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kk „ f ~ 10- 4kTc A S S O C I A T E S I N C . The MTBE concentrations in MW5 and MW8 were fairly consistent during the June 3, 2004, September 13, 2004 and December I3, 2004 groundwater monitoring events. MTBE concentrations in MW5 and MW8 may be declining but the concentrations are too low to have any degree of confidence at this time. In EWI, MTBE concentrations decreased from June 3, 2004 (13,000 ug/L) to December 13, 2004 (9700 ug/L). Although MTBE concentrations are decreasing with time, fluctuating conditions are evident. If the trend is actually declining and it is associated with the ongoing remediation, some mixing may also have occurred around the time of the August 13, 2004 monitoring event. DO has increased after system start-up in EWI from 0.52 mg/L on June 3, 2004 to 2.29 mg/L on December 13, 2004. Since DO also increased significantly after system start-up the results may be due to remediation system operations. The change in areal extent of MTBE from June 3, 2004 monitoring event to the December 13, 2004 monitoring event appears similar(Figure 22). 4.5.3 TPHg During this monitoring period, the groundwater samples collected from onsite wells, MW1 through MW3, MW5 and EWI contained detectable concentrations of TPHg. However, TPHg was not detected in MW4 and MW8. TPHg concentrations in MW 1, and EW1 appeared to decline or remain fairly consistent. MW3, MW5 and show overall increases of TPHg when compared with concentrations from September 13, 2004. The TPHg concentrations decreased significantly in MW2 and MW4. TPHg in MW4 was reduced to a non-detection level (<50 ug/L)during this monitoring period. The successive decline of TPHg in MWI from 54,000 ug/L on June 3, 2004 in each of the monitoring events following (i.e., July 16, 2004, August 13, 2004, September 13, 2004 and December 13, 2004) was significant. During the 13 December event, TPHg was reported at 670 ug/L for a removal efficiency of 98.8% after approximately 6 months of operation. An overall decline may be indicated in TPHg in MW2 from 1,500 ug/L on June 3, 2004 to 880 ug/L on December 13, 2004). However, this decline apparently followed some mixing during the July 16, 2004 and August 13, 2004 groundwater monitoring events. If the reduction in concentration is a result of system operation this reduction equates to a removal efficiency of 41.3% after approximately 6 months of operation. The concentration of TPHg increased in MW3 from 2400 ug/L on September 13, 2004 to 2700 ug/L on December 13, 2004. DO also increased from 0.28 mg/L on June 3, 2004 to 1.25mg/L on September 13, 2004. A decline of TPHg in MW4 has accelerated from 4800 ug/L on June 3, 2004 to <50 ug/L on December 13, 2004. If this reduction was attributed solely to system activities this would equate to a removal efficiency of 99.0% after approximately 6 months of operation. s:1268231QM reports12044 4QR.doc 11