Laserfiche WebLink
amec— <br /> screened <br /> from approximately 50 — 65 feet bgs, and were designed to extract groundwater at <br /> rates of approximately 30 gpm each, as part of the Water Board-approved Interim Remedial <br /> Action Plan (TRAP) for TCE-affected groundwater between 50 and 60 feet bgs (ECM, 2007). <br /> Extraction of B-zone groundwater was implemented offsite, at 519 Stockton Avenue in 1999. <br /> Groundwater was extracted at rates ranging from approximately 60 to 120 gpm from well E-2 <br /> which is screened from approximately 47.8 — 65.5 feet bgs. This well was installed to address <br /> historically high offsite concentrations of TCE reported for samples from well M-813 (the source <br /> for TCE near M-813 is likely the historical exfiltration of dissolved TCE discharged to the <br /> sewer). A deeper extraction well (E-6) was installed in 2001 and began operational in mid- <br /> 2004 to capture VOC-affected groundwater (primarily cDCE and TCE) that appears to have <br /> originated near the City of Ripon Wastewater Treatment Facility, at the southern end of <br /> Stockton Avenue. Extraction well E-6 is screened from 143 — 158 feet bgs and has been <br /> pumped at annual average rates of approximately 200 - 400 gpm. <br /> 3.2 HYDROSTRATIGRAPHY <br /> The hydrostratigraphy of the Site has been characterized in detail to a depth of approximately <br /> 120 feet bgs using the results of 18 CPT borings, 13 of which were located onsite and 5 which <br /> were located in the parking area to the west across Industrial Avenue (Figure 3). Cross- <br /> sections of physical soil characteristics, based on the CPT borings to approximately 120 feet <br /> bgs are included as Figures 4 and 5, and are summarized as: <br /> • Silts and sands to approximately 35 —40 feet bgs (referred to as the B-zone in Site <br /> reports). This zone includes the water table at a depth of approximately 30 feet bgs. <br /> Although coarse-grained sediments have been logged by both CPT and manual <br /> methods in the B-zone, the sediments generally are characterized as finer-grained <br /> compared to deeper water bearing zones of the Upper Aquifer. A tracer dilution test <br /> performed at well M-1 B was used to estimate a groundwater velocity of <br /> approximately 1 ft/d (Geomatrix, 2006). <br /> • Fine-grained sediments (silts and clays) are generally encountered from <br /> approximately 40— 50 feet bgs (40 —55 feet bgs further west at CPT-6). <br /> • Sands and gravels with silt and clay interbeds are encountered from approximately <br /> 50 to 95 feet bgs. No formal designation was applied to this stratigraphic interval <br /> during the early hydrogeologic investigations completed during the RI because no <br /> wells were installed within this zone until December 2007. The revised SCM, as <br /> presented herein, divides this depth interval into the Upper A-zone (approximately <br /> 50 — 80 feet bgs) and Middle A-zone (approximately 80 — 95 feet bgs) for <br /> consistency with the long-established classification scheme. Generally, two <br /> apparent fining-upward sequences can be observed in the Upper A-zone CPT logs <br /> where the Middle A-zone appears to be a coarse-grained unit (sand and or gravel) <br /> of relatively consistent vertical thickness and physical properties (Figures 4 and 5). <br /> These coarse-grained water-bearing zones are highly productive, where pumping <br /> AMEC Geomatrix, Inc. <br /> hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 16 <br />