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'• 18 July 2002 AGE-NC Project No 96-0254 Page 15 of 17 6 2 4 Duration ' Cleanup durations for in-situ bioremediation will vary,depending upon specific site characteristics However,an 18 to 36 month treatment period is commonly achievable Since dissolved hydrocarbon concentrations at the site are moderate to high, a cleanup period of 24 to 36 months may be projected 6 2 5 Cost tBioremediation of ground water can be a very cost-effective method oftreatment With little on-site equipment required beyond the initial inoculation, the cost for in-situ bioremediation at the site would be between$40,000 and$60,000 However,monitoring will vary depending upon regulatory requirements,which could significantly increase the total cost of the project ' Typical costs for monthly monitoring of an in-situ bioremediation system, in addition to standard quarterly sampling, would be approximately$1,500 per month Based on a one and one-half year treatment period, the total cost for in-situ bioremediation at the site would be between$70,000 and lie $95,000 I6 3 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 or ground water Essential I 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 I 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 ofhydrocarbons have remained relatively stable m monitoring • wells MW-1B and MW-4 overtime,continued groundwater monitoring would be required to show decreasing hydrocarbon concentrations ' Advanced CeoEnnronmenial,Inc