2007-11-20_REVISION - M1982015 (14)

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9.3 Groundwater and Surface Water Monitorinq Schedule The new wells were sampled in March and April 2007 for purposes of obtaining baseline data and to evaluate the fate and transport of BTEX compounds in groundwater. E-21 recommends resuming groundwater sampling in June 2007 and then performing future sampling on a quarterly basis (i.e. June, September, December and March). At this time, we propose to continue to sample all the existing mentoring wells and all new wells proposed in the addendum. If a well is determined to be free of BTEX compounds for four consecutive quarters and is not a downgradient wells, E-21 proposes that sampling of such a well would be discontinued. However, we recommend that the four surface water body sampling locations be performed on a quarterly basis until remediation is completed. E-21 proposes that closure monitoring may be initiated once BTEX concentrations in all wells at the site are below the Colorado Maximum Contaminant Limits (MCLs). If the wells remain below the MCLs for four consecutive quarters, E-21 will petition for "no further action" and well decommissioning will be performed if closure is granted by CDPHE. 10.0 Groundwater Remediation Plan E-21 recommends performing air-sparge/ soil vapor extraction (AS/SVE) in the dissolved phase plume to remediate petroleum impacts in groundwater. Based on the pilot testing results, positive pressure (>0.10 inches of water) for sparging was generated up to 50 feet from AS pilot test well. Vacuum was generated (>1.0 inches of water) over 50 feet ' from the SVE pilot test well. The location and radius of influence estimated by the remediation wells is shown on Figure 9. The SVE wells will be 2-inch diameter and the AS wells will be 1-inch diameter. ' Due to the construction activities at the site and changing nature of the landscape, we recommend using one or two trailer-mounted remediation systems to allow for moving ' the remediation equipment if needed. The equipment would be supplied by a generator for continuous operation and adjustments to the AS/SVE program could be made relatively easily. Air flow to the wells will be conveyed by reinforced flexible hose, thus eliminating the need for pipe trenching. E-21 estimates the AS/SVE system will need to operate for at least six months to reduce the dissolved phase plume. The quantity of VOCs will be measured weekly to confirm ' remediation progress. Soil pressure and vacuum will be measured by monitoring from selected monitoring wells. Quarterly groundwater sampling will be performed during the remediation phase to determine the progress of the groundwater cleanup and to allow ' adjustments. Initially, all the remediation wells are expected to be used. As the plume is reduced, the system will be modified in the field to concentrate on areas containing increasing or elevated BTEX concentrations. Once BTEX concentrations are reduced to below the MCLs, the system will be removed from the site and closure monitoring will ' commence. Based on the above described remediation program, E-21 estimates that the ' remediation phase of the project will require one year from initiation of remediation. If remediation is successful, closure monitoring will require one additional year. Thus, if remediation is successful, a petition for no further action may be submitted in approximately two years from the initiation of remediation. 12