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Yf_ ' Mr Ronald Rowe C �1 M B R I A August 26, 1999 To determine whether the hydrocarbons were Figure o biodegrading, we graphed hydrocarbon rPHg in Groundwater concentrations for a time period where the IW ell MW-3) C 80 000 water table was relatively stable As shown s0 0oo !qfil 010Figures D and E, hydrocarbon concentrations m q0000 ��,,,, 20 V 20 000 30 were decreasing during 1992 through 1994, 'o 0 50 when the water table was stable Therefore, it is ati 41, SP a'b op S�' difficult to determine whether the hydrocarbon1g-1 Vol `�®� PO Ile 0 decreases that occurred as the water rose were rTop of Screen -Exon (TPHg) due to the rising water table or due to continued Figure s biodegradation Benzene in Groundwater (well MW-3) Cq 000a+rn�+e.�n�awr 0 The extent of h drocarbons in oundwater is s aoo �� -i 0 " y gT a 2,000 30 20 defined by wells MW-7, MW-13, MW-14, o 1 00040 W MW-16 and MW-17 The primary area where o so the distribution of hydrocarbons in groundwater ��CP,a 1 has not been completed is to the direction of ®Top of Screen Expon (TPFIg) well MW-15 The hydrocarbon plume appears to be stable and, if the hydrocarbon concentration decreases are due to natural attenuation, the plume is shrinking The SJCEHD indicated in their letter that the hydrocarbon plume was migrating However, we did not locate any data to support this conclusion The analytic data indicate that there is no significant methyl-tertiary-butyl-ether(MTBE) mass in groundwater The only wells with consistent MTBE detections are MW-10 and MW-11 MTBE analysis for these wells has been by EPA Method 8020 Therefore, as discussed below, we recommend analyzing water samples using EPA Method 8260 to determine actual MTBE and other oxygenate concentrations Estimated Hydrocarbon Source Area Based on the distribution of hydrocarbons in soil and groundwater, it appears that the source of the hydrocarbons was the tanks and dispenser islands Since the tanks and dispenser islands are immediately adjacent to each other,we can address this as one source area PROPOSED SCOPE OF WORK The SJCEHD requested additional investigation to further define the horizontal and vertical . extent of hydrocarbons in soil and groundwater As discussed above, the horizontal extent of hydrocarbons in soil is adequately defined The vertical extent of hydrocarbons in soil also 5