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31 January 2013 AGE-NC Project No. 02-0926 Page 17 of 21 9.3.6. Cost Installation of an in-situ air sparging system and soil vapor extraction system will not be required as both systems and utility infrastructure is currently installed at the site. However, a minimum of two additional air sparging wells and one vapor recovery well should be installed to more rapidly remediate the residual hydrocarbon impact at the site. In general sparging equipment maintenance and monitoring would range from $15,000 to $20,000 annually. In addition to sparging maintenance additional utility and monitoring cost for operation of the carbon based SVE system at the site would be accrued. It is expected that an annual cost of $10,000 to $20,000 would be needed to maintain and monitor the onsite vapor extraction system. Based upon a treatment period of 12 to 24 months, and including replacement of any carbon that would be expended during the remedial process, the total costs would be in the range of $35,000 to $50,000; these costs would not include the installation of the proposed three remedial wells. 9.4. MONITORED NATURAL ATTENUATION (MNA) Natural attenuation, or passive bioremediation, is an alternative relying on natural processes to degrade hydrocarbons. Biodegradation is the most important natural attenuation mechanism. No additional bacteria or nutrients are added to the soil or groundwater . Essential nutrients required for biodegradation are commonly naturally occurring in the subsurface. Dispersion of contamination is an effective physical mechanism to allocate mass into the area of naturally occurring bacteria which consume hydrocarbons. However, where high concentrations of hydrocarbons are present at the site, nutrients may be completely consumed before hydrocarbon remediation is complete. Oxygen depletion in the subsurface is a characteristic of biodegradation. The core of a contaminant plume is typically under anaerobic conditions and only the margins are aerobic. Therefore, even though anaerobic biodegradation is much slower than aerobic biodegradation, anaerobic processes may dominate the degradation of contaminants. Groundwater movement will generally assist in the transport of nutrients in the saturated zone. Natural attenuation is normally used where low concentrations of hydrocarbons are present, following completion of another form of remediation or in an area where no domestic wells or shallow receptors are threatened. In regards to the site, both the soil and groundwater plumes are both highly concentrated in the UST area and downgradient of the former UST area. Impacted soils continue to intensify the groundwater plume, which is evident by the high concentrations encountered Advanced GeoEnvironmental, Inc.