LEAVE IT ALONE! A CASE STUDY FOR NATURAL ATTENUATION OF A TRICHLOROETHENE PLUME WITHOUT ENHANCEMENT

摘要

This paper presents a case study of natural attenuation of the Zone 16 Plume, a large (0.4 by 2.4 kilometers [km]) dissolved-phase trichloroethene (TCE) plume at Hill Air Force Base (AFB) Operable Unit (OU) 5. Evidence indicates that natural attenuation processes may be sufficient to remediate the Zone 16 TCE Plume. Natural attenuation was evaluated through examination of degradation products, geochemical data (e.g., dissolved oxygen, nitrate, oxidation-reduction potential, etc.), and contaminant time-series trends. A groundwater flow and contaminant transport model (MODFLOW and MT3D) was constructed to predict plume longevity. Both site data evaluation and modeling results suggest that the TCE plume will continue to attenuate through time to below the maximum contaminant level (MCL) (5 micrograms per liter [μg/l] for TCE) in less than 50 years. The Zone 16 TCE Plume is shallow (water table is 1 to 6 meters [m] below ground surface [bgs]), lies beneath residential areas, and has hydraulic and geotechnical properties that preclude conventional engineered remediation (e.g., pump and treat). While innovative remedial technologies such as enhanced biodegradation can be an effective means to reduce remediation times, unacceptable side effects need to be carefully considered prior to application. Laboratory microcosm and column studies performed with aquifer material from OU 5 have shown that biotransformation rates can be increased through biostimulation (e.g., addition of organic carbon) and bioaugmentation (I.e., addition of microbes). However, the formation of methane and odors were observed in laboratory experiments in sufficient quantities to be of concern for application in a residential setting. In addition, due to the high amorphous iron content of the aquifer material, enhanced biodegradation would mobilize iron and other metals such as arsenic causing further groundwater degradation. Natural attenuation may be the preferred remedial approach over either conventional engineered technologies or enhanced bioremediation, due to physical limitations as well as potential adverse impacts associated with these technologies.