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SECTION 5 CONCLUSIONS AND RECOMMENDATIONS <br /> R=1+PL Kd <br /> nc <br /> where: <br /> R = Retardation Factor <br /> Ka=Distribution coefficient(3.2 L/Kg) <br /> pr, = Soil dry bulk density(1.9 g/cm3) <br /> n, =Effective porosity(0.40) <br /> A Kd of 3.2 L/Kg relates to a lumped TPH-gas retardation coefficient of 16.The ratio of the <br /> ground-water velocity to the velocity at which a compound is transported is frequently <br /> referred to as the retardation factor,R. The actual values of R for a particular compound <br /> depend on aquifer properties,such as porosity and organic carbon content. For example,a <br /> compound that moves at one-half the velocity of the ground water has an R value of 2. A <br /> retardation factor of 16 means that the dissolved-phase portion of the TPH-gas moves 16 <br /> times slower than the average linear groundwater flow velocity. <br /> 1 <br /> Typical gasoline compounds,such as BTEX,have R values that can range from 1.1 to about <br /> 2.0 (Squillace,et al). This provides additional evidence that the lumped TPH-gas <br /> distribution coefficient is conservative,and that the estimate of sorbed mass of TPH-gas is <br /> likely a worst-case estimate. <br /> 5.2 Recommendations <br /> Monitoring wells <br /> Additional groundwater monitoring wells are needed to monitor the cross-gradient and <br /> downgradient extent of the hydrocarbon/1,2-DCA plume present at the Disco site. The <br /> presence of the 1,2-DCA in groundwater is a concern.Numerous studies confirm that <br /> 1,2-DCA will readily degrade via the process of reductive dechlorination.Typical electron <br /> donors and carbon sources are BTEX compounds. Reductive dechlorination-of the 1,2-DCA <br /> is likely occurring within the BTEX plume at this site,for we see a large decline in 1,2-DCA <br /> concentration within the BTEX plume. Additional monitoring over time, and for analyses of <br /> biodegradation parameters and 1,2-DCA daughter products are needed to ensure that this <br /> process is occurring,and assess degradation rates.and mechanisms. <br /> A work plan will be prepared for agency review and approval for installation of four(4)to <br /> t five (5)monitoring wells to be installed at cross-gradient and downgradient locations. The <br /> exact number,locations,and depths of the monitoring wells to be installed,and a <br /> groundwater sampling plan will be decided after concurrence with PHS/EHD. <br /> Meeting <br /> Based on the results of this investigation,and mass calculations estimating up to 39,000 <br /> pounds of gasoline may be present beneath the Disco site,Del Monte feels that installation <br /> of an interim remedial system may be warranted.Del Monte requests that a meeting <br /> between Del Monte,CH2M HILL,PHS/EHD,and CVRWQCB be scheduled within two to <br /> three weeks after receiving this report to discuss the findings of this investigation,potential <br /> monitoring well locations,and potential IRM systems. <br />