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' S [EC0R 1 521 Distribution of TPHg in Groundwater 5 2 1 1 A Monitoring Zone ' Historically, TPHg had not been reported in any of the A monitoring zone wells (MW-1 through MW-4) prior to the fourth quarter of 2003 with the exception of one detection in well MW-3 at a concentration of 67 ppb during the first quarter 2001 During the second quarter 2005, TPHg ' was reported in three of the four A monitoring zone wells at a maximum concentration of 1,600 ppb MW-2 could not be sampled during the second quarter 2005 because the well was inaccessible due to drilling activities at the site Based on the isoconcentration map included as Figure 8, TPHg concentrations are likely being affected by the upgradient source from ARCO Station 4932 Currently, the TPHg plume in the A monitoring zone to the northeast ', (downgradient) is delineated by ConocoPhdlips well MW-2 (less than the reporting limit, 1 Q05) and ARCO well MW-11 ' 5 2 1 2 B Monitoring Zone TPHg historically has been reported in on-site wells MW-5, MW-6, MW-8, MW-9, and MW-10 at concentrations ranging from 88 ppb to 22,000 ppb MW-8 could not be sampled during the + second quarter 2005 because the well was inaccessible due to drilling activities at the site Off- site wells MW-11 through MW-14, which were installed in 2003, initially had no detectable ' concentrations of TPHg in groundwater Since the initial sampling event for the offsite wells, � TPHg has been reported at all four wells at concentrations ranging from 51 ppb to 120 ppb During the second quarter 2005, TPHg was detected in groundwater at only one off-site well ' (MW-13 at 71 ppb) Based on the isoconcentration map included as Figure 9, TPHg is delineated to nondetect in the B monitoring zone in the downgradient direction by well MW-14, upgradient by ARCO well DMW-3, and cross-gradient to low concentrations (less than ' laboratory reporting limit to 71 ppb) by wells MW-11, MW-12, and MWA3 5 21 3 C Monitoring Zone r x ' During the initial sampling event for MW-15, which was installed in November 2004, TPHg was not detected MW-15 was sampled during the second quarter 2005 event (April 12, 2005) and was also sampled following well development on April 20, 2005 TPHg was detected in both + samples at concentrations of 59 ppb and 89 ppb, respectively MW-16 and MW-17 were sampled for the first time following well development on April 20, 2005 TPHg (reported as GRO) was detected at a concentration of 410 ppb in the sample collected from MWA 6 and at a concentration of 30,000 ppb in the sample collected from MWA 7 TPHg is not delineated in any direction in the C monitoring zone (Figure 10) 622 Distribution of Benzene in Groundwater 5 2 2 1 A Monitoring Zone Historically, benzene had not been reported in any of the A monitoring zone wells (MW-1 through MW-4) prior to the first quarter of 2004 with the exception of one detection in well MW-3 at a concentration of 0 61 ppb during the first quarter 2000 During the first and third quarters of 2004, benzene was reported in well MW-2 at a concentration of 0 96 ppb and 0 65 ppb, respectively During the second quarter of 2004 benzene was reported in well MW-4 at a ,r § concentration of 0 57 ppb During the second quarter 2005, benzene was not detected in any of " the A monitoring zone wells As shown on Figure 11, the benzene plume in the A monitoring Updated SCM 2005 doc 12 ' 77cP 6.4409 05 0005 -- '