David Lerner, Stephen Thornton and Ruth Davison, Groundwater Protection and Restoration Group, University of Sheffield
Groundwater pollution problems
Contaminated land and groundwater remain a large liability on the balance sheets of many companies, local authorities and for UK plc. Groundwater pollution is particularly difficult to clean up and researchers have often argued that no severely polluted site has been successfully restored by engineered intervention (National Research Council, 1994).
The underlying problem is inaccessibility of the pollutants. Bypasses for flow, diffusion of pollutants into low permeability materials and long term sorption, all mean that the time scales for clean-up are controlled by diffusion, a very slow process. This has been recognised in the US, where it is now possible to obtain a Technical Infeasibility Waiver for some sites. Pump and treat, which is the standard approach when active restoration is undertaken, is best seen as a containment approach.
Until now, most 'owners' of polluted groundwater have only dealt with the problems when they wish to sell land, or they have such a serious incident that the Environment Agency becomes involved. This is going to change. Recent and imminent legislation will create a new contaminated land regime, which will require every contaminated site to be identified and evaluated for the risk it may cause to controlled waters. The Environment Agency is now empowered to require clean-up where groundwater is polluted but not linked to any contaminated land. And the new IPPC legislation will require landholders to put baseline surveys of the environmental condition of their sites on the public register. Then there will be public information about which sites are polluted and powers to require clean- up - ignorance and inaction will no longer be options.
There is a potential conflict between the increasing pressure on landholders to act on groundwater pollution and the acknowledged difficulties of engineered clean-up. Natural attenuation is a potential strategy to resolve this conflict for many sites - if natural processes can be demonstrated to be reducing risks to acceptable levels, then active clean-up will be unnecessary.
This article deals with the concepts and philosophy of natural attenuation and discusses whether the UK needs a formal protocol to guide consultants and landholders.
Natural attenuation concepts
Natural attenuation of polluted groundwater refers to the suite of '. . . physical, chemical or biological processes that act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil and groundwater. . .' (US EPA, 1998). These insitu processes include biodegradation, dispersion, dilution, sorption and volatilisation and chemical or biological stabilisation, transformation, or destruction (US EPA, 1999). It is also called intrinsic (bio)remediation, and the term 'monitored natural attenuation' (MNA) is becoming standard, as it emphasises that monitoring must be undertaken if MNA is to be accepted a strategy for any site.
The processes of natural attenuation are shown schematically in Figure 1. With time, polluted groundwater moves from the source (A), through intermediate points in the plume (B), to the receptor (C). MNA will be successful if the flux and concentration of pollutants is reduced enough to make the risks to the receptor acceptably low. For organic pollutants, biodegradation is usually the most important process. Anaerobic degradation will occur inside the plume but is slow and limited for many situations. Usually aerobic and nitrate reducing degradation are the most important reactions but can only take place at the fringe where background waters mix slowly into the plume. The plume will grow until the influx of pollutants at the source is balanced by the rate of destruction of the combined fringe mixing and the slower internal reactions.
When is MNA appropriate?
The use of MNA has increased substantially over the last decade. It has, for example, been implemented in 23% of US cases in 1997 compared with only 5% in 1991 (Kremer, 1998a). MNA was preferred for the remediation of 46% of petroleum hydrocarbon-contaminated sites and was also widely used for inorganic contaminants and chlorinated solvents (35%).
The exploitation of MNA as a remediation technology is supported by the US EPA, within the context 'of a carefully controlled and monitored site clean-up approach that will reduce contaminant concentrations to levels that are protective to human health and the environment within a reasonable time frame' (US EPA, 1999). This framework dictates that MNA is neither a 'walk away' or presumptive approach to site remediation. It requires sound technical justification within a site-specific decision process, that allows for contingency measures in the event that its predicted performance fails to meet regulatory clean-up criteria (Wilson, 1998a). This implies a sophisticated site assessment.
MNA has many potential advantages, but also some possible limitations, when compared with conventional engineered approaches (Table 1). It is not for those in a hurry, or for landholders who want to install equipment for public relations reasons. Nor is it for all compounds. For example, chlorinated solvents only degrade anaerobically and incomplete degradation produces vinyl chloride which is more toxic than the parent compounds.
MNA is a viable option for compounds which do attenuate and for sites with enough travel distance and time to allow the processes to operate. If these hurdles are passed, non-technical factors are considered, such as the institutional controls over long periods, the availability of adequate funding for reliable monitoring and performance evaluation and public acceptance of the extended timescale required for remediation.
The cost effectiveness of MNA depends on the technical feasibility of achieving clean-up targets at comparable or lesser cost than active schemes. A recent study (Ellis, 1997) found that there is only an incremental increase in site investigation costs required for the technical appraisal of MNA, compared to pump and treat schemes, but significant cost savings if MNA is subsequently implemented (Table 2).
How to demonstrate MNA
Natural attenuation is evaluated using multiple lines of evidence (Wiedemeier et al, 1999). These distinct but converging lines of evidence are based on hydrochemical and microbiological data that document the occurrence and extent of insitu contaminant removal processes. The nature of the information required to demonstrate MNA of organic contaminants includes:
Primary lines of evidence such as historical trends in contaminant data showing plume stabilisation and/or contaminant mass loss over time;
Secondary lines of evidence showing that insitu hydrogeochemical conditions are suitable for biodegradation and that active biodegradation has occurred. This is based on geochemical indicators of naturally occurring contaminant biodegradation, such as the depletion of electron acceptors and donors, increased metabolic by-product concentrations, decreasing parent compound concentrations and increased daughter compound concentrations;
Tertiary or optional lines of evidence that prove the processes and estimate rates.
The process of decision making is shown in Figure 2. Primary and secondary lines of evidence are obtained by sampling groundwater monitoring boreholes along the plume flowpath, including the source area, and uncontaminated zones of the aquifer. They are used to show that a plume is shrinking, stable or only growing at a rate slower than that predicted by conservative groundwater flow calculations.
They also demonstrate that this behaviour is consistent with the hydrochemical environment. Tertiary lines of evidence include data from microcosm studies and are used to support an MNA assessment, when the other evidence is inconclusive, or when information is required on a specific degradation mechanism or on environmental factors that may limit biodegradation processes. Plumes are often simulated with analytical or numerical solute transport models. Evaluating the effectiveness of MNA is an ongoing commitment and a long term monitoring strategy is required to ensure that initial predictions of its performance meet the site-specific clean-up objectives.
Development of protocols for MNA
The emergence of MNA as a remediation technology has resulted in the development (which is ongoing) of technical protocols for its application. These are guidance documents which present good practice in site investigation and analysis of data required to implement and monitor MNA. The development of these protocols has been driven by American experiences, producing guidance for petroleum hydrocarbons (Wiedemeier et al, 1995) and chlorinated organics (Wiedemeier et al, 1997).
Protocols have also been developed by the American Society for Testing & Materials (ASTM, 1998) and private sector companies in response to specific contaminant problems (eg Buscheck and O'Reilly, 1995). Significantly, there is little technical guidance for the application of MNA in many European countries, although this is presently under consideration in Denmark (P Bjerg) and in the Netherlands (A Sinke). A guidance document, commissioned by the Environment Agency, for the assessment of MNA in the UK is currently under development.
UK need for a MNA protocol
The US protocols have proved very useful. They have introduced a common language and philosophy to MNA assessments. They have made studies more cost effective and enabled MNA to be widely accepted, reducing the costs of contaminated groundwater to society.
However the UK is not the same as the US. The legal and administrative arrangements are different; for example the concept of a legally binding Record of Decision, used in the US to define the clean-up plan, does not exist in the UK. The hydrogeology is also different, with greater interest in the UK in consolidated aquifers such as the chalk and Sherwood Sandstone. The knowledge base among UK regulators and consultants is weak, partly due to the culture of secrecy and only a few well-documented case studies underlain by good scientific investigation and interpretation.
A UK protocol for MNA would rapidly increase knowledge levels in the industry and produce a common language. It would probably raise standards of investigation, a subject of much concern in the profession. It is strongly recommended that the protocol includes a peer review. More than anything, this will raise standards of investigation and interpretation and share experience among the community. It will help with the problem of Environment Agency decision making staff often having very low levels of knowledge and experience in science of biodegradation in groundwater.
Without such transparency there is a serious risk that the public will see MNA as a do nothing, money saving option which is justified in a secret report. MNA is more than this, it is a valuable addition to a repertoire of solutions to the massive problems of contaminated land and groundwater.
The Network on Natural Attenuation in Groundwater and Soils (NNAGS) is hosted by the Groundwater Protection and Restoration Group and funded by EPSRC to promote research in the field. NNAGS runs courses, workshops and an annual conference. Anyone can join by visiting the website, http://www.shef.ac.uk/~nnags/. Stephen Thornton is the Environment Agency sponsored research fellow on natural attenuation at the University of Sheffield and can be contacted for advice and further information.
American Society for Testing & Materials (ASTM) (1998), ASTM guide for remediation by natural attenuation at petroleum release sites, ASTM, Philadelphia.
Buscheck T and O'Reilly K (1995). Protocol for monitoring intrinsic bioremediation in groundwater. Chevron Research &Technology Company, Health, Environment and Safety Group. pp20.
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Wiedemeier TH, Swanson MA, Moutoux DE, Wilson JT, Kampbell DH, Hansen, JE and Haas P (1997). Overview of technical protocol for the natural attenuation of chlorinated aliphatic hydrocarbons in groundwater under development for the US Air Force Center for Environmental Excellence. Proc. Symposium on Natural attenuation of chlorinated organics in groundwater, 37-61. U.S. EPA/540/R-97/504.
Wiedemeier TH, Rifai HS, Newell CJ and Wilson JT (1999). Natural attenuation of fuels and chlorinated solvents in the subsurface. Wiley & Sons.
Wilson JT (1998). Risk management of monitored natural attenuation. Seminars on monitored natural attenuation for groundwater, 6-3 to 6-17. US EPA/625/K- 98/001.