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Subsurface Investigation December 12, 1990 <br /> Former Unocal Station No. 2859, Stockton, California AGS 38012-5 <br /> Results of eight quarterly monitoring events(Table 5)indicated concentrations of TPHg and <br /> BTEX are highest in samples from monitoring well MW-2, downgradient of a former <br /> dispenser island.Concentrations of TPHg have increased each monitoring event in this well. <br /> The concentrations of BTEX compounds over eight monitoring events in well MW-3 ranged <br /> from below detection limits to 2.1, 9.0, 8.4 and 17 ppb, respect: Ay. <br /> Results indicated the benzene concentration in wells MW-1, MW-4 and MW-5 were below <br /> detection limits since the monitoring program began for each well. The concentrations of <br /> toluene, ethylbenzene, and total xylene isomers in these three wells ranged from below <br /> detection limits to 1.7, 8.1 and 1.9 ppb, respectively. <br /> Wells MW-1 through I4iW-3 were not sampled during the third quarter 1990 monitoring <br /> event because the wells were dry. Results of laboratory analyses of samples from wells <br /> MW-4 and MW-5 indicated TPHg and BTEX concentrations have remained the same or <br /> increased since the May 30, 1990 sampling event. TPHg was detected in the samples from <br /> wells MW-4 and MW-5 at concentrations of 130 and 180 parts per billion (ppb), <br /> respectively. The benzene concentration was below detection limits in the two samples. <br /> Concentrations of toluene, ethylbenzene,and total xylene isomers ranged from less than 0.5 <br /> ppb to 4.1 ppb in the two samples. <br /> 15 <br /> APpl ed tae®Syete,77s <br />