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Interim Remedial Action Test Report,Former Unocal#0187,Stockton,CA
<br /> August 28,2001
<br /> on July 17, 2001 This is a decrease of at least 94% There was a dramatic decrease, to 773 ppb,
<br /> approximately one month after beginning the test Since that time levels have fluctuated between 60 6
<br /> and 2800 ppb In well NP-1-111, the TPHg concentration went from 170 ppb at the test baseline to less
<br /> than 50 0 ppb on January 9, 2001,and has remained below detection limits since that time
<br /> The screen interval of well NP-1-22 begins approximately 40 feet above the level of the sparge points
<br /> The TPHg concentration reported at the baseline was 12,000 ppb The concentration reported on July 17,
<br /> 2001 was 5,700 ppb Although not as dramatic as the decreases in wells NP-1-65 and NP-1-111, this is
<br /> still a decrease of 53% from the baseline concentrations, and represents a very significant downward trend
<br /> in TPHg concentrations It is particularly evident when compared to the historical quarterly monitoring
<br /> data in Appendix E
<br /> Analysis of sample results from well NP-2 does not point to any significant conclusions There appears
<br /> to be general downward trend in TPHg concentrations, but there are no major decreases
<br /> On-site monitoring well U-3 is within the treatment area The well is screened from 30 to 55 feet bgs
<br /> On November 20, 2000, the last quarterly-sampling event before the test, TPHg concentrations were
<br /> reported to be 12,300 ppb This was consistent with previous results for this well For the May 14, 2001
<br /> sampling event, after approximately five months of system operation, the well was non-detect for TPHg
<br /> This is a decrease of over 99%
<br /> On-site monitoring wells U-1 and U-2 are upgradient of the treatment area On November 20, 2000,
<br /> TPHg were non-detect in both wells For the May 14, 2001 sampling event, TPHg concentrations had
<br /> increased in wells U-I and U-2 to 1380 and 376 ppb, respectively This result was not expected
<br /> Benzene
<br /> At the test baseline, benzene concentrations in wells NP-1-22, NP-1-65 and NP-1-I 11 were 740, 2,000,
<br /> and 4 8 ppb, respectively On July 17, 2001, benzene concentrations in wells NP-1-22 and NP-1-65 were
<br /> reported to be 240 and 88 ppb, respectively, and benzene was non-detect in well NP-1-1 11, which are
<br /> decreases of 68%,96%, and over 90%respectively
<br /> A review of sample results from well NP-2 does not show any specific,consistent concentration changes
<br /> For well U-3, the benzene concentration was reported at 515 ppb on November 20, 2000 For the May
<br /> 14, 2001 sampling event, benzene was non-detect in well U-3 This is a decrease of over 99%
<br /> Monitoring wells U-1 and U-2 showed increases in benzene concentrations similar to the increases in
<br /> TPHg concentrations On November 20, 2000 benzene levels in both wells were reported to be less than
<br /> the detection limit of 0 500 ppb For the May 14, 2001 sampling event the benzene levels had increased
<br /> to 19 1 ppb and 14 4 ppb respectively
<br /> Discussion of Results
<br /> The following observations can be made based on the sampling data presented above
<br /> • 1 Sample results from wells NP-1-65, NP-1-111, and U-3 demonstrate that the treatment is very
<br /> effective at decreasing petroleum hydrocarbon concentrations in groundwater near the sparge points
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