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24100 South Lammers Road, Tracy - 2 - 27 December 2000 <br /> CAP Review <br /> Based on the soil and groundwater investigation completed to date, Kloberdanz used the EPA Bioscreen <br /> Natural Attenuation Decision Support System model to evaluate the following four corrective action <br /> alternatives: <br /> 1. Low-risk site closure. <br /> 2. Excavation of source material. <br /> 3. Enhanced aerobic biodegradation (ORC injection). <br /> 4. Excavation of source material with enhanced aerobic biodegradation. <br /> As presented in the CAP, modeling parameters used in the Bioscreen model were either estimated or <br /> derived from avaitable-site data: KI'oberd-anz includzd a-rfio ei-itput data-table�in the-CAP;-however,`they did not provide references for the data values, and different values were used in the Bioscreen <br /> modeling spreadsheets. Kloberdanz listed a hydraulic conductivity of 0.001 cm/sec and a porosity of <br /> 30% in their data table, and used a hydraulic conductivity of 0.0001 cm/sec and a porosity of 20% in <br /> their modeling spreadsheets. A discussion for this change was not presented. Hydraulic conductivity is <br /> a sensitive parameter, and change from 0.001 to 0.0001 cm/sec equates to a seepage velocity change <br /> from 10 to 1 feet per year. Hydraulic conductivities for fate and transport modeling should be derived <br /> from site-specific aquifer testing, or from-slug testi-rig. <br /> The retardation factor and the solute half-life are also sensitive parameters when modeling high <br /> solubility constituents like MtBE, and again Kloberdanz did not provide references for their input <br /> values. Kloberdanz used a retardation factor of 1.1 for MtB , which was the same value used for the <br /> BTEX modeling. This value is conservative for both MtBE and BTEX; however, lacking site-specific <br /> data, a retardation factor of 1.0 is more appropriate for MtBE_. Kloberdanz used a solute half-life of 5.0 <br /> years for MtBE when modeling the low-risk closure, and a solute half-life of 2.0 years when modeling <br /> the MtBE after ORC injection. Published solute half-life values for MtBE range from 1 to 27 years; <br /> thus, appropriate values need to be referenced. <br /> Kloberdanz calculated that the BTEX plume would stabilizc at 40 to 120 feet in 3 to 15 years, and the <br /> _ MtBE would_stabilize atJb9 to_Z70 feet in 21 to 30 years, d pending on the alternative initiated. Base_d <br /> on the supporting data submitted, Board staff~do not agree with the conclusions. <br /> Discussion <br /> Using the Bioscreen model, Kloberdanz recommended low 'sk site closure. The low-risk site closure <br /> alternative is not a feasible alternative due to incomplete groundwater assessment and the presence of <br /> MtBE in groundwater. Due to the estimated hydraulic conductivity, retardation factors, and solute half- <br /> life values used in the fate and transport modeling, the results of the modeling are not acceptable for <br /> determining the future extent of groundwater contamination. The three existing monitoring wells are <br /> placed too far from the source area, and screened too shallow to effectively monitor contaminant trends. <br /> Monitoring wells have not been installed to discretely mom' or the identified sand units, and there are no <br /> data from the lower sand unit to determine whether it is a conduit to the drinking water wells. <br /> The excavation of source material is the only alternative presented that will protect future beneficial uses <br /> of the groundwater at this site. By removing a large volume of hydrocarbon-impacted soil at the former <br />