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.. ... . F 27 June 1946 AGE-NC Project No. 95-0118 Page 22 of 23 10.0. RECOMMENDATIONS FOR REMEDIATION OF HYDROCARBON-IMPACTED a ' GROUND WATER F; Ground water gradient calculations for the February 1996 sampling event indicated that MW-4 was `:' the down-gradient well. As in the three previous sampling events, the highest concentration of dissolved hydrocarbons were detected in MW-2, northwest of the former UST excavations; slight benzene concentrations were detected in MW-4. However,no hydrocarbons were detected in MW-5 or MW-6. -: An in-situ method would be more effective in cost and in operation over a"pump and treat"method. Both in-situ methods described in this CAP could be equally effective in remediating the site. However, based upon limited regulatory acceptance and uncertain monitoring requirements, in-situ '{ bioremediation of ground water may not be the most cost-effective remedial alternative for the Elkhorn site. :j Combined with SVE, the IAS method "mechanically/physically" strips hydrocarbons without the addition of any chemicals or microbe solution into the environment. It is a better known and more widely accepted method of volatilizing hydrocarbons. As IAS has few,if any,adverse environmental =` effects, it is likely to be more readily approved for implementation by regulatory agencies. Based upon these considerations and the presence of relatively sandy soil at the Elkhorn site, we recommend the installation of an IAS system for remediation of ground water..A 12 to 18 month remediation period can be expected. The use of a combined SVE/IAS system at the Elkhorn site has the potential to remove 95 to 99+ percent of petroleum hydrocarbons from ground water at the site. Remaining concentrations, if any, Would be very low and it is unlikely that any of,the most volatile components of gasoline (i.e,, benzene)would remain. Ground water remediation alternatives, estimated durations and associated costs for the Elkhorn site are summarized on Table 7. 3 Installation of an IAS system at the Elkhorn site should include the installation of three additional- _J y sparging points (wells): one IAS point should be installed northwest of MW-2; a second IAS point should be installed northwest of MW-3; a third IAS point should be installed south of VW-1. The locations of the proposed remedation wells/points are depicted in Figure 5 - Renxediation Plan. Extraction from MW-2, MW-3 and the proposed extraction wells should be sufficient for removal of volatilized hydrocarbons (Section 9.0) as the result of operation of an IAS system. Air should be supplied to each of the IAS points via underground piping by a compressor or blower-driven IAS unit capable of approximately'8 to 14 scfm. A Remedial Action Work Plan detailing the proposed installation and operation of the IAS system should be prepared. .:1