Natural attenuation is the combination of naturally-occurring processes that act without human intervention to reduce the risks posed by contamination in soil or groundwater. The way forward? Phil Morgan, senior environmental consultant with Eutech, on not getting involved.The most widely recognised definition is given by the US Office of Solid Waste and Emergency Response (US EPA OSWER 1999):
"The 'natural attenuation processes' that are at work in such a remediation approach include a variety of physical, chemical or biological processes that, under favourable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume or concentration of contaminants in soil or groundwater. These in-situ processes include biodegradation, dispersion, dilution, sorption, volatilisation; and chemical or biological stabilization or destruction of contaminants."
Extensive field studies indicate that natural attenuation can provide an effective remedial approach to soil and groundwater contamination. It can be applied either as a stand-alone solution or in conjunction with other methodologies. Its correct application may offer significant financial benefits to all stakeholders. The UK Network on Natural Attenuation in Ground Water, for example, used the following illustration to demonstrate the potential savings to be gained from natural attenuation: "Assuming that 2,000 sites require remediation over the next five years, and that the average cost per site is £500,000, the total expenditure on site remediation would be £1,000m.
"Now assume that natural attenuation is appropriate at 25% of these sites and that the saving from natural attenuation per site is 75% of the average cost. Then the potential saving is significant - £200m over the five year period.
Historically, the UK has been cautious in its endorsement of natural attenuation as a remedial measure - and justifiably so. The very nature of UK geology and hydrogeology necessitates specific considerations. However, the regulatory framework now in place in the UK means that natural attenuation can now be recognised as an acceptable remedial approach where it can be proved to be effective. The Environment Agency will shortly be issuing guidelines to facilitate the assessment of natural attenuation for such cases.
The most successful applications of natural attenuation to date have involved petroleum hydrocarbons. However, understanding of natural attenuation for other contaminants, such as phenols and chlorinated solvents, is steadily growing. Landfill leachate, pesticides, chlorinated aromatics and certain inorganics are also treatable by natural attenuation.
Landfill leachate: the components arising from decomposition of organic compounds in a typical landfill are readily biodegradable. Studies also indicate that anthropogenic organics, which may occur in leachates, are biodegradable under the conditions prevailing downstream. This, coupled with the fact that leachate itself is a good stimulator of microbial activity in situ, renders natural attenuation a key process for many landfill plumes.
Petroleum hydrocarbons: highly vulnerable to natural degradation processes in soil and water. Consequently, natural attenuation can provide an effective risk management approach and covers a wide range of applications. Dojka et al (1998) calculated that, as of mid-1998, natural attenuation was the remedy approved by the US EPA for hydrocarbon arising from leaking underground storage tanks at about 47% of sites with groundwater contamination and 28% with soil contamination.
Chlorinated solvents: their excellent solvent properties, chemical stability and relatively low hazard lend them to diverse applications. Chlorinated solvents may be found as contaminants in soil and groundwater where the compounds have been manufactured, used or stored, or where waste has been disposed. Although the application of natural attenuation for chlorinated solvents is at an earlier stage of development compared to petroleum hydrocarbons, all chlorinated solvents are vulnerable to biodegradable destruction and a number of natural attenuation cases have been reported. Therefore, monitored natural attenuation can be a feasible remedial alternative for these compounds at some sites.
Best practice in the evaluation and documentation of natural attenuation is still evolving. Currently, all guidance uses the same fundamental approach, namely "lines of evidence". Here, several complimentary data sets should be collated to establish not only that the processes of natural attenuation are occurring, but that they will protect human health and the environment.
MNA does have its limitations. It can only be considered where the process will adequately address risk and may be difficult to establish for complex geology. The results can take some time to materialise by comparison to some active approaches. There is, however, substantial evidence to suggest that the benefits are numerous. MNA can protect human health and the environment and prove extremely cost-effective. Moreover, it happens anyway - so take care that other remedial approaches don't do anything to disrupt it.