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I <br /> the continuing source of the contamination, it was addressed first. <br /> Approximately 3 , 000 cubic yards of contaminated soil were excavated <br /> and treated ex-situ (above ground) . After levels of contamination <br /> I in the soil reached non-detect (ND) , it was used to backfill the <br /> original excavation. <br /> After the treated soil was backfilled, groundwater samples <br /> collected during the regular quarterly monitoring showed dramatic <br /> decreases in concentrations of all hydrocarbons over a 10-month <br />' period (see Appendix E) . Levels of hydrocarbons decreased from <br /> 7 , 500 parts per billion (ppb) to 1, 500 ppb during that period. No <br /> other groundwater treatment was used at this site. <br /> I In-situ bioremediation of groundwater at the Geweke site can <br /> probably be achieved more quickly and at lower cost than required <br /> for a pump and treat method. Moreover, the potential for achieving <br /> non-detect results is greater than in most pump and treat <br /> operations. <br /> 6.0 RECOMMENDED REMEDIATION METHODS <br /> 6. 1 In-Situ Bioremediation <br /> GeoAudit concludes that bioremediation will be the most technically <br /> feasible and cost effective method to mitigate soil and groundwater <br /> contamination at the Geweke site. All work performed by GeoAudit <br /> will be done under the supervision of a Microbiologist and a <br /> Registered Geologist. <br />' 6. 1. 1 General Considerations <br /> The following general considerations of the properties of <br /> I bioremediation provide the basis for the course of action <br /> recommended in the following sections. <br /> I ■ The same biological cultures can be used to treat the soil as <br /> to treat the groundwater. <br /> ■ The bacteria are non-pathogenic and indigenous to soil. They <br /> have been cultured to consume hydrocarbons, and without an <br /> available hydrocarbon food source, die back to population <br /> levels reflective of indigenous soil populations. <br /> ■ Unlike other methods, bioremediation has the potential to <br /> remediate soil and groundwater to non-detectable levels. <br /> I ■ Bioremediation of the soil and groundwater is accomplished <br /> without extensive above ground-equipment and without labor <br /> intensive maintenance or monitoring. <br /> 10 <br />