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L LLe City of Stockton A68107.01B LJuly 18, 1997 Page 10 The soil samples collected from soil probe locations SP-1 and SP-2 suggest the maximum L vertical extent of TPH-D migration in soil is to 17 feet BSG. Therefore it appears there is approximately 23 feet of uncontaminated soil between the deepest TPH-D detected in soil and the groundwater surface. LThe groundwater sampling technique specified for this work(i.e., grab sampling through an open borehole) can produce false positive results. Diesel contaminated soil and/or water may have fallen into the bottom of the open borehole from above. The presence of the "perched" water zone in the bottom of the former UST excavation increases the possibility this may have occurred. Based on this, the analytical results for the groundwater sample L may not be representative of actual site conditions. Additional groundwater assessment involving the installation and sampling of groundwater monitoring well(s) would be necessary to confirm groundwater contamination. L, Based on the length of time the contamination source has been removed from this site (over two years), the relatively low volume of contaminant remaining in the soil, the thickness of uncontaminated soil above the groundwater, and the possibility of a false positive groundwater sample analytical result, Twining considers it possible that the groundwater beneath this site is not contaminated with TPH-D. LVMPW 6.4 Additional Inv -estanon and Propt►sed Corrective Action: Based on the results of the assessment additional investigation and/or corrective action would appear to be unwarranted. In the event additional assessment is required, Twining would recommend confirmation of groundwater impact and a risk assessment to determine if the site could be classified as "low risk". L 7.0 CONCLUSIONS L • TPH-D was detected at a concentration of 4,100 mg/kg in a soil sample collected at 13 feet BSG beneath the former UST. TPH-D was detected at a concentration of L11 mg/kg in a soil samples collected at 16 feet beneath the former UST and at the same concentration at a depth of 7 feet BSG adjacent to the former UST on the east Lside; • The deepest dectection of PPCs in soil was at 16 feet BSG. Based on this, there L • appears to be more than 20 feet of soil with no detectable concentrations of PPCs above the groundwater surface at 39 feet; • Approximately 650 cubic feet of soil appear to be impacted with TPH-D in Lconcentrations ranging from 4,100 to 11 mg/kg. A conservative estimate based on the highest concentrations detected, suggest that approximately 40 gallons of diesel L144W fuel is present in the contaminated soil volume. L