Natural source zone depletion (NSZD) acts to reduce the mass and/or toxicity of light non-aqueous phase liquids (LNAPL) in the shallow subsurface. Several indirect measurement techniques are commonly used to estimate LNAPL mass depletion based on CO2 generation by hydrocarbon mineralization, heat production by exothermic biodegradation processes, and subsurface gas concentration gradients. This paper presents data on LNAPL density and viscosity and shows changes that result from NSZD processes. Increasing trends in LNAPL density and viscosity were observed that provided a qualitative line-of-evidence for NSZD. When LNAPL density and viscosity were combined with contemporaneous LNAPL chemistry analysis it was possible to correlate changes in the physical properties to depletion of alkanes from the LNAPL in a manner that would inform a quantitative NSZD assessment. LNAPL density and viscosity measurements are relatively cheap and reproducible, and increasing trends in LNAPL density and viscosity provide additional lines-of-evidence for NSZD.

Understanding anaerobic biodegradation of ether oxygenates beyond MTBE in groundwater is important, given that it is replaced by ETBE as a gasoline additive in several regions. The lack of studies demonstrating anaerobic biodegradation of ETBE, and its product TBA, reflects the relative resistance of ethers and alcohols with a tertiary carbon atom to enzymatic attack under anoxic conditions. Anaerobic ETBE- or TBA-degrading microorganisms have not been characterized. Only one field study suggested anaerobic ETBE biodegradation. Anaerobic (co)metabolism of ETBE or TBA was reported in anoxic microcosms, indicating their biodegradation potential in anoxic groundwater systems. Non-isotopic methods, such as the detection of contaminant loss, metabolites, or ETBE- and TBA-degrading bacteria are not sufficiently sensitive to track anaerobic biodegradation in situ . Compound- and position-specific stable isotope analysis provides a means to study MTBE biodegradation, but isotopic fractionation of ETBE has only been studied with a few aerobic bacteria (εC −0.7 to −1.7‰, εH −11 to −73‰) and at one anoxic field site (δ 2 H-ETBE +14‰). Similarly, stable carbon isotope enrichment (δ 13 C-TBA +6.5‰) indicated TBA biodegradation at an anoxic field site. CSIA and PSIA are promising methods to detect anaerobic ETBE and TBA biodegradation but need to be investigated further to assess their full potential at field scale. Graphical abstract

Natural source zone depletion (NSZD) is increasingly being considered as a risk-management option at sites impacted with light non-aqueous phase liquids (LNAPLs). NSZD can be applied in isolation or in combination with active remediation techniques, depending on site-specific risk-management requirements. A case study of the transition from active remediation to passive NSZD is presented for a petroleum-impacted site in NW Europe. This transition was supported by multiple lines of evidence/management options, including: the introduction of institutional controls on groundwater and land-development restrictions; the results from a residual-NAPL risk assessment; monitoring to establish that the LNAPL plume is reducing in size; a LNAPL transmissivity assessment; a CO 2 equivalent assessment of remediation options; and a LNAPL recovery diminishing returns model. Through application of local sustainable remediation principles consistent with ISO/SuRF-UK sustainable remediation frameworks and tools, regulatory approval was obtained for a partial closeout of the remediation system. By the final year of operation, NSZD rates in the portion of the site on which transition to NSZD has been agreed were over three times greater than active LNAPL recovery rates (12 000 l/ha/a for NSZD; 3800 l/ha/a for active LNAPL recovery). In the remaining active remediation areas total fluids extraction currently outperforms NSZD and will be continued until a comparable point is reached when NSZD removal exceeds active remediation. At that point transition to NSZD alone will be considered as the most sustainable risk-based approach.

Remediation JournalVolume 32, Issue 1-2 p. 129-132 COMMENTARY Sustainable remediation column: Sustainable remediation tools Jonathan Smith, Jonathan Smith Shell Global Solutions (UK) Ltd, London, UKSearch for more papers by this authorPaul Bardos, Paul Bardos r3 Environmental Technology Ltd, Reading, UKSearch for more papers by this authorFrank Evans, Frank Evans National Grid Properties, Warwick, UKSearch for more papers by this authorAlan Thomas, Alan Thomas ERM Ltd, Oxford, UKSearch for more papers by this authorHayley Thomas, Hayley Thomas Shell Nederland Verkoopmaatschappij B.V., Rotterdam, The NetherlandsSearch for more papers by this authorGerlinde Wolf, Corresponding Author Gerlinde Wolf gerlinde.wolf@ramboll.com Ramboll UK Ltd, London, UK Correspondence Gerlinde Wolf, Ramboll UK Ltd., London, UK. Email: gerlinde.wolf@ramboll.comSearch for more papers by this authorJoe Ricker, Joe Ricker WSP, USASearch for more papers by this author Jonathan Smith, Jonathan Smith Shell Global Solutions (UK) Ltd, London, UKSearch for more papers by this authorPaul Bardos, Paul Bardos r3 Environmental Technology Ltd, Reading, UKSearch for more papers by this authorFrank Evans, Frank Evans National Grid Properties, Warwick, UKSearch for more papers by this authorAlan Thomas, Alan Thomas ERM Ltd, Oxford, UKSearch for more papers by this authorHayley Thomas, Hayley Thomas Shell Nederland Verkoopmaatschappij B.V., Rotterdam, The NetherlandsSearch for more papers by this authorGerlinde Wolf, Corresponding Author Gerlinde Wolf gerlinde.wolf@ramboll.com Ramboll UK Ltd, London, UK Correspondence Gerlinde Wolf, Ramboll UK Ltd., London, UK. Email: gerlinde.wolf@ramboll.comSearch for more papers by this authorJoe Ricker, Joe Ricker WSP, USASearch for more papers by this author First published: 11 March 2022 https://doi.org/10.1002/rem.21709Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume32, Issue1-2Spring 2022Pages 129-132 RelatedInformation

This paper describes the proceedings of a Special Session on Natural Source Zone Depletion (NSZD) at the AquaConSoil conference, held virtually in June 2021. It spans research across Europe, the United States of America and Australia. NSZD has been described as the ‘combination of processes that reduce the mass of LNAPL light non-aqueous phase liquid) in the subsurface’. LNAPL NSZD research and investigations have been focused on a range of hydrocarbon products, such as gasoline, diesel, jet fuel, as well as crude oil. Key NSZD processes include aerobic biodegradation, fermentation and methanogenesis of LNAPL constituents, dissolution of LNAPL constituents into groundwater and volatilization of LNAPL constituents into the unsaturated zone. In turn the generated methane, carbon dioxide and dissolved/volatilized constituents can be cycled and biodegraded in the unsaturated and saturated zones. Importantly these physical, chemical, and biological processes can act without human intervention to reduce the NAPL mass and toxicity. Over time NSZD can both reduce LNAPL mass, and change its chemical composition resulting in risk reduction, and ultimately source depletion. Methanogenesis of organic materials has long been recognized in municipal landfills and natural anoxic environments, such as peat and wetlands. Recognition of similar processes in LNAPL source zones in the past decade along with high rates of aerobic biodegradation observed in unsaturated zones above LNAPL-impacted areas, has significantly revised the conceptual model of LNAPL source zone behaviour and persistence. Several NSZD monitoring approaches have been developed and are being applied in field studies. While the quantitative NSZD rates derived can vary between techniques, they all demonstrate that NSZD LNAPL removal can exceed that delivered by engineered LNAPL recovery techniques, particularly for mature LNAPL bodies.

Electrokinetics (EK) was applied to enhance biodegradation of toluene in the low hydraulic conductivity (K) zone of a physically heterogeneous water-saturated granular porous media. The hypothesis tested was that EK transport processes, which operate independently of advection, can deliver a limiting amendment, nitrate, across a high-K–low-K boundary to stimulate bioremediation. Two types of experiment were evaluated: (1) bench-scale tests that represented the active EK system and physically heterogeneous sediment configuration; (2) microcosms that represented biodegradation in the bench-scale tests under ideal conditions. The bench-scale experiment results showed a rapid decrease in toluene concentration during the application of EK that was attributed to electroosmotic removal from low-K zones. Comparison of toluene removal rates by electroosmosis and biodegradation (microcosm) confirmed that electroosmosis was the most effective mechanism under the conditions evaluated. Overall, this work challenges the original hypothesis and indicates that, at the field scale, the most favourable conditions for biodegradation are likely to be achieved by applying EK to increase contaminant flux across the low-K–high-K boundary (out of the low-K zone) and allowing biodegradation to occur in the high-K zone either by natural attenuation or enhanced by amendment addition. Supplementary material: Supplementary material is available at https://doi.org/10.6084/m9.figshare.c.5174554

Welcome (Fig. 1) to Volume 54 of the Quarterly Journal of Engineering Geology and Hydrogeology (QJEGH), the first to be published online only, after 53 years of printed copies and 20 years of publishing both versions. While some readers will undoubtedly miss the familiar blue volume dropping through the letterbox, the move to online only for all Geological Society journals is more sustainable, environmentally friendly and allows continuous publication, thus avoiding the current backlog of online first articles. This change is fitting and forward looking, as 2022 will be the Geological Society’s Year of Sustainability, following the Year of Space in 2021. Geoscientists have an important role to play in a sustainable low-carbon future, through developing renewable geothermal energy resources, managing risks of spent nuclear fuels, and carbon mitigation measures such as carbon capture and sequestration (e.g. Younger 2013; Brabham et al. 2019; Patton et al. 2019). For engineeringand hydrogeologists, inclusion of sustainability into projects has become increasingly common in recent years (Bardos et al. 2011, 2018), and the launch of the United Nations Sustainable Development Goals (SDG) in 2015 provided added impetus to drive transformational change to meet the targets of the 17 SDGs. Of particular relevance to QJEGH are SDG6 Clean Water and Sanitation, SDG7 Affordable Clean Energy, SDG9 Industry, Innovation and Infrastructure, SDG11 Sustainable cities and communities and SDG13 Climate Action. Inherent in the SDGs, although not a specific goal, is pollution. Soil and groundwater contamination are very much within the scope of QJEGH and the focus of this year’s editorial by Jonathan Smith, (Fig. 2) Assistant Scientific Editor for Hydrogeology. Soil and groundwater contamination can pose potential risks to human health, ecology and the wider environment, hinder the beneficial re-use of land, and create legal and reputational issues for those responsible. Government policies have been developed around the world since widespread recognition of the issue in the latter part of the twentieth century. These policies reflect societal expectations, cultural norms, scientific understanding, and the degree of public outrage about contamination events in a country. Such outrage is typically associated with catastrophic and highprofile events, such as those at Love Canal (USA) and Lekkerkerk (The Netherlands). When public concern was raised by very serious events, the political response was often to develop a policy that required removal of all the contamination, and to restore the site to pristine conditions. In other situations, a more pragmatic fit-for-use approach was adopted, where the presence of residual contamination was tolerated, provided that the land posed no unacceptable risk to human health or the environment, and did not unduly constrain the future beneficial use of the land. A generalised evolution of contaminated land management since the 1950s is illustrated in Figure 3. The timing of this process varied across the world, and some societies jumped straight to later stages, having recognised the importance of the issue later, but able to draw on international experience to speed their policy development. Figure 3 also illustrates the technical awareness, political response and land management solutions associated with each step. The latest developments in this evolution are in the field of sustainable remediation (e.g. CL:AIRE 2010; Bardos et al. 2011; ISO 2017). Sustainable remediation principles require that unacceptable risks to human health and the environment are

The scale of land-contamination problems, and of the responses to them, makes achieving sustainability in contaminated land remediation an important objective. The Sustainable Remediation Forum in the UK (SuRF-UK) was established in 2007 to support more sustainable remediation practices in the UK. The prevailing international consensus is that risk assessment is the most rational approach for determining remediation needs and urgency. Sustainability in this context is related to the effectivedeliveryof whatever risk management is necessary to protect human health or the wider environment. SuRF-UK suggests that decisions made at the project planning stage, and also in the choice of remediation approach used to reach particular objectives decided upon, are both opportunities for sustainability gain. In 2011, SuRF-UK issued a set of wide-ranging indicators to support sustainability assessments made during project planning and remediation option appraisal. This advice was reviewed over 2018-2020 and new guidance on process and indicators has been released. Within this guidance, SuRF-UK has provided a checklist of possible sustainability indicators/criteria that can be used to benchmark the scope of sustainability assessment for remediation projects. These indicators are divided into 15 overarching ("headline") categories, divided in a balanced way across the three elements of sustainability: Environmental (emissions to air, soil and ground conditions, groundwater and surface water, ecology, and natural resources and waste); social (human health and safety, ethics and equity, neighborhoods and locality, communities and community involvement, and uncertainty and evidence); and economic (direct economic costs and benefits, indirect economic costs and benefits, employment and employment capital, induced economic costs and benefits, and project lifespan and flexibility). The majority of this study explains these categories and their various considerations in more depth and provides the supporting rationale that led to their inclusion in the revised SuRF-UK guidance.

Sustainable remediation concepts have evolved during the decade 2007-2017. From the establishment of the first Sustainable Remediation forum (SURF) in 2007, to publication of ASTM and ISO standards by 2017. Guidance has been developed around the world to reflect local regulatory systems, and much has been learned in applying sustainability assessment to contaminated site management projects. In the best examples, significant improvements in project sustainability have been delivered, including concurrent reduction of the environmental footprint of the remediation program, improved social performance, and cost savings and/or value creation. The initial advocates for the concept of sustainable remediation were quickly supported by early adopters who saw its potential to improve the remediation industry's performance, but they also had to overcome some inertia and scepticism from other parties. During the debates and discussions that occurred at numerous international conferences and SURF workshops around the world, various opinions were formed and positions stated. Some proved to be correct, others not so. With the recent publication of ISO Standard 18504 and the benefit of a decade's-worth of hindsight on sustainable remediation programs implementation and project delivery, this paper summarizes a number of myths and misunderstandings that have been stated regarding sustainable remediation and seeks to debunk them. Sustainable remediation assessment shows us how to manage unacceptable risks to human health and the environment in the best, that is to say the most sustainable, way. It provides the contaminated land management industry a framework to incorporate sustainable development principles into remediation projects and deliver significant value for affected parties and society more broadly. In dispelling some myths about sustainable remediation set out in this paper, it is hoped that consistent application of ISO18504/SuRF-UK (or equivalently robust guidance) will facilitate even wider use of sustainable remediation around the world.

Sustainability considerations have become widely recognised in contaminated land management and are now accepted as an important component of remediation planning and implementation around the world. The Sustainable Remediation Forum for the UK (SuRF-UK) published guidance on sustainability criteria for consideration in drawing up (or framing) assessments, organised across 15 headline categories, five for the environment element of sustainability, five for the social, and five for the economic. This paper describes how the SuRF-UK indicator guidance was developed, and the rationale behind its structure and approach. It describes its use in remediation option appraisal in the UK, and reviews the international papers that have applied or reviewed it. It then reviews the lessons learned from its initial use and the opinions and findings of international commentators, and concludes with recommendations on how the indicator categories might be further refined in the future. The key findings of this review are that the SuRF-UK framework and indicator guidance is well adopted into practice in the UK. It is widely recognised as the most appropriate mechanism to support sustainability-based decision making in contaminated land decision making. It has influenced the development of other national and international guidance and standards on sustainable remediation. However, there is room for some fine tuning of approach based on the lessons learned during its application.

Sustainable remediation is the elimination and/or control of unacceptable risks in a safe and timely manner while optimizing the environmental, social, and economic value of the work. Forthcoming International Organization for Standardization (ISO) Standard on Sustainable Remediation will allow countries without the capacity to develop their own guidance to benefit from work done over the past decade by various groups around the world. The ISO standard has progressed through the committee draft (ISO/CD 18504) and draft international standard (ISO/DIS 18504) stages. The risk-based approach to managing the legacy of historically contaminated soil and groundwater has been incorporated into policy, legislation, and practice around the world. It helps determine the need for remediation and the end point of such remediation. Remediation begins with an options appraisal that short lists strategies that could deliver the required reduction in risk. A remediation strategy comprises one or more remediation technologies that will deliver the safe and timely elimination and/or control of unacceptable risks. The ISO standard will help assessors identify the most sustainable among the shortlisted, valid alternative remediation strategies. Practitioners presenting case studies claiming to constitute sustainable remediation should now report how they have aligned their work with the new standard. Indicators are used to compare alternative remediation strategies. The simplest metric that allows a characteristic to act as an indicator should be chosen. Weightings indicators can become a contested exercise and should only be undertaken where there is a clear desire for it by stakeholders and a clear need for it in identifying a preferred strategy. The simplest means of ranking alternative remediation strategies should be adopted.

Fatty acid methyl esters (FAME) are a group of organic compounds that can be synthesized through the process of esterification of fatty acids with methanol. With the increasing use of FAME in biodiesel, there is interest in the fate and effects of FAME in the environment. Single FAME compounds are of low aqueous solubility, low volatility and low mobility but the mechanisms of autoxidation and hydrolysis may result in the generation of more mobile but equally biodegradable components. The FAME types that have been studied in the peer-reviewed literature do not appear to enhance the solubility of hydrocarbons. FAME are widely reported to be readily biodegradable under both aerobic and anaerobic conditions, although rates may vary from site to site. In the majority of studies, biodiesel FAME biodegradation occurred more rapidly than petroleum diesel biodegradation. At sites with limited electron acceptors and macronutrients, microorganisms that degrade FAME have the potential to deplete available electron acceptors and nutrients, resulting in an extended time for diesel biodegradation. As with other labile biofuels, anaerobic biodegradation of FAME may result in significant methane generation. Overall, natural attenuation would appear to be significant in controlling the fate, behaviour and potential risks posed by biodiesel.

Large scale landfarming experiments, using an extensive range of treatments, were conducted in the Niger-Delta, Nigeria to study the degradation of oil in contaminated soils. In this work the effect of nutrient addition, biosurfactant, Eisenia fetida (earthworm) enzyme extract, bulking and sorption agents and soil neutralization were tested. It was found that these treatments were successful in removing up to 53% of the total petroleum hydrocarbon in the soil within 16 weeks. A comparison between treatments demonstrated that most were no more effective than agricultural fertilizer addition alone. One strategy that did show better performance was a combination of nutrients, biochar and biosurfactant, which was found to remove 23% more Total Petroleum Hydrocarbons (TPH) than fertilizer alone. However, when performance normalized costs were considered, this treatment became less attractive as a remedial option. Based on this same analysis it was concluded that fertilizer only was the most cost effective treatment. As a consequence, it is recommended that fertilizer is used to enhance the landfarming of hydrocarbon contaminated soils in the Niger Delta. The attenuation rates of both bulk TPH and Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) fractions are also provided. These values represent one of the first large scale and scientifically tested datasets for treatment of contaminated soil in the Niger Delta region. An inverse correlation between attenuation rates and hydrocarbon molecular weight was observed with heavy fractions showing much slower degradation rates than lighter fractions. Despite this difference, the bioremediation process resulted in significant removal of all TPH compounds independent of carbon number.

The effect of physical heterogeneity on the electrokinetic (EK) transport of nitrate, an electron acceptor frequently used for anaerobic biodegradation, was investigated experimentally within saturated granular porous media comprising two layers of high and low hydraulic conductivity (K) material. Two hypotheses were tested: firstly, that the presence of layered physical heterogeneity will generate non-uniformities in the electric field; and secondly that this would create non-uniform electromigration of ions resulting in an additional nitrate flux into the lower-K layer. Experiments were conducted in bench-top test cells that contained electrode and sediment chambers. An aqueous nitrate solution (0, 0.1, 1 and 5g-NO3L−1) was added at the cathode and the experiments run with an idealised mixture of glass beads and kaolinite, and natural sediment and kaolinite. A constant current (1.6Am−2) was applied in all experiments. Results showed elevated voltage differences between layers in heterogeneous experiments compared to equivalent homogenous experiments. Furthermore, nitrate concentrations are elevated in the low-K material in heterogeneous compared with homogeneous systems. Using predicted values this is shown to be a function of a transverse flux associated with the voltage difference between layers. The importance of this phenomena at field scale for delivery of an amendment (i.e., electron acceptor, donor or nutrient) by EK for bioremediation is presented in an electron balance model. Overall, this research establishes and quantifies a previously unreported important phenomenon in the electrokinetic transport literature that enhance the application of this technology for bioremediation of contaminated aquifers.

Sustainable management practices can be applied to the remediation of contaminated land to maximise the economic, environmental and social benefits of the process. The Sustainable Remediation Forum UK (SuRF-UK) have developed a framework to support the implementation of sustainable practices within contaminated land management and decision making. This study applies the framework, including qualitative (Tier 1) and semi-quantitative (Tier 2) sustainability assessments, to a complex site where the principal contaminant source is unleaded gasoline, giving rise to a dissolved phase BTEX and MTBE plume. The pathway is groundwater migration through a chalk aquifer and the receptor is a water supply borehole. A hydraulic containment system (HCS) has been installed to manage the MTBE plume migration. The options considered to remediate the MTBE source include monitored natural attenuation (MNA), air sparging/soil vapour extraction (AS/SVE), pump and treat (PT) and electrokinetic-enhanced bioremediation (EK-BIO). A sustainability indictor set from the SuRF-UK framework, including priority indicator categories selected during a stakeholder engagement workshop, was used to frame the assessments. At Tier 1 the options are ranked based on qualitative supporting information, whereas in Tier 2 a multi-criteria analysis is applied. Furthermore, the multi-criteria analysis was refined for scenarios where photovoltaics (PVs) are included and amendments are excluded from the EK-BIO option. Overall, the analysis identified AS/SVE and EK-BIO as more sustainable remediation options at this site than either PT or MNA. The wider implications of this study include: (1) an appraisal of the management decision from each Tier of the assessment with the aim to highlight areas for time and cost savings for similar assessments in the future; (2) the observation that EK-BIO performed well against key indicator categories compared to the other intensive treatments; and (3) introducing methods to improve the sustainability of the EK-BIO treatment design (such as PVs) did not have a significant effect in this instance.

The scale of land-contamination problems, and of the responses to them, makes achieving sustainability in contaminated land remediation an important objective. The Sustainable Remediation Forum in the UK (SuRF-UK) was established in 2007 to support more sustainable remediation practice in the UK. The current international interest in ‘sustainable remediation’ has achieved a fairly rapid consensus on concepts, descriptions and definitions for sustainable remediation, which are now being incorporated into an ISO standard. However the sustainability assessment methods being used remain diverse with a range of (mainly) semi-quantitative and quantitative approaches and tools developed, or in development. Sustainability assessment is site specific and subjective. It depends on the inclusion of a wide range of considerations across different stakeholder perspectives. Taking a tiered approach to sustainability assessment offers important advantages, starting from a qualitative assessment and moving through to semi-quantitative and quantitative assessments on an ‘as required’ basis only. It is also clear that there are a number of ‘easy wins’ that could improve performance against sustainability criteria right across the site management process. SuRF-UK has provided a checklist of ‘sustainable management practices’ that describes some of these. This paper provides the rationale for, and an outline of, and recently published SuRF-UK guidance on preparing for and framing sustainability assessments; carrying out qualitative sustainability assessment; and simple good management practices to improve sustainability across contaminated land management activities.

This study investigates and quantifies the influence of physical heterogeneity in granular porous media, represented by materials with different hydraulic conductivity, on the migration of nitrate, used as an amendment to enhance bioremediation, under an electric field. Laboratory experiments were conducted in a bench‐scale test cell under a low applied direct current using glass bead and clay mixes and synthetic groundwater to represent ideal conditions. The experiments included bromide tracer tests in homogeneous settings to deduce controls on electrokinetic transport of inorganic solutes in the different materials, and comparison of nitrate migration under homogeneous and heterogeneous scenarios. The results indicate that physical heterogeneity of subsurface materials, represented by a contrast between a higher‐hydraulic conductivity and lower‐hydraulic conductivity material normal to the direction of the applied electric field exerts the following controls on nitrate migration: (1) a spatial change in nitrate migration rate due to changes in effective ionic mobility and subsequent accumulation of nitrate at the interface between these materials; and (2) a spatial change in the voltage gradient distribution across the hydraulic conductivity contrast, due to the inverse relationship with effective ionic mobility. These factors will contribute to higher mass transport of nitrate through low hydraulic conductivity zones in heterogeneous porous media, relative to homogeneous host materials. Overall electrokinetic migration of amendments such as nitrate can be increased in heterogeneous granular porous media to enhance the in situ bioremediation of organic contaminants present in low hydraulic conductivity zones.