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Stockton Scavenger Association Facility <br /> November, 1997 <br /> Page 7 <br /> 5 0 GROUNDWATER REMEDIATION EVALUATION <br /> Information regarding the operation of the groundwater pump and treat system was gathered from <br />' self monitoring reports (sanitary sewer discharge) and groundwater monitoring reports prepared <br /> by MARK during the operation of the groundwater pump and treat system through December <br />' 1996 <br /> The groundwater pump and treat system in well MW-2 shut down in December 1996 and was not <br /> restarted On behalf of Norcal, ENSR notified the San Joaquin County Public Health Services, <br />' Environmental Health Division (EHD) by telephone on January 9, 1997 that the system was not <br /> operating The EHD approved of leaving the system shut down until Norcal could evaluate its <br /> effectiveness and whether continued operation was warranted The EHD requested written <br />' justification for discontinuing operation and dismantling the system On the basis of the following <br /> evaluation and on behalf of Norcal, ENSR requests approval to dismantle the groundwater pump and <br /> treat system <br /> 51 Mass Removal <br /> Data collected during operation of the groundwater pump and treat system is summarized in Table 3 <br /> Table 3 includes a summary of gallons pumped each month, the analytical results of groundwater <br /> samples collected from MW-2, and the calculated petroleum mass removal As shown in Table 3, the <br /> groundwater pump and treat system installed in well MW-2 removed 10 pounds of TPHg, 2 1 pounds <br /> of TPHd, 2 5 pounds of benzene, and lesser amounts of toluene, ethylbenzene, and xylenes from <br /> well MW-2 A sample calculation is included in Attachment C <br /> Figure 6 depicts cumulative gallons removed over time As shown on Figure 6, the pump and treat <br /> system operated more efficiently during the last year of operation removing well over one half of the <br /> total volume pumped Between April 1996 and December 1996, the system removed 266,000 <br /> Igallons of the total 592,000 gallons removed <br /> 5 2 Petroleum Hydrocarbon Concentrations <br />' Figure 7 depicts TPHg and TPHd concentrations in well MW-2 over time and includes the results <br /> from the latest groundwater monitoring event During operation of the pump and treat system <br /> between July 1994 and April 1996, the TPHg concentration was variable and the TPHd <br />' concentration did not exhibit a definitive trend Between April 1996 and July 1997, however, the <br /> TPHg concentration decreased dramatically from 2 3 to less than 0 05 mg/I This reduction is <br /> apparently associated with more efficient operation of the pump and treat system The TPHd <br />' concentration did not exhibit a reduction during this time period, but has remained below 1 mg/l since <br /> January 1994 <br /> Figure 8 depicts STEX concentrations in well MW-2 over time and includes the results from the latest <br /> groundwater monitoring event During operation of the pump and treat system the benzene <br /> concentration declined steadily Between April 1996 and July 1997, the benzene concentration <br /> declined more dramatically from 0 51 to 0 027 mg/I Similarly to the reduction of TPHg, this reduction <br /> appears to be associated with more consistent operation of the groundwater pump and treat system <br /> sazraaz c,Rr <br />