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' 01 June 1999 AGE-NC Project No 95-0137 Page 23 of 26 6 3 5 Cost Bioremediation of ground water can be a very cost-effective method of treatment 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 $95,000 ' 6 4 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 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 Ground water 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 nearby wells Concentrations of hydrocarbon have varied in monitoring wells MW-1 and MW-2 over time Dissolved hydrocarbon concentrations ' have increased over the short tern in vapor wells VW-1 and especially VW-2 Continued ground water monitoring would be required to show a stable plume and decreasing hydrocarbon ' concentrations The duration for natural attenuation can be many years 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 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 ground water plume Advanced GeoEn%ironmenlal,Inc