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' Quarterly Groundwater Monitorin • lg Report Frontier Transportation ' 0998-011 October 16, 1998 ' respectively. Groundwater analyzed from MW-3 identified a maximum BTEX concentration of 5,971 ug/L. See figure 4 for TPH-g Isoconcentration Map and Figure 5 for Benzene Isoconcentration Map. ' MTBE was identified in groundwater from two monitoring wells only. Groundwater samples collected from monitoring wells MW-1 and MW-3 identified MTBE at 63 ug/L and 13,604 ug/L, respectively, using EPA method 8020. MTBE concentrations were quantified using EPA method 8260. The laboratory used up all the sample from MW-3 to acquire a satisfactory ' dilution factor for EPA methods 8015 and 8020, therefore, EPA method 8260 was not performed for that well. EPA method 8260 groundwater sample analysis identified the following (See Table 2): • MW-1 -4 38 ug/L as MTBE only. NIW-2 -+ 5 ug/L as benzene and I 1 ug/L as 1,2,4-Trimethylbenzene only. ' MW-4 -4 1,600,ug/L as 2-Butanone (MEK), 7 ug/L as Ethylbenzene, 120 ug/L as 4 Methyl-2-pentanone (MIBK), 12 ug/L as Toluene, and 48 ug/L as Xylene. • MW-5 -4 320 ug/L as MEK, 9I ug/L as MIBK, 5 ug/L as Toluene, and 7 ,ug/L as ' . 1,2,4-Trimethylbenzene. • MW-6 .4 No detectable concentrations of analytes, • MW-7 -4 210 ug/L as MEK, 21 ug/L as MIBK, and 8 ug/L as Xylene. 5.0) CONCLUSIONS AND RECOMMENDATIONS ' Based on groundwater sample analysis results, the dissolved petroleum hydrocarbons appear to be localized near the former tanks location and monitoring well MW-3 in shallow groundwater and pose no significant threat to the environment. Groundwater collected from ' monitoring wells MW-I, MW-2, MW-4, MW-5, MW-6, and MW-7 identified low level to no detectable concentrations of petroleum dissolved hydrocarbons (TAH-d, TAH-g, BTEX, and MTBE) which supports that the dissolved phase plume is confined to the site. The shallow groundwater beneath the site is designated as non beneficial use and no municipal drinking water wells are screened in the shallow groundwater zone within 2,000 feet of the site. ' Characteristically, shallow groundwater contains relatively high dissolved oxygen concentrations and is rich in nutrients to sustain intrinsic biodegradation. In addition, lateral migration of the dissolved plume is not a significant transport mechanism beneath the site since the gradient flow is ' relatively stagnant (0.002 to 0,004 feet per foot) and the fluctuations in the flow direction. Therefore, BEI, Inc. concludes that the existing dissolved petroleum hydrocarbons will remain confined to the site and safely biodegrade intrinsically. BEI, Inc. recommends that three additional groundwater monitoring episodes be 4