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11 February 2003 • AGE-NC Project No. 98-0520 Page 19 of 21 7.4. MONITORED NATURAL ATTENUATION Natural attenuation, or passive bioremediation, is a remedial alternative that relies on natural ' processes to degrade hydrocarbons. Biodegradation is' the most important natural attenuation mechanism. No additional bacteria or nutrients are added to the soil of ground water. Essential nutrients required for biodegradation are commonly naturally present in the subsurface. However, •• where high concentrations of hydrocarbons are present, nutrients may be 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 or irrigation . wells are threatened. Concentrations of petroleum hydrocarbons and BTEX compounds have ' declined noticeably in monitoring well-MW-5, at the former gasoline UST location (Figures-8 and 9);all BTEX compounds are below the California's Department of Health Services' Maximum Contaminant Levels (MCLs) for drinking water. Costs for natural attenuation are.generally lower than most active remediation options; however, costs .can be highly variable depending on'the duration and type of monitoring required. The ' duration can be many years; for instances based on preliminary TPH-g natural attenuation curves, the TPH-g concentration in ground water at the UST locations are theoretically projected to decline to non-detectable levels by.year 2006 to 2008 (Figure 8). Fate and Transport modeling may be used to demonstrate the potential for hydrocarbons to impact . nearby ground water receptors.Quarterly ground water monitoring may also be used to demonstrate a stable and declining groundwater plume. ` 8.0. RECOMMENDATIONS ' ' The following recommendations for remediation are based upon data collected during the investigation of the property and predicted responses of the site conceptual model' Advanced Geo$nvironmerital,Inc.