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a a : t r 28 February1997 AGE-NC Project No 95-0144 Page 18 of 20 concentrations at the site are low, a cleanup period of 18 months is likely f ; r 6 3 6 COST 1 , Bibremediation 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 Chase Chevrolet- Madison site would be between $40,000 00 and $60,000 00 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 00 per month Based on a one and one-half year treatment period, the total cost for in-situ bioremediation at the Chase Chevrolet - Madisowsite would be between $70,000 00 and $95,000 00 4 ' ,6 4 NATURAL ATTF,NUATION t 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 Essential nutrients required for biodegradation are usually 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 1 r Natural attenuation is normally used where law concentrations of hydrocarbons are present or following completion of another form of remediation Concentrations of hydrocarbon have varied ' in monitoring well MW-I over time, but showing a general decrease Hydrocarbon concentrations have shown a general decrease in MW-2 and MW-4 Natural attenuation appears to have occurred ' near the former location of USTs No 3 through 6 Hydrocarbon concentrations have increased in monitoring wells MW-5 and MW-13, but have remained relatively low Continued ground water 'monitoring would e 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 , ti 1 f