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29 v <br /> The data indicate that, while the vertical extent of groundwater impacts have not been <br /> precisely delineated, concentrations of TPHg and benzene are significantly attenuated with <br /> depth <br /> ' 6.5 INTRINSIC DEGRADATION OF SITE PETROLEUM HYDROCARBONS <br /> ' 6.5.1 Site Concentrations Declining <br /> ' An examination of wells that have a long-term monitoring record indicates that <br /> concentrations of petroleum hydrocarbon constituents at the Site have been declining over the <br /> ' 11-year penod in which monitoring has been conducted The well with the highest recorded <br /> concentrations at the Site is monitoring well MW-2 where TPHg was recorded at a <br /> ' concentration of 480,000 ug/L in March, 1993 Concentrations of TPHg in well MW-2 <br /> generally ranged in the vicinity of 100,000 to 200,000 ug/L at the beginning of the monitoring <br /> ' record TPHg concentrations in well MW-2 declined to below 100,000 ug/L after mid-1997 <br /> TPHg concentration in MW-2 was 50,000 ug/L in the February 4, 1999 monitoring event <br /> MSmmdarly,benzene concentrations in well MW-2 were recorded as high as 27,000 ug/L in 1990 <br /> and in 1995, with high concentrations typically ranging between 10,000 to 20,000 ug/L <br /> ' Benzene concentration in MW-2 declined to below 5,000 ug/L in the third quarter of 1995 <br /> and, with one exception in November, 1995, have remained there The February 4, 1999 <br /> ' benzene concentration in MW-2 was 2,200 ug/L <br /> ' The declining concentrations over the period of the monitoring record provide primary <br /> ' evidence that intrinsic degradation of petroleum hydrocarbons is occurring at the Site <br /> i <br /> ' 6.5 2 Stable Plume <br /> ' Further examination of the 11-year monitoring record indicates that the front of the <br /> plume, as indicated by contouring of the Site TPHg and benzene concentrations, is stable <br />