Natural subsurface processes such as dilution, volatilization, biodegradation, adsorption, and chemical reactions with subsurface materials are allowed to reduce contaminant concentrations to acceptable levels. Natural attenuation is not a 'technology' per se, and there is significant debate among technical experts about its use at hazardous waste sites. Consideration of this option usually requires modeling and evaluation of contaminant degradation rates and pathways and predicting contaminant concentration at down gradient receptor points, especially when plume is still expanding/migrating. The primary objective of site modeling is to demonstrate that natural processes of contaminant degradation will reduce contaminant concentrations below regulatory standards or risk-based levels before potential exposure pathways are completed. In addition, long term monitoring must be conducted throughout the process to confirm that degradation is proceeding at rates consistent with meeting cleanup objectives.
Natural attenuation is not the same as 'no action,' although it often is perceived as such. CERCLA requires evaluation of a 'no action' alternative but does not require evaluation of natural attenuation. Natural attenuation is considered in the Superfund program on a case-by-case basis, and guidance on its use is still evolving. It has been selected at Superfund sites where, for example, removal of DNAPLs has been determined to be technically impracticable (Superfund is developing technical impracticability (TI) guidance); and where it has been determined that active remedial measures would be unable to significantly speed remediation time frames. Where contaminants are expected to remain in place over long periods of time, TI waivers must be obtained. In all cases, extensive site characterization is required.
Compared with other remediation technologies, natural attenuation has the following advantages:
- Less generation or transfer of remediation wastes;
- Less intrusive as few surface structures are required;
- May be applied to all or part of a given site, depending on site conditions and cleanup objectives;
- Natural attenuation may be used in conjunction with, or as a follow-up to, other (active) remedial measures;
- Overall cost will likely be lower than active remediation.
Target contaminants for natural attenuation are VOCs and SVOCs and fuel hydrocarbons. Fuel and halogenated VOCs are commonly evaluated for natural attenuation. Pesticides also can be allowed to naturally attenuate, but the process may be less effective and may be applicable to only some compounds within the group. Additionally, natural attenuation may be appropriate for some metals, when natural attenuation processes result in a change in the valence state of the metal that results in immobilization (e.g., chromium).
Factors that may limit applicability and effectiveness include:
- Data used as input parameters for modeling need be collected.
- Intermediate degradation products may be more mobile and more toxic than the original contaminant.
- Natural attenuation is not appropriate where imminent site risks are present.
- Contaminants may migrate before they are degraded.
- Institutional controls may be required, and the site may not be available for reuse until contaminant levels are reduced.
- If free product exists, it may have to be removed.
- Some inorganics can be immobilized, such as mercury, but they will not be degraded.
- Long term monitoring and associated costs.
- Longer time frames may be required to achieve remediation objectives, compared to active remediation.
- The hydrologic and geochemical conditions amenable to natural attenuation are likely to change over time and could result in renewed mobility of previously stabilized contaminants and may adversely impact remedial effectiveness; and
- More extensive outreach efforts may be required in order to gain public acceptance of natural attenuation.
The extent of contaminant degradation depends on a variety of parameters, such as contaminant types and concentrations, temperature, moisture, and availability of nutrients/electron acceptors (e.g., oxygen, nitrate).
Although many potential suppliers perform the modeling, sampling, and sample analysis required for monitoring natural attenuation, the evaluation of natural attenuation is often not straightforward and will require expertise in several technical areas including microbiology/bioremediation, hydrogeology, and geochemistry. When available, information to be obtained during data review includes:
- Soil and ground water quality data:
- Three-dimensional distribution of residual-, free-, and dissolved-phase contaminants. The distribution of residual- and free-phase contaminants will be used to define the dissolved-phase plume source area.
- Groundwater and soil geotechnical data.
- Historical water quality data showing variations in contaminant concentrations through time.
- Chemical and physical characteristics of the contaminants.
- Geochemical data to assess the potential for biodegradation of the contaminants.
- Geologic and hydrogeologic data:
- Lithology and stratigraphic relationships.
- Grain-size distribution (sand versus silt versus clay).
- Aquifer hydraulic conductivity.
- Flow gradient.
- Preferential flow paths.
- Interaction between ground water and surface water.
- Location of potential receptors:
- Ground water wells.
- Surface water discharge points.
The operation and maintenance (O&M) duration is determined from natural attenuation evaluation and regulatory requirements. The process is expected to continue for several years until desired degradation levels are achieved. The duration of O&M is dependent on all of the data and information listed above.
Natural attenuation has been selected by AFCEE for remediation at 45 sites.
There are costs for modeling and monitoring. Modeling determines whether natural attenuation is a feasible remedial alternative. The most significant costs associated with natural attenuation are most often due to monitoring requirements, which include two major parts - site characterization and performance monitoring. Site characterization determines the extent of contamination and contaminant degradation rates. Performance monitoring tracks down contaminants migration and degradation and cleanup status.