ARCHIVED REPORTS XR0000118 - Laserfiche WebLink

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' Mr Ron Rowe December 3, 1997 ' groundwater, illustrated in Figure 4 and Figure 5, appear to have decreased approximately one-half to two-thirds in area compared to the plumes measured in December 1996 The historic high TPH-G and benzene concentrations shown in Figure 6 also illustrate the ' significant decreases that these parameters have currently achieved TBA, MTBE, ETBE, and TAME concentrations were not detected in any groundwater sample DIPE was only detected in the samples collected from monitoring wells MW-7 ' and MW-12 at concentrations of 12 micrograms per liter (pg/L) and 7 9 p,g/L, respectively ' SITE CONDITIONS A further discussion of site groundwater and subsurface soil discussions follows below ' Groundwater Elevation Versus Analytical Results Monitoring wells MW-1, MW-4, and MW-13 were chosen to graphically represent ' historic groundwater elevations versus analytical results because samples from these three wells have generally contained the highest petroleum hydrocarbon concentrations Table 7 lists historic benzene concentrations and contains graphs of these concentrations versus associated groundwater elevations ' Generally, as the groundwater elevations have slowly increased from the drought conditions experienced during the early 1990's,the petroleum hydrocarbon ' concentrations in groundwater samples have shown a measurable decrease It is not sufficiently evident whether the petroleum hydrocarbon decrease is related more to the increase in groundwater level or to natural intrinsic biodegradation The graphs illustrate that the decreasing trend of petroleum hydrocarbon concentrations is much more dramatic than the increase in groundwater level Furthermore, benzene concentration peaks have occasionally occurred during periods of depressed groundwater levels ' Data previously submitted to the SJCEHD in the Quarterly Groundwater Monitoring and Natural Biodegradation Assessment Report—Fourth Quarter 1996 indicates that intrinsic biodegradation is occurring at this site Direct relationships were established between the petroleum hydrocarbon concentrations in groundwater and the biodegradation indicator parameters dissolved oxygen, redox potential, nitrate, and sulfate TPH Concentrations in Soil Samples Two cross-sections showing soil profiles and soil sampling locations with historic analytical concentrations are included as Figure 8 and Figure 9 The cross-section locations are shown in Figure 7 Most of the soil samples collected along these profiles did not contain measurable petroleum hydrocarbon compounds The highest TPH-G concentrations detected in soil were found in samples collected from boring MW-1 at a depth of 55 feet(120 mg/kg) and from boring MW-2 at a depth of 30 feet(130 mg/kg)