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Interim Remedial Action Test Report,Former Unocal#0187,Stockton,CA August 15,2001 Discussion of Results The following observations can be made based on the sampling data presented above 1 Sample results from wells NP-1-65, NP-1-111, and U-3 demonstrate that the treatment is very effective at decreasing petroleum hydrocarbon concentrations in groundwater near the sparge points All three wells are within the projected radius of horizontal influence of one or more sparge points Wells NP-1-65 and NP-1-111 are at the same depth as the sparge points The screen interval in well U-3 begins approximately ten feet above the shallow sparge point depth Over the test period,TPHg and benzene concentrations decreased by at least 90% in these wells Decreases of over 99% were seen 2 The screen interval in well NP-1-22 begins approximately 30 feet above the shallow sparge points The soil types between the sparge point and this well are generally described as fine grained silts and sandy silts TPHg and benzene concentrations in well NP-1-22 have decreased by 53% and 68%, respectively, during the test, indicating that the system is providing treatment over a large vertical distance 3 Although petroleum hydrocarbon concentrations in well IVP-2 show a slight downward trend, the concentrations are not significantly lower than those reported prior to beginning the test Well NP-2 is approximately 75 feet from the sparge points It is too early to determine whether or not this system will provide treatment at that distance under these site conditions 4 There have been increases in petroleum hydrocarbon concentrations in wells U-1 and U-2, located near the treatment area in the upgradient direction This is believed to be due to desorption of hydrocarbons from soil caused by the aggressive mechanical scrubbing action of the microsparge bubbles In terms of overall treatment, this desorption is necessary to achieve effective, long-term treatment However, there must be sufficient treatment capacity to prevent stimulation of downgradient contaminant migration 5 Well NP-2 will be critical in determining whether or not the current system provides sufficient treatment to prevent off site migration Sample results to date indicate a slight downward trend developing There are two possible conclusions First, there might be no significant off site migration and the on site treatment is sufficient The other possibility is that the potential contaminant migration created by on site desorption has not yet migrated to well NP-2 Continued sampling should clarify this issue CONCLUSIONS AND RECOMMENDATIONS Ozone Microsparging The ozone microsparging system has been successful in significantly reducing dissolved hydrocarbon concentrations in the groundwater at this site There are indications that the treatment system might be reducing concentrations in well NP-2, located across California Street approximately 75 feet downgradient from current sparge point locations Small increases in dissolved hydrocarbon concentrations have been observed in two upgradient wells It is believed that these increases are due to accelerated desorption caused by the aggressive mechanical 24004209-1 9