants.
Chlorinated volatile organic compounds (VOCls) are by far the most prevalent organic contaminants in the subsurface throughout the world . Among these compounds, PCE, TCE cis-DCE and vinyl chloride (VC) are the main representatives . Since VOCls are potentially carcinogenic, especially VC has been classified by International Agency for Research on Cancer as a human carcinogen , their presence in groundwater is considered as a threat to public health.
Together with the concerns on toxic impacts, the removal of VOCls is of importance and necessary as well. Due to the recalcitrant nature of VOCls, they are among the most difficult contaminants to be cleaned up and characterised, especially when they exist as dense non aqueous-phase liquid (DNAPL). Therefore, conventional techniques such as pump-and-treat, soil vapour extraction and soil excavation are either too costly or inefficient to properly remediate VOCls contaminants, while bio-based techniques become more and more attractive, such as monitored natural attenuation and enhanced bioremediation . It is reported that remediation based on biological transformation and biodegradation with proper stimulation approaches can be an effective way to reduce organic contaminants . Hence a combination of natural attenuation and (bio)stimulation by existing engineering system, in this case ATES system, could a very promising integrated technique for remediation of VOCls.
Several factors are proved to be important for biological natural attenuation, which also apply in enhanced bioremediation of VOCls, included temperature, availability of electron donor and nutrients, redox conditions, presence of specific microorganisms and pH . When these conditions are suitable in the environment, VOCls can be biologically reduce to ethane completely . Such anaerobic process is called reductive dechlorination. However, natural reductive dechlorination is normally limited by one or more factors that mentioned above, resulting in no or incompletely biodegradation of VOCls in the subsurface. For instance, due to lack of “Dehalococcoides ethenogenes”, the only species that can perform full reduction of VOCls to ethene, often cis-DCE and VC accumulate in contaminated subsurface; and electron donor and extra pH most often should be added as part of an engineered bioremediation scheme to fulfil reductive dechlorination .
ATES as an engineered system that locate at similar depth in the subsurface where VOCls also present , could possibly change the environmental conditions for VOCls biodegradation, when ATES is used as a tool for bio-stimulation implementations. ATES is expected to highly influence the temperature and groundwater flow in the subsurface. Temperature is already known as a significant factor for the activities of microorganisms. The tolerance, metabolic activity of microorganisms, and interactions with other microorganisms are influenced by temperature. For biological systems, if rate-limited by enzyme activity, the conversion rates increase by a factor of 1.5 to 2.5, when the temperature of the system increases by 10°C . Moreover, the transport of large volume groundwater (ATES systems have a typical flow rate up to several hundred cubic meters of groundwater per hour) can alter other conditions in the subsurface. Generally seasonal change of groundwater flow direction could be simplified as a homogeneous and redistributive effect in the ATES area, especially on some important geochemical solutes like SO42-, NO3- and HCO3-, microorganism, dissolved organic carbons and nutrients, leading to larger biodegradation area. The enhanced dissolution effect on DNAPL could also be benefit for its bioavailability when ATES is located in DNAPL layers. The dissolution can as well reduce the toxic effect of DNAPL on microorganism. Therefore with proper designs and operation, ATES can be further used to apply bioaugmentation or chemical injection and implemented to stimulate bioremediation.
On the other hand, the functioning of ATES can negatively affect bioremediation of VOCls, and vice versa. The possible disturbance on reductive dechlorination from external unsuitable groundwater with high redox state due to groundwater movement, and biological clogging on ATES wells due to biomass growth from bio-stimulation approaches are so far the most concerns for the application of ATES on contaminated fields. As a result, combing ATES and bio-stimulation as an enhanced bioremediation technique or system requires both comprehensive study of the biogeochemical aspects on different processes as well as characterization of subsurface conditions, and later optimization of engineered system design.
This PhD project aims to investigate the feasibility of combining ATES and bioremediation of VOCls, and study the mutual effects between them. This lecture will provide an overview of the whole PhD project, including latest results, conclusions and future perspective, from not only lab experiments (both batches and columns), but also from numerical models.
Chlorinated Volatile Organic Compounds (CVOCs) are often remediated by means of Enhanced Reductive Dechlorination (ERD). While the working principle and the design criteria are well known in the remediation sector, construction and operation details, which could seem trivial in the design phase, may lead to a significant increase in the Operation and Maintenance (O&M) costs if not properly addressed. Moreover, an adaptive approach, oriented to the optimization in every project phase can contribute to achieve the remediation goals in the planned timeframes. An ERD project led by ARCADIS in an urban area in Germany is used as an example to show how the combination of the adaptive approach with the optimization of technical details could lead to a successful remediation even with a limited accessibility of the contaminated area that is densely covered with buildings.
The contamination at the site was caused by unknown volumes of CVOCs, released by a metalworking factory. The CVOCs are present both in the shallow, low conductive porous aquifer and, in lower concentrations, in a second deeper aquifer in the fractured bedrock. The site was first remediated starting at the end of the 1980s using a combination of Pump and Treat in the aquifer and SVE in the vadose zone. These remediation measures were unsuccessful; ARCADIS was therefore commissioned with a new remediation plan. After integrative investigations and the definition of the groundwater model, ERD was chosen as most feasible remediation method. According to the new remediation plan, diluted molasses would have to be used as organic substrate to promote the biological degradation of the CVOCs. At the beginning of the remediation, concentrations of CVOCs up to 20 mg/L were present in the porous aquifer, and 3 contaminated hot spots were known.
From the very start of the ERD-remediation the injection and monitoring have been managed in a flexible way to address the critical contaminated hot spots. Between the regular monitoring campaigns on all available groundwater wells, more frequent samplings of critical wells were performed and through their evaluation the next injection round was adaptively planned. Through this approach an optimal molasses concentration and optimal injection parameters were selected and further used.
After approximately two years of active remediation only one critical contaminated hot spot was left, which is for the most part covered by a building. After having tried to address this hot spot by increasing the injection volumes in the injection points upstream (to increase the radius of influence and get to deliver the substrate to the soil volume underneath the building), a new injection well in the proximity of the building was installed. The well construction was used as a chance to gain more information about the source area: soil samples were taken and hydrophobic dye soil-water shake tests were carried out to assess the possible presence of residual DNAPL. Through the information collected it was possible to better localize the main contamination source, on which the last remediation efforts were concentrated.
Meanwhile the daily operation of the injection plant proved the efficacy of some design choices. For example, diluting the molasses on site by pumping it through a static mixer in the drinking water stream, biofouling was substantially reduced in comparison to plants in which the mixing happens beforehand, the maintenance was reduced and the molasses could be preserved undiluted for months. Other aspects were optimized during the remediation. For instance, instead of continuously injecting for a long time, the remediation plant was activated intermittently to reduce the pressure within the pipes. Appropriate remediation steering avoided any problems concerning outgassing in the urban remediation area.
The combination of the adaptive approach with the technical optimization allowed reducing the contaminant mass flux to values below the regulatory limit. The remediation was therefore ended, exactly as planned, after four years. Currently the site is monitored to check whether rebound occurs and that the expected transition to the natural attenuation of the residual contamination takes place as postulated.
Bioremediation of chlorinated ethenes and many other chlorinated compounds is optimal at neutral pH with pH’s below 6.0 considered problematic for bioremediation. For example, complete biodegradation of chloroethenes to ethene, is often inhibited below pH 6.0. Given that both reductive dechlorination and fermentation of commonly used electron donors are acid generating processes, enhanced bioremediation has the potential to decrease pH into the inhibitory range, even if prior to biostimulation, pH was acceptable.
In recent years, modifying aquifer pH using buffering agents such as sodium bicarbonate and various commercial formulations has become increasingly common. Aquifer pH modification has met with varying
degrees of success depending on application method, site geology and geochemistry but is generally considered challenging. Effective alternatives or complimentary approaches would be welcome and could improve bioremediation outcomes.
In certain cases, especially where pH is near or slightly below 6.0, the use of bioaugmentation cultures acclimated to lower pH has the potential to reduce the need for aquifer neutralization. Increasing evidence indicates that complete dechlorination to ethene is possible below pH 6.0 with pH tolerant bioaugmentation cultures. Also, previous studies have indicated that certain electron donors have a reduced acidification impact, particularly formate (McCarty et al., 2007) which generates neutralizing bicarbonate alkalinity upon fermentation. The use of electron donors with reduced pH impact, combined with pH tolerant bioaugmentation cultures has the potential to achieve successful bioremediation results with reduced need for aquifer pH adjustment.
The development and field use of low pH acclimated cultures will be discussed and case studies presented. At a Site in Florida, with the pH ranging between 5.5 and 6.0, PCE, TCE and cDCE were completely dechlorinated to VC and ethene within 6 months of bioaugmentation with a low pH tolerant bioaugmentation culture.
Trichloroethene (TCE) is a priority pollutant and among the most frequently detected contaminants in groundwater. The currently available bioremediation measures have certain drawbacks like e.g. the need for auxiliary substrates. Oxidation of chloroethenes under aerobic conditions represents a promising way to deal with the shortcomings of anaerobic reductive dechlorination.
Aerobic oxidation is possible via metabolic degradation with the pollutant serving as growth substrate as well as via cometabolic degradation depending on the presence of a suitable auxiliary substrate. Metabolic pollutant degradation is superior for field applications compared to cometabolic degradation since it does not require the presence of oxidisable auxiliary substrates within the plume. Furthermore, additional oxygen consumption by the auxiliary substrates is avoided. Thus, aerobic metabolic degradation can occur even in aquifers with low organic carbon content, in which cometabolic aerobic degradation as well as anaerobic reductive dechlorination will be hampered due to limited availability of auxiliary substrates.
In our study, the aerobic biodegradation of TCE as the sole growth substrate was demonstrated. This new process of metabolic TCE degradation was first detected in laboratory groundwater microcosms. Further experiments with the enriched mixed bacterial culture in mineral salts medium showed sustained long-term TCE biodegradation down to concentrations below the detection limit. Aerobic TCE degradation resulted in stoichiometric chloride formation and bacterial growth (increase of DNA). Stable carbon isotope fractionation was observed providing a reliable analytical tool to assess this new biodegradation process at field sites. Please refer to Schmidt K. R., Gaza S., Voropaev A., Ertl S., Tiehm A. (2014) Aerobic biodegradation of trichloroethene without auxiliary substrates. Water Res. 59: 112-118 for further information.
Further studies are currently conducted to prove field applicability of the aerobic metabolic TCE biodegradation by a field test with in-situ delivery of oxygen into a joint aquifer.
Aerobic metabolic TCE degradation might represent a new and promising concept for monitored natural attenuation (MNA) approaches or engineered bioremediation of contaminated sites (enhanced natural attenuation, ENA). Those remediation approaches should have advantages compared to reductive dechlorination or cometabolic degradation and can be considered cost-efficient and environmental safe.
Based on these results the assessment of aerobic metabolic TCE degradation at field sites is highly recommended. The observed stable carbon isotope fractionation provides a reliable analytical tool to monitor and quantify aerobic oxidative degradation pathways in the field.
Financial support provided by the German Federal Ministry of Economics and Technology (grant number: 16224 N) is acknowledged. The authors thank Michael Deusch, Siegmund Ertl, Markus Friedrich, Holger Hansel, Michael Heidinger, and Andrey Voropaev for their support.
In situ bioremediation of chlorinated solvents faces particular challenges at operational facilities posed by practicability issues, most notably accessibility. This paper will examine two case studies where remedial strategies employed complementary approaches or treatment trains to address historic losses of tetrachloroethene (PCE) and trichloroethene (TCE) at operational sites in the UK.
At the first site, TCE concentrations averaging 2900µg/l showed little evidence of degradation, with degradation products being less than an order of magnitude (cis -1,2-dichloroethene (cis-DCE) being <0.01mg/l in source zone and vinyl chloride (VC) below detection) . Groundwater conditions were mostly aerobic, so conditions were unsuitable for reductive dechlorination and elevated concentrations of competing electron acceptors were also present such as sulphate. The remedial strategy consisted of injection of a lactate-based hydrogen release compound (HRC®, manufactured by Regenesis) accompanied by HRC primer to acclimatise the groundwater for reductive dechlorination, which was duly demonstrated by successive increases , followed by decreases in the degradation products cis-DCE, VC and ethene. An order of magnitude reduction in total chlorinated ethenes was achieved in two years, and despite the initial absence of any significant reductive dechlorination, no inoculation with Dehalococcoides was necessary. A soil vapour extraction scheme, undertaken in parallel with the groundwater successfully reduced vadose zone concentrations of TCE in soil above the impacted area, the aeration having no effect on redox conditions and no inhibitory effects on reductive dechlorination proceeding within the underlying groundwater.
The second site by contrast had been subject to a much greater degree of impact (and by PCE as well as TCE), including the presence of localised DNAPL in a more cohesive geological formation consisting of a gravelly clay. Reductive dechlorination was already well advanced but access to a significant area of the source of the contamination was restricted, so the overall aim was to mitigate the potential for any off-site migration as a first priority whilst achieving a reasonable degree of mass removal within the source and plume subject to the constraints of on-site operations.
A three-fold strategy was therefore implemented involving (i) periodic injection of HRC as a ‘barrier’ hydraulically down gradient at the site boundary to protect off-site receptors, (ii) application of a percarbonate-based chemical oxidation reagent, Regenox followed by HRC in the source area and (iii) HRC alone in the plume, the HRC applications comprising both a primer and an extended release formulation HRC-X. The purpose of the Regenox was to achieve some immediate mass reduction and/or conversion to more labile intermediates but also to enhance the availability of the contaminant for subsequent degradation. Application of both Regenox (involving three successive rounds of injection over three months) and HRC resulted in a significant mobilisation of PCE or TCE from sorbed or localised DNAPL into the aqueous phase, which subsequently underwent rapid conversion to cis -1,2-dichloroethene followed by a slower reduction to vinyl chloride and then through to ethene.
Notwithstanding some continuing dissolution into the aqueous phase within the source area and immediately down hydraulic gradient from it, degradation has been proceeding at a steady rate in localities towards the boundary. Here, following initial mobilisation, total chlorinated hydrocarbons in groundwater rose from tens of thousands to over 100,000µg/l before being reduced by two-orders of magnitude to concentrations of less than 1,000µg/l, five years after treatment commenced.
The paper will discuss the variation in CHC behaviour in response to active remedial intervention according to its nature, severity and distribution and how remedial progression can be assed using the’ chloride index’, in conjunction with mass removal.