Risk-Based Closure Considerations For Monitored Natural Attenuation

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Courtesy of Weston Solutions, Inc


Recent advances in both the chemistry- and hydrogeology-based aspects of monitored natural attenuation (MNA) and in toxicology research have changed some of the traditional approaches to MNA for petroleum and chlorinated groundwater plume management. For MNA to be approved as a remedial approach, human health and the environment must still be protected. To demonstrate protection, alternate exposure pathways (such as groundwater-to-air transfer) have been explored and recommended by regulatory agencies.

For example, although degradation of chlorinated plumes is largely desirable, the classic conversion of parent products (perchloroethylene, or PCE, and/or trichloroethylene, or TCE) and accompanying transformation products to vinyl chloride is largely undesirable due to toxicity considerations. MNA projection of degradation to (or through) vinyl chloride has thus been an important consideration in modeling vapor transfer from an affected groundwater zone through vadose zone soil to environmental receptors via inhalation. However, vinyl chloride has recently been found by the National Center for Environmental Assessment (NCEA) at the United States Environmental Protection Agency (USEPA) to be ten to forty times less toxic (via inhalation) than originally thought. Therefore, many chlorinated plumes may, in fact, be safely degrading through the vinyl chloride transformation product, although migration of the plumes and protection for all exposure pathways must still be considered.

Similarly, benzene toxicity factors have been revised to reflect a decreased inhalation toxicity over those initially published in the Integrated Risk Information System (IRIS). As with vinyl chloride, estimates of inhalation risks during the MNA process would also decline and therefore, MNA goals may be set slightly higher for BTEX plume MNA.

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