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TreadweII &RDII0 A LAN6AN COMPANY Request for Low-Threat Case Closure 6 June 2013 RWQCB case#391149 and#391162 Page 4 420 and 423 N Madison Street Stockton,Caltfomla Project:731589001 were approximately 1.5 feet lower than the A zone, indicating that a downward gradient was present in 2001 (AGE, 2002). Vertical gradients of 0.02 to 0.06 were calculated using data from between May 2001 and September 2001 and are presented in Attachment 2. Historical groundwater monitoring performed by AGE indicates the groundwater flow direction to be consistently towards the north-northeast in the A zone. Groundwater flow direction in the B zone is variable and has been toward the west-northwest, southwest, and southeast(AGE, 2002). The magnitude of the groundwater gradient during 2001 and 2002 ranged from approximately 0.002 to 0.003 feet per foot in the A zone and approximately 0.001 to 0.01 in the B zone. Extent of Releases to Soil and Groundwater 420 N Madison Street Soil and groundwater at Carando have been impacted by TPHg,TPHd, MTBE, and BTEX. Releases of petroleum hydrocarbons are likely due to historical operations at the Site which included fuel USTs, piping, and dispensers. The highest concentrations of petroleum hydrocarbons in soil and groundwater were detected in samples from borings CSB-2 in January 2004 and B-3 in June 2003. Both of these borings are located in the former UST area in t ersidewalk at the northern edge of the Site along West Fremont Street(Figure 2). After three remedial excavations were performed in 2004 and 2005,TPH remained in soil at three locations as shown on Figure 6. The highest remaining TPHg and TPHd concentrations in soil were 3,100 mg/kg and 520 mg/kg, respectively, at 30.5 feet bgs in boring CSB-;. __---*The highest concentrations of TPHg and TPHd in groundwater were also at boring CSB-2 at 34,000 micrograms per liter(µg/L) and 440,000 pg/L, respectively. However, after this same wel as collected, the remedial excavation was performed in this area in June 2004 and 20,000 gallons of groundwater was removed. The remaining concentrations of TPHg and TPHd in groundwater in the bottom of the June 2004 excavation were 16,000 pg/L and 2,700 pg/L, indicating excavation and groundwater removal significantly reduced the mass of TPH in the source area (Figure 7). MTBE was detected in remaining soil at concentrations up to 0.024 mg/kg with the highest concentration at 10 feet bgs in boring B-4. MTBE was also detected at 10 feet bgs in borings B-2 and B-5. MTBE was not detected in groundwater at the site. Benzene was not detected in any soil samples, and was detected in groundwater at the base of the June 2004 excavation at 46 pg/L. Toluene was detected in soil at concentrations up to 0.051 mg/kg, in the June 2004 excavation confirmation sample. Ethylbenzene and xylenes were detected in soil at concentrations up to 77 mg/kg and 150 mg/kg, respectively, with the highest concentrations in the sample from 30.5 feet bgs at boring CSB-2. Toluene, ethylbenzene,and xylenes were also detected in groundwater at CSB-2 at 260 pg/L, 2,500 pg/L,and 7,100 pg/L, respectively. 423 N Madison Street Soil and groundwater at 423 N Madison Street have been impacted by TPHg,TPHd,TPHmo, and ethylbenzene and xylenes related to UST B-6. Excavation of UST B-6 and surrounding soil in 2004 removed the majority of TPH mass in soil. Based on excavation confirmation samples, remaining soil left in-place contained TPHg up to 2,800 mg/kg and TPHd up to 1,300 mg/kg at BOT-17 and ESW-15, respectively(Figure 8).