ARCHIVED REPORTS XR0001313 - Laserfiche WebLink

Home Browse Search

S E C 0 R Ms Lori Duncan ARCO Station 4493 Site Closure Report • July 31,2006 Page 3 In November 2002, URS supervised the installation of dual-completion groundwater monitoring well set MWA 1 MID downgradient from the site Groundwater analytical data suggest that groundwater in the vicinity of MW-11 M is Impacted with TPHg and MtBE MtBE has been detected In well MW-11 D during four of five sampling events since the fourth quarter of 2002, but none of the detections have been above the MCL of 13 micrograms per liter(Ng/L) (URS, 2003) In December 2003, one deep groundwater monitoring well (MW-12D) was Installed under the direction of URS In the planterwest of North Center Street to assess the vertical extent of petroleum hydrocarbon contamination In groundwater Immediately downgradient from the former pump Islands (URS, 2004x) On May 3, 2004, one dual-completion groundwater monitoring well set(MW-1 3M/D)was Installed under the direction of URS in the State of California parking lot at 31 East Channel Street,west of EI Dorado Street, to assess the lateral and vertical extent of petroleum hydrocarbon contamination In groundwater downgradient from the former ARCO station pump Islands Groundwater samples collected from these wells were non-detect for gasoline-related compounds and MtBE (URS, 2004b) Quarterly groundwater sampling data is summarized in Tables 1 and 2 Boring logs for the groundwater monitoring wells are presented in Attachment B Well construction details are . summarized in Table 3 SITE GEOLOGY AND HYDROGEOLOGY Soil encountered during previous Investigations consisted of silt, silty sand, sandy clay, sandy gravel, and clayey silt Geologic cross sections of the site are Included In Attachment C Boring logs for the monitoring wells at this site are Included In Attachment B Depth to groundwater beneath the site has ranged from approximately 10 85 (12/2001) to 27 26 (9/2004)feet top of casing (TOC) Based on quarterly water level measurements at this site from February 1999 through March 2006 the Inferred groundwater flow direction is generally to the northeast at a gradient of 0 003 to 0 01 feet/foot Groundwater gradients and flow directions are summarized In Table 4 ; DISCUSSION In 1999, relatively low petroleum hydrocarbon concentrations In groundwater were reported in the samples collected from the site monitoring wells TPHg was reported at a maximum concentration of 15,000 micrograms per liter pg/L in February 1999 (Well MW-4) ri During the first quarter 2006 TPHg, benzene, toluene, ethylbenzene, total xylenes (BTEX), MtBE, and TBA were detected above laboratory reporting limits in groundwater samples collected from site wells during this quarter Additionally, tetrachloroethene (PCE),and TCE were detected above laboratory reporting limits In groundwater samples collected from site wells this quarter Low-level concentrations of TCE In MW-1, MW-2, MW-7, MW-10M, MW-10D, MW-11 D, MW-13M,and MW- z • 13D were detected Y