Surface and subsurface soils have different characteristics in natural attenuation. Mobile contaminants in subsurface soils diffuse into soil vapor and aqueous phase and thus are relatively easily subject to natural subsurface processes that can attenuate these contaminants. Most high-molecular-weight (persistent) organic and many inorganic contaminants will be immobilized in the subsurface soil matrix. These persistent organic contaminants often are difficult to degrade and the inorganic metals are conserved. Without probable exposure routes, however, they do not represent significant risk unless unlikely events such as fresh solvent releases, chemical or biochemical transformation or physical disturbances that increase their mobility or open exposure routes. Thus natural attenuation in subsurface soils may be considered on a site-specific-basis as a possible remedy.
In contrast, mobile contaminants in surface soils (typically 1 to 2 feet of soil at sites) usually have already degraded, volatilized or leached from the soil unless weathered free product has entrapped the contaminants. In addition, immobilized contaminants persist in surface soils for decades and thus, if undisturbed, are only slowly removed or not removed at all by natural attenuation processes. Unfortunately, wind or water erosion may mobilize the persistent or conserved contaminants through soil transport into surface waters, because of bioaccumulation of contaminants and biomagnification in the watershed food chain, already has produced wide spread human health and ecosystem risk. In addition, the resulting contaminated sediments are often uncontrollable, since storms and natural events such as earthquakes can resuspend or distribute contaminants over wide areas. Because of these risks, contaminated surface soils will in addition require very long-range and effective containment of soils before being considered for natural attenuation.
For natural attenuation in soils, natural biotransformation processes such as dilution, dispersion, volatilization, biodegradation, adsorption, and chemical reactions with soil materials can reduce contaminant concentrations to acceptable levels. Natural attenuation may be considered for remediation of contaminants in soils if site-specific factors support its use. The factors include:
Protection of potential receptors during attenuation,
Favorable geological and geochemical conditions,
Documented reduction of degradable contaminant mass in a reasonable time frame in the surface and subsurface soils,
Confirmation in microcosm studies of contaminant cleanup,
For the persistent or conserved contaminants, ensured containment during and after natural attenuation.
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 in appropriate time frame before potential exposure pathways are completed. In addition, sampling and sample analysis must be conducted during the process to confirm that clean up is proceeding at rates consistent with 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, PCBs are strongly sorbed to deep subsurface soils and are not migrating; where 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, as in the first two examples, TI waivers must be obtained. In all cases, extensive site characterization and continued monitoring are required.
The O&M duration of natural attenuation is determined from site evaluation and regulatory requirements. The process is expected to continue for several years until desired degradation levels are achieved. Currently, there is virtually no guidance available on the use of natural attenuation in soil.
Target contaminants for natural attenuation are VOCs and SVOCs and fuel hydrocarbons. Fuel and halogenated VOCs (chlorinated solvents) are so far the most 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.
Factors that may limit the applicability and effectiveness of the process include:
Extensive site characterization with modeling and long-term monitoring may be more costly than active remediation.
Toxicity of degradation and transformation products may exceed that of the original contaminants.
High risks occur at sites where geological characteristics such as fracture bed rock or Karst landscapes may prevent assessment of stable plume control for contaminants leached from soil.
Contaminants may migrate (erosion, leaching, volatilization) before they are degraded or transformed.
Ground water at the site contaminated by the soil source will not be available for an extended period of time.
Extensive free product, as nonaqueous phase liquids, may have to be removed before natural attenuation can restore soil in a reasonable time frame.
Conservative metals may only be temporarily immobilized with remobilization when natural attenuation reestablishes oxygenated soil conditions.
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.
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 and nitrate). 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.
Ground water and soil geochemical data.
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 vs. silt vs. clay).
Preferential flow paths.
Interaction between ground water and surface water.
Location of potential receptors: ground water, wells, and surface water discharge points.
Natural attenuation has been selected by AFCEE for remediation at 45 USAF sites.
There are costs for modeling contamination degradation rates to determine whether natural attenuation is a feasible remedial alternative. Additional costs are for appropriate containment, surface and subsurface sampling and sample analysis (potentially extensive) to determine the extent of contamination and confirm contaminant degradation rates and cleanup status. Skilled labor hours are required to conduct the modeling, sampling, and analysis. O&M costs would be required for monitoring to confirm that contaminant migration has not occurred.