Executive Summary of Natural Attenuation

Many sites consist of groundwater contaminated with petroleum hydrocarbons and/or chlorinated ethenes (solvents). The source of this contamination is usually a fuel spill in the case of the former, and improper disposal of waste solvents in the case of the latter. Formerly, the most common method of remediating groundwater with these contaminants was pump and treat. These systems were forecast to operate for decades before contaminant concentrations were significantly reduced, if ever. As a consequence, the cost of remediating contaminated groundwater at any one site was substantial, and when summed across similar sites throughout the United States was astronomical.

Monitored natural attenuation (MNA) was developed as a method to provide remediation of groundwater at significantly reduced costs while still being protective of human health and the environment. MNA is accepted by the United States Environmental Protection Agency and all states when supported by field data. Monitored natural attenuation has as its basis the degradation of groundwater contaminants by naturally occurring microorganisms. Implementation is usually straightforward and operation/maintenance is simple.

Computer-based models are available for free to allow sites to be screened for their potential to support MNA as either the sole remediation, or as part of a more complex remediation strategy. If conditions for biodegradation are favorable MNA may be implemented with periodic monitoring of groundwater to verify that contaminant concentrations and mass are diminishing. In some cases it may be effective to enhance MNA by adding amendments to the groundwater. Usually this is not required on a continuous basis.

In summary, where MNA potential is high, and where an imminent risk to human health and the environment does not exist, MNA may be the remediation method of choice for groundwater contaminated with petroleum hydrocarbons or chlorinated ethenes.

Overview

Natural Attenuation has emerged in recent years as a viable approach for remediating groundwater at many contaminated sites for numerous reasons. This remedial approach involves the reduction of contaminant concentrations and mass by relying on attenuation mechanisms that occur without human intervention. These mechanisms typically involve both nondestructive and destructive processes. Some of the advantages of natural attenuation include the fact that it is non-intrusive, cost-effective, and, typically does not involve the transfer of contamination from one medium or phase to another. Similar approaches have been used for years to successfully control water quality in rivers, streams, lakes, and estuaries. For example, waste loading permits for industrial and municipal wastewater discharges are generally based on the ability of the water body to assimilate the proposed waste load in such a way that significant depletion of oxygen is prevented.

Researchers and practitioners have, in recent years, sought an understanding and acceptance of natural attenuation for contaminants in groundwater similar to that which exists for surface water. This process is complicated by the nature of groundwater systems and the behavior of the contaminants. Contaminants in the subsurface distribute themselves among the different media (soil, water, and soil vapor) and can exist in different phases (e.g., nonaqueous, aqueous, and vapor phases). Additionally, a large number of processes such as sorption, dispersion, dilution, and biodegradation control their fate and transport. Significant progress in evaluating and quantifying natural attenuation of contaminants in groundwater systems has been made over the past decade and numerous case studies are available.

Natural attenuation is not appropriate for all sites but in many cases it can be appropriate and effective in removing contaminant mass from the subsurface. Field have been developed to aid in evaluating natural attenuation. Use of these protocols in conjunction with direct reading meters and field test kits for biogeochemical parameters (e.g., dissolved oxygen, nitrate, iron, sulfate, oxidation-reduction potential) allows real-time plume delineation which reduces the cost and time required to determine plume configuration. In addition analytical and numerical >models have been developed to evaluate the potential for further attenuation and to simulate the contaminant plume in future years. In addition, cost and performance summary reports have been prepared. These efforts have allowed the scientific, technical and regulatory communities to make informed decisions regarding the use of natural attenuation as a remedial approach for contaminated groundwater.

In many cases if natural attenuation is to remain as the selected remedial alternative, long term monitoring may be required to make sure natural attenuation remains protective. It is estimated that natural attenuation has saved the Air Force and US taxpayers $525,000,000 dollars while still affording an equal level of protection of human health and the environment. The AFCEE natural attenuation initiative was instrumental in gaining a better understanding of natural attenuation and has been working in that field since 1993. If you would like to find out more about natural attenuation click on the links above and then click here.

Background

Natural attenuation is a term that refers to the observed reduction in contaminant concentrations as contaminants migrate from the source area in environmental media without human intervention. This reduction in concentration is primarily due to a number of fate and transport processes that can be defined as nondestructive and destructive. Nondestructive attenuation mechanisms bring about a reduction in contaminant concentration but not contaminant mass and include simple dilution, dispersion, sorption, and volatilization. Destructive attenuation mechanisms bring about both a reduction in contaminant concentration and contaminant mass and include biotic and abiotic transformations. In many cases, natural attenuation will reduce dissolved contaminant concentrations below regulatory standards such as maximum contaminant levels (MCLs) or other remediation goals before the contaminant plume reaches potential receptor exposure points.

The use of natural attenuation as a remedial approach, referred to as "monitored natural attenuation" by the United States Environmental Protection Agency (USEPA), implies a plume management approach that involves long-term monitoring of the plume. USEPA (1999) defines monitored natural attenuation as follows:

The term "monitored natural attenuation," as used in this Directive, refers to the reliance on natural attenuation processes (within the context of a carefully controlled and monitored site cleanup approach) to achieve site-specific remedial objectives within a time frame that is reasonable compared to that offered by other more active methods. The "natural attenuation processes" that are at work in such a remediation approach include a variety of physical, chemical, or biological processes that, under favorable conditions, act without human intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil and groundwater. These in-situ processes include biodegradation; dispersion; dilution; sorption; volatilization; and chemical or biological stabilization, transformation, or destruction of contaminants.

In practice, natural attenuation also is referred to as intrinsic remediation, intrinsic bioremediation, natural restoration, or passive bioremediation.

Development History

The AFCEE Technology Transfer Division was instrumental in developing our current understanding of natural attenuation. Beginning in the early 1990s Technology Transfer, with the participation of the US EPA, undertook an extensive field investigation of natural attenuation at petroleum-contaminated Air Force sites. One outcome of this effort was the development of a detailed field protocol for conducting and interpreting natural attenuation studies. Subsequent to this Technology Transfer undertook a similarly extensive field investigation of natural attenuation at Air Force sites contaminated with chlorinated aliphatic hydrocarbons (CAH), such as trichloroethene. This effort led to the development of a field protocol for conducting natural attenuation studies at chlorinated-contamination sites.

For a brief history of the development of our understanding of natural attenuation and the development of the numerous protocols available for evaluating natural attenuation click here.