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Treadwell&Rollo
<br /> A LANSAN COMPANY
<br /> Request for Low-Threat Case Closure 6 June 2013
<br /> RWQCB case#391149 and#391162 Page 3
<br /> 420 and 423 N Madison Street
<br /> Stockton,Califomia
<br /> Project:731589001
<br /> information regarding its past use and contents was not available in historical records; however, gasoline
<br /> and diesel were likely stored in the UST based on detected concentrations of TPHg,TPHd, ethylbenzene
<br /> (E) and xylenes(X) detected in soil beneath the UST. Benzene(B), toluene (T), methyl tertiary butyl
<br /> ether(MTBE), and fuel oxygenates were not detected in soil samples (T&R, 2004a).
<br /> UST B-6 was removed on 2 April 2004 and soil was over-excavated under oversight by the EHD. Soil
<br /> confirmation samples were collected at the bottom and side-walls of the excavation and samples were -51
<br /> oo�S
<br /> analyzed for TPHg,TPHd,TPHmo, polycyclic aromatic hydrocarbons(PAHs), BTEX, MTBE, fuel
<br /> oxygenates, lead scavengers, lead, and arsenic. Approximately 3,800 cubic yards of soil was removed
<br /> during the over-excavation of UST B-6 (T&R, 2005b).
<br /> Soil and grab groundwater samples have been collected by T&R and others prior to and during
<br /> remediation. Periodic groundwater monitoring at the site has been performed by AGE under the
<br /> oversight of the EHD using 15 groundwater monitoring wells (14 on-site and one off-site).
<br /> �.Y n December 2003,T&R submitted a well destruction work plan to the EHD which was accepted on 12
<br /> January 2004. All wells were destroyed in March 2004 according to the procedures described in the work
<br /> C & plan (T&R, 2005b).
<br /> AGE conducted a final groundwater monitoring event in August 2003, prior to well destruction. The ,
<br /> locations of the groundwater monitoring wells (former wells MW-1, MW-2, MW-4, MW-5, MW-8, MW-11,
<br /> MW-12, MW-13, MW-18, MW-19A, MW-19B, MW-20A, MW-20B, MW-21A and MW-21B)are shown on
<br /> Figure 3. Former well MW-21A was located near former UST No. 8 and had the greatest concentrations
<br /> of TPHg,TPHd, total petroleum hydrocarbons as motor oil (TPHmo), and BTEX at the Site. Former well
<br /> MW-21B,also located near former UST-8 had the greatest concentrations of TPHg, TPHd,TPHmo,
<br /> ethylbenzene, and xylene in the deeper groundwater zone. Other former wells (MW-1, MW-2, MW-4,
<br /> MW-5, MW-8, MW-11, MW-12, MW-13, MW-19A, MW-19B, MW-20A, and MW-20B) including those near
<br /> UST B-6 did not have TPHg detections.
<br /> Soil and groundwater data for 423 Madison are included in Attachment 2.
<br /> Lithology and Hydrogeology
<br /> Soil borings at Carando show sandy fill material from ground surface to approximately 8 feet bgs.
<br /> Beneath the fill material are silts and clays with laterally discontinuous sand lenses to the total depth
<br /> explored, 38 feet bgs(Figure 4). Groundwater has been encountered in soil borings at Carando at 19 to
<br /> 20 feet bgs.
<br /> Soil borings at 423 Madison have also encountered sandy fill material from ground surface up to
<br /> approximately 8 feet bgs. Beneath the fill material are fine-grained materials to about 15 feet bgs with
<br /> sand and silty sand below (Figure 5). A low permeability clay layer has been identified at approximately
<br /> 40 to 45 feet bgs which separates the shallow aquifer into an A zone (0 to 40-feet bgs) and a B zone
<br /> (deeper than 45 feet) (T&R, 2009).
<br /> Groundwater has generally been encountered in wells at 423 Madison at approximately 16 to 18 feet bgs.
<br /> There were previously three nested well locations at 423 Madison that had both A and B zone wells (MW-
<br /> 19A& MW-19B, MW-20A&MW-20B, and MW-21A& MW-21B). Groundwater elevations in the B zone
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