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'Subsistence Warehouse Soils -3- 12 March 1993 <br /> DDRW Tracy <br /> Methodology, all of the soil samples would be considered "designated waste", and <br /> therefore, a threat to groundwater. However, it should be noted that the method <br /> detection limit for the arsenic DI-WET of 50 ppb is too high, because the detection <br /> limit is greater that the SDL of 5 ppb. Three of the arsenic DI-WET sample results <br /> were reported as non-detect, and therefore, inconclusive as to whether they are <br /> actually above or below the SDL for arsenic. Detection limits should be lower than <br /> the SDLs, if possible. This should be considered when developing method detection <br /> limits for the comprehensive site-wide remedial investigation. <br /> When the DI-WET analytical results for the 17 samples from the stockpiled soil are <br /> compared with the SDL for lead of 150 ppb, seven of the lead DI-WET sample results <br /> are above the SDL for lead. These seven samples ranged in concentrations from 162 <br /> to 4080 ppb. Based on The Designated Level Methodology, these soil samples would be <br /> considered "designated waste", therefore, a threat to groundwater. None of the lead <br /> DI-WET sample results were below the method detection limit for the lead DI-WET. <br /> METHOD TWO: THE WATER QUALITY SITE ASSESSMENT METHOD <br /> The enclosed RWQCB staff draft report, Water Quality Site Assessment for Soils and <br /> Groundwater, outlines the steps necessary to evaluate the impact of soil <br /> contaminants on groundwater quality. This method uses site specific data to <br /> calculate the relative attenuation required to protect background concentration <br /> levels and beneficial uses. This attenuation may not be reasonable available in the <br /> actual area of the contaminated soil . The Water Quality Site Assessment Table <br /> (attached) provides a step-by-step method for calculating these relative attenuation <br /> values. This method requires determining: water quality limits, soil and <br /> groundwater background concentrations, site soil and groundwater concentrations, <br /> site and background soil leachate concentrations. Because a comprehensive site-wide <br /> remedial investigation has not been completed, several assumptions had to be made to <br /> estimate background soil and groundwater concentrations. The Water Quality Site <br /> Assessment Table contains the data used to evaluate the Subsistence Warehouse Soil . <br /> The assumptions made to complete the evaluation are as follows: <br /> ► The water quality limit selected for arsenic is the Proposition 65 <br /> regulatory level of 5 ppb, and the water quality limit selected for lead <br /> is the Primary MCL of 15 ppb. <br /> ► Monitoring well LM-11 is up-gradient from the Subsistence Warehouse, and <br /> was selected for background groundwater concentrations. Quarterly <br /> groundwater sampling data from 20 June 1991 through 11 March 1992 were <br /> evaluated for dissolved arsenic and lead concentrations. Dissolved arsenic <br /> concentrations in the groundwater did not exceed the method detection limit <br /> for the dissolved arsenic analysis, therefore, the detection limit of 5 ppb <br /> was selected as background. Dissolved lead was detected at 5.59 ppb, <br /> therefore, this value was selected as background. <br /> ► Monitoring wells LM-13, LM-14, LM-21 and LM-22 are in the vicinity and <br /> relatively down gradient of the Subsistence Warehouse, and were selected to <br /> represent the site's background groundwater. Quarterly groundwater sampling <br /> data from 20 June 1991 through 11 March 1992 were evaluated for dissolved <br /> arsenic and lead concentrations. Dissolved arsenic and lead concentrations <br /> in the groundwater did not exceed the method detection limit for the <br />