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Site Characterization - Second Event Page 12 <br /> Former Karlson Trucking Facility <br /> Project No. 191.2 <br /> April 27, 2000 <br /> 3. The lower detectable levels of diesel range petroleum hydrocarbons from <br /> groundwater samples collected from MW-4 relative to those collected from <br /> Geo-Probe soil boring SB-10 seem to suggest that the contaminant is <br /> relatively immobile. The 15 foot groundwater sample from Geo-Probe <br /> boring SB-10 was collected directly from the formation, while the <br /> groundwater sample collected from MW-4 was collected from water <br /> entering the well from the formation. If the contaminant were mobile, it <br /> would be expected that groundwater sample concentrations from the <br /> monitoring well and that from the soil boring would be similar, when in <br /> fact those from the monitoring well were approximately an order of <br /> magnitude lower than those collected during the installation of the Geo- <br /> Probe boring. <br /> This also is reflected by the observation that contaminant levels from the <br /> shallow subsurface were much greater than those from deeper in the <br /> formation. Groundwater contaminant levels from samples collected from <br /> SB-10 decreased by two orders of magnitude between 14 and 19 feet, one <br /> order of magnitude between 19 and 24 feet and a half order of magnitude <br /> between 24 and 28 feet. This could suggest that the reason groundwater <br /> samples from MW-4 were an order of magnitude lower in hydrocarbon <br /> ' concentration than those from the shallow sample in SB-10 is that <br /> formation water from 9-20 feet below grade is mixing in the well, with the <br /> most contaminated groundwater coming from near the surface of <br /> groundwater and contributing to most of the detectable contamination. <br /> 4. Groundwater samples from SB-10 collected at varying depths below grade <br /> displayed a dramatically decreasing contaminant concentration with <br /> depth. As mentioned above, the groundwater sample from the 14 foot <br /> interval contained 133 mg/1 diesel range petroleum hydrocarbons while <br /> the sample from the 19 foot interval contained 2.82 mg/1 diesel range <br /> petroleum hydrocarbons. The samples from 24 feet and at 28 feet below <br /> grade contained 0.588 mg/1 and 0.133 mg/1 diesel range petroleum <br /> hydrocarbons, respectively. This suggests that the groundwater <br /> contamination attenuates rapidly with depth, even through relatively <br /> permeable sediments as were observed at the site. (See following graph <br /> which shows how contamination decreases with depth). <br /> 2937 Veneman Avenue,Suite B240 Geo-Phase Environmental Inc. Phone: (209)569-0293 <br /> Modesto, Ca 95356 Fax: (209)569-0295 <br />