The approach presented in this guide is a practical and streamlined process for determining the appropriateness of remediation by natural attenuation and implementing remediation by natural attenuation at a given petroleum release site. This information can be used to evaluate remediation by natural attenuation along with other remedial options for each site.

In general, remediation by natural attenuation may be used in the following instances:

As the sole remedial action at sites where immediate threats to human health, safety and the environment do not exist or have been mitigated, and constituents of concern are unlikely to impact a receptor;

As a subsequent phase of remediation after another remedial action has sufficiently reduced concentrations/mass in the source area so that plume impacts on receptors are unlikely; or

As a part of a multi-component remediation plan.

This guide is intended to be used by environmental consultants, industry, and state and federal regulators involved in response actions at petroleum release sites. Activities described in this guide should be performed by a person appropriately trained to conduct the corrective action process.

The implementation of remediation by natural attenuation requires that the user exercise the same care and professional judgement as with any other remedial alternative by:

Ensuring that site characterization activities focus on collecting information required to evaluate and implement remediation by natural attenuation;

Evaluating information to understand natural attenuation processes present at the site;

Determining whether remediation by natural attenuation is the most appropriate and cost-effective remedial alternative with a reasonable probability of achieving remedial goals; and

Monitoring remedial progress.

Application and implementation of remediation by natural attenuation is intended to be compatible with Guide E1739 or other risk-based corrective action programs.

This guide does not address specific technical details of remediation by natural attenuation implementation such as site characterization (see Guide E1912), sampling, data interpretation, or quantifying rates. For additional discussion and guidance concerning these technical issues for remediation by natural attenuation see Appendix X1 through Appendix X7.

This guide does not specifically address considerations and concerns associated with natural attenuation of non-petroleum constituents, such as chlorinated solvents. Care must be taken to ensure that degradation by-products will not cause harm to human health or the environment. In addition, if constituents are present which do not readily attenuate, such as methyl-t-butyl ether (MTBE), remediation by natural attenuation may not be a suitable remedial alternative or may need to be supplemented with other remedial technologies.

This guide is intended to be consistent with Guide E1599 and U.S. EPA guidance for implementation of remediation by natural attenuation (U.S. EPA, 1995, Chapter 9).

1.1 This is a guide for determining the appropriateness of remediation by natural attenuation and implementing remediation by natural attenuation at a given petroleum release site, either as a stand alone remedial action or in combination with other remedial actions.

1.2 Natural attenuation is a potential remediation alternative for containment and reduction of the mass and concentration of petroleum hydrocarbons in the environment to protect human health and the environment. Remediation by natural attenuation depends upon natural processes such as biodegradation, dispersion, dilution, volatilization, hydrolysis, and sorption to attenuate petroleum constituents of concern to achieve remedial goals.

Note 1—Remedial goals must be established through another process as determined by the appropriate regulatory agency.

1.3 In general, remediation by natural attenuation should not be considered a presumptive remedy. A determination of whether remediation by natural attenuation is appropriate for an individual petroleum release site, relative to site-specific remedial goals, requires site characterization, assessment of potential risks, evaluation of the need for source area control, and evaluation of potential effectiveness similar to other remedial action technologies. Application and implementation of remediation by natural attenuation requires demonstration of remedial progress and attainment of remedial goals by use of converging lines of evidence obtained through monitoring and evaluation of resulting data. When properly applied to a site, remediation by natural attenuation is a process for risk management and achieving remedial goals. Monitoring should be conducted until it has been demonstrated that natural attenuation will continue and eventually meet remedial goals.

1.3.1 The primary line of evidence for remediation by natural attenuation is provided by observed reductions in plume geometry and observed reductions in concentrations of the constituents of concern at the site.

1.3.2 Secondary lines of evidence for remediation by natural attenuation are provided by geochemical indicators of naturally occurring degradation and estimates of attenuation rates.

1.3.3 Additional optional lines of evidence can be provided by microbiological information and further analysis of primary and secondary lines of evidence such as through solute transport modeling or estimates of assimilative capacity.

1.4 The emphasis in this guide is on the use of remediation by natural attenuation for petroleum hydrocarbon constituents where ground water is impacted. Though soil and ground water impacts are often linked, this guide does not address natural attenuation in soils separate from ground water or in situations where soils containing constituents of concern exist without an associated ground water impact. Even if natural attenuation is selected as the remedial action for ground water, additional remedial action may be necessary to address other completed exposure pathways at the site.