Principle
In natural attenuation (NA), the pollution level in the soil (soil, ground-water and soil air) is reduced via natural processes, without human intervention, within a reasonable timeframe compared to other more active approaches. These processes may be physical, chemical or biological in nature. They cause the concentration, mass, volume, mobility and/or toxicity of the pollution in the ground-water and/or soil to fall.
When a soil clean up project is performed on the basis of natural attenuation rather than available technology, then this is considered to be guarded or monitored natural attenuation (MNA). MNA is a method for soil clean up that is based on understanding and quantitatively documenting natural processes at a polluted site.
It must be proven that these natural processes provide protection to human and ecological receptors, against harmful exposure to pollutants.
Implementation area and implementation conditions
Natural attenuation is well documented for a limited number of pollutant types, like BTEX. For most other contaminants, there is less experience with NA and its chance of success. The low chance of success is sometimes attributable to the fact that immobilisation and degradation are dependent upon very specific soil conditions. If toxic intermediaries are formed or if there is lack of knowledge about the possible by-products, the attainability of natural attenuation should be regarded as being low.
At each location where natural attenuation is being considered as a ground-water purification technique, the reference processes responsible for natural degradation need to be identified and documented. It is insufficient to establish that, over time, there will be a fall in contaminant concentrations.
The following is meant by reference processes: Well documented and well know reaction mechanisms (such as BTEX and VOCL) and the appearing process and end products. Only by establishing the correlation between the disappearance of the original pollution and (1) the forming of degradation products and (2) the use of carbon sources or electron acceptors (and in accordance with forming of CO2 or CH4 and the reduced forms of the electron acceptors) can the correct level of evidence be attained.
A conceptual model of the site and the pollution needs to be set out, which provides an indication of the long-term evolution during natural attenuation. The model should contain: Ground-water flow, substance transport in source and plume zones (all relevant parameters, also process and end products). During monitoring, this model should always be tested and adjusted for newly received information. The more complicated the site, the greater the required effort – due to higher uncertainty in the original conceptual model.
A long-term monitoring plan, linked to the original conceptual model, must be established in detail. The required monitoring is determined by the level of uncertainty (for complex soil structure, proximity of potential receptors and difficult to degrade compounds, much more extensive monitoring is required than in simple cases). In any case, one should consider a monitoring period ranging from years to decades. For stabilisation, monitoring that lasts for centuries could be required One should also implement an alternative approach, in the event that monitoring shows that
natural attenuation is not running successfully.
In a densely populated region like Flanders, the cadastral land parcels are often small. This may limit the applicability of natural attenuation as an alternative for a more active remediation or risk management, in comparison with more expansive regions that could be encountered in other countries, like the USA. Thus, the option of natural attenuation will often remain limited to large cadastral lands and/or to plumes that are stable or reducing in size.
Preferably, natural self-cleaning is recommended in combination with other remediation approaches. For example, by:
- active remediation of the source(s) (e.g. via excavation) with a treatment on the edge of the plume with reactive walls; or
- as post-treatment of the plume after the main share of the waste content has been removed via an active in-situ clean up technique (e.g. multi-phase extraction).
Removal of the source can influence the geo-hydrological conditions which determine the NA process. One should consider this when the conceptual model for MNA is designed.
Costs
The costs for a prior attainability investigation and for long-term monitoring after implementing natural attenuation, could be very high. In addition, there is always a risk that natural attenuation will not prove effective at a future point in time. During this long period, there will be a mortgage on the sale value of the terrain. Thus, the financial risks from this technique are large; which is why one should be particularly carefully when evaluating it.
Environmental burden and measures to be implemented
Natural attenuation is the most extensive remediation option that can be selected; though there are very few energy costs.