ESN Northwest

Natural Attenuation

Natural Attenuation of both petroleum and halogenated hydrocarbons has received increasing acceptance at sites where plumes are not adversely affecting off-site receptors. In general, natural attenuation may be considered as an acceptable alternative for all or part of a contaminant plume where several criteria are met:

Natural attenuation includes a variety of physical, chemical, and biological processes that, under the appropriate conditions, act without intervention to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in soil or groundwater. Most regulatory authorities accept the use of natural attenuation, but require that it be monitored. Monitoring is ideally conducted in such a way to maximize the accuracy of determining that natural attenuation is appropriate and that the process continues as predicted.

ESN is fully equipped to install systems to monitor prior to and during natural attenuation. Wells are installed to obtain water and vapor samples upgradient plus across and down the plume, plus downgradient, in three dimensions. Prior to determining that natural attenuation is the preferred alternative, analysis of groundwater (and to an extent vapors and soils) for some or all of the following analytes is undertaken. Samples are taken in water at 2-3 levels within the aquifer (plus in vapor at 1-2 depths). The exact list of analytes is determined on a project specific basis. These are in addition to the targeted contaminants.

Those constituents that are starred (*) should always be done on-site to provide for maximum quality control. The others generally are the types of analytes reported from ESN mobile labs on a routine basis, so that an integrated analytical product is convenient and inexpensive on-site, whether fuels or VOCs.

Because the analytical results are available in real time, a preliminary determination can be made as to the applicability of natural attenuation on the site. In addition, it is possible to determine which sampling locations are best for extended monitoring. These can be converted into small bore, permanent monitoring wells during the testing phase. By doing so, and using Direct Push, two significant advantages are obtained.

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