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Petroleum hydrocarbons were detected in MW-DIS-8D during the two sampling events
<br /> following installation of the well (March 1995 and June 1995),and have not been detected in
<br /> the last 10 sampling events.
<br /> Cross-gradient Wells-MW-DIS-3,-9,and-10.
<br /> TPH as gasoline and BTEX compounds were not detected in groundwater samples collected
<br /> from cross-gradient wells MW-DIS-3,-9,and-10.
<br /> Petroleum hydrocarbons have not been detected in MW-DIS-3 in the last 6 sampling events.
<br /> Petroleum hydrocarbons have not been detected in MW-DIS-9 and--10 in the last 5
<br /> sampling events.
<br /> Wells Within the Plume-MW-DIS-1, MW-DIS-2, NP-1-70', NP-1-80', NP-1-90', SVE-South and
<br /> SVE-North
<br /> TPH as gasoline and BTEX compounds were detected in groundwater samples collected
<br /> i from monitoring wells MW-DIS-1,MW-DIS-2,NP-1-70',NP-1-80', NP-1-90',SVE-South and
<br /> SVE-North. The concentrations detected in wells NP-1-70',NP-1-80',NP-1-90',SVE-South
<br /> + and SVE-North are consistent with previous sampling events(see Table 4).
<br /> The concentrations of petroleum hydrocarbons detected in monitoring wells MW-DIS-1 and
<br /> -2 have been steadily decreasing (see Table 4). Concentrations detected in MW-DIS-1
<br /> during this monitoring event were detected at their lowest levels since monitoring began.
<br /> TPH as gasoline was detected in MW-DIS-1 at a concentration of 84 gg/L and benzene,
<br /> toluene, ethylbenzene,and total xylenes were detected concentrations of 4.1, 12,1.5,and
<br /> 11 gg/L,respectively.
<br /> TPH as gasoline was detected in MW-DIS-2 at a concentration of 95 gg/L and benzene,
<br /> toluene,ethylbenzene,and total xylenes were detected concentrations of 5.8, 1.3,0.83,and
<br /> 1.1 µg/L,respectively.
<br /> Oxygenates
<br /> Methyl-cert-Butyl Ether(MTBE), tert-Butanol (TBA),Di-isopropyl ether(DIPS),Ethyl-tert-
<br /> butyl ether(ETRE),and Cert-Amyl Methyl ether(TAME)were not detected in any of the
<br /> wells during this monitoring event.
<br /> During the re-sampling event on March 19, 1999,MTBE was detected in MW-DIS-2 by EPA
<br /> Method 8020 at a concentration of 21 gg/L. This result was not confirmed by GC/MS(EPA
<br /> Method 8260). Because MTBE has not been detected in this well in the 7 previous sampling
<br /> events, the detection using EPA Method 8020 is anomalous,and appears to be a "false-
<br /> positive" result. False-positives for MTBE commonly occur when analyzed by EPA Method
<br /> 8020 as discussed below. This is particularly true when TPH is present in the groundwater
<br /> sample,as is the case for MW-DIS-2. The groundwater sample collected from MW-DIS-2 on
<br /> February 11,1999 did not contain MTBE, and it was analyzed by both EPA Methods 8020
<br /> and 8260. Therefore,the detection of MTBE in MW-DIS-2 is being treated as a non-detect.
<br /> The susceptibility of false-positives for MTBE by EPA Method 8020 is well documented. A
<br /> report published by Lawrence-Livermore Laboratory(June 1998) entitled:An Evaluation of
<br /> MTBE Impacts to California Groundwater Resources,June 11, 1998,notes that EPA Method 8020
<br /> t is sensitive to matrix effects and oxygenate analysis may be impacted significantly in the
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