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r <br /> ` y <br /> 'f <br /> i <br /> l <br /> 15 February 2000 <br /> AGE-NC Project No 98-0448 <br /> Page 29 of 33 <br /> i <br />' contaminants and-nutrients, temperature and the permeability of soil Theoretically, the radius of <br /> influence of the inoculation well should increase rapidly at first, then gradually decline as the <br /> affected volume increases and the bacteria encounter decreasing hydrocarbon concentrations toward <br />' the margins of the hydrocarbon plume Shorter-chain hydrocarbons are typically consumed fust and - <br /> longer-chain hydrocarbons later Based upon the above mentioned factors, a duration of 24 to 36 <br /> months is estimated for successful in-situ bioremediation <br /> 1 <br /> 9 3 5 Cost <br />' Due to the low number of hydrocarbon-degrading organisms at the site, the cost for in-situ <br /> bioremediation is uncertain and will depend upon the amount of inoculant required and the duration <br />' of bEoremedial activity With little on-site equipment required beyond the initial inoculation, the cost <br /> for in-situ bioremediation at the site could range between $25,000 and $65,000 However, <br /> monitoring costs will vary depending upon regulatory requirements, which could significantly <br /> increase the total cost of the project Typical costs for monthly monitoring of an in-situ <br /> bioremediation system, in addition to standard quarterly sampling, would be approximately$1,500 <br /> per month Based on a 24 to 36 month treatment period, the cost for monthly monitoring and <br /> quarterly sampling events could range between $36,000 and $54,000 Therefore, the total cost for <br /> in-situ bioremediation at the site could range between $61,000 and $119,000 <br /> 1 <br /> 9 4 NATURAL ATTENUATION <br /> Natural attenuation, or passive bioremediation, is a remedial alternative that relies on natural <br /> processes to degrade hydrocarbons Natural attenuation is an"inactive"remediation method and does <br />' require a high degree of site characterization and long-tern monitoring <br />' 9 4 1 Principles <br /> As with natural attenuation in soil, biodegradation is the most important natural attenuation <br />' mechanism for impacted ground water No additional bacteria or nutrients are added to the ground <br /> water Essential nutrients required for biodegradation are usually naturally present in the saturated <br /> zone However,where high concentrations of hydrocarbons are present, nutrients may be'consumed <br />' before hydrocarbon remediation is complete Oxygen depletion in ground water is another <br /> characteristic of passive biodegradation Natural attenuation is normally used where low <br />' concentrations of hydrocarbons are present or after completion of another method of remediation <br /> i <br /> Advanced GeoEnvironmental,Enc <br /> 1 <br />