SITE INFORMATION AND CORRESPONDENCE

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page 2, 24100 Lammers Road The site cross section that was presented in the CAP does not accurately depict the boring logs. The cross section shows the three mo iitoring wells completed to 25-feet bsg with 20-feet of screen interval, when in fact they were all built to a total depth of 20-feet with 15-feet of screen interval. In addition, severalof the boring log lithological intervals do not agree with the cross section. For example, tie cross section depicts boring B-319 as being silty clay from 15 to 30-feet bsg, while the boring log shows a sandy silt interval at 20-feet bsg. Several such errors/inconsistencies were noted. The CAP evaluated four corrective action alternatives utilizing the EPA Bioscreen Natural Attenuation Decision Support System model. The four alternatives were: 1. Low-risk site closure. i 2. Excavation of source material. 3. Enhanced aerobic biodegradation ( RC injection). i 4. Excavation of source material with enhanced aerobic biodegradation. As presented in the CAP, modeling parameters USEd in the Bioscreen model were either i estimated or derived from available site data. MJK included a model input data table in the CAP; however, they did not provide references or the data values, and different values were used in the Bioscreen modeling sprea sheets. MJK listed a hydraulic conductivity of 0.001 cm/sec and a porosity of 30% in their data table. However, a hydraulic conductivity of 0.0001 cm/sec and a porosity of 20% were used in their modeling spreadsheets. A discussion for this change was not presented. Hydraulic conductivity is a sensitive parameter, and a change from 0.001 to 0.0001 cm/sec equates to a seepage velocity change from 10 to 1 feet per year. Hydraulic conductivities for fate and transport modeling should be derived from site-specific aquifer testing. The retardation factor and the solute half-life are also sensitive parameters when modeling high solubility constituents like methyl terliary butyl ether(MtBE), and again MJK did not provide references for their input values. MJK used a retardation factor of 1.1 for MtBE, the same value used for the BTEX modeling This value is conservative for both MtBE and BTEX; however, lacking site-specific da t , a retardation factor of 1.0 is more appropriate for MtBE. MJK used a solute half-life of 5:0 years for MtBE when modeling the low-risk closure and a solute half-life of 2.0 yeas when modeling the MtBE after ORC injection. Published solute half-life values for MtBE range from 1 to 27 years; thus, appropriate values need to be referenced. MJK calculated that the BTEX plume would stabilize at 40 to 120 feet in 3 to 15 years, and the MtBE would stabilize at 160 to 270 feet in 21 to 30 years, depending on the alternative initiated. Based on the supporting data submitted, HS/EHD and the CVRWQCB staff do not agree with the conclusions. Discussion Using the Bioscreen model, MJK recommended to -risk site closure. The low-risk site closure alternative is not a feasible alternative due o incomplete groundwater assessment and the presence of MtBE in groundwater. Due to he estimated hydraulic conductivity, retardation factors, and solute half-life values used in the fate and transport modeling, the results of the modeling are not acceptable for dete mining the future extent of groundwater contamination. The three existing monitoring wells on site are placed too far from the source area, and screened too shallow to effectively monitor contaminant trends.