WORK PLANS - Laserfiche WebLink

Home Browse Search

Mr.Nuel Henderson,P.G., SJCEHD September 21,2005 ARCO Station 6100,Tracy,California Project No.U6100-15-1 Additional Site Assessment Work Plan Page 5 Site Hydrogeology Based on information collected during the drilling of boring B-32, the regional water table appears to be deeper than approximately 202 feet bgs at the site. The on-site water supply well is screened between approximately 396 and 416 feet bgs and 496 to 536 feet bgs, with a gravel pack installed from approximately 35 to 540 feet bgs. The static water level in the water supply well has been measured at approximately 45 feet bgs. Based on this information, the water supply well appears to be screened within a confined regional water supply aquifer. Historical depth to groundwater measurements and groundwater elevations are presented in Table 3. The source of the groundwater observed in the monitoring and vapor extraction wells beneath the site cannot be positively determined at this time. In the site conceptual model (SCM), Stratus identified percolation of meteoric water into the subsurface, leakage from on-site water pipes, and recharge from the on-site water supply well as possible sources of the encountered groundwater. Stratus has previously reported that the water encountered in the shallow-screened wells was likely meteoric water. The basis of this interpretation was a general increase in water levels in the shallow screened wells during the winter and spring months, and decrease in water levels during the summer and autumn months. Groundwater elevations in the shallow screened wells were measured between approximately 223 and 258 feet msl at the time of the second quarter 2005 monitoring event. This significant variation in the water level measurements suggests that groundwater in the shallow screened wells is perched and localized. Depth to groundwater was measured between approximately 181 and 185 feet msl in the deeper screened monitoring wells located south of Patterson Pass Road. A review of Figure 6 shows approximately evenly spaced potentiometric surface lines, with a groundwater surface sloping towards the east-southeast at an average gradient of 0.031 ft/ft. Based on this observation, an upgradient source of groundwater recharge, providing relatively constant head pressure, appears to be supplying groundwater to sand zone 2. Given the groundwater elevations observed in wells MW-1, MW-12, MW-13, and MW-15, and the close, upgradient position of the on-site water supply well relative to these wells, it appears likely that groundwater recharge into sand zone 2 has occurred from the on-site water supply well. Since the well is gravel packed up to 35 feet bgs, and has a static water level of approximately 45 feet bgs, static water from the water supply well should come into contact with sand zone 2. However, the consistent absence of groundwater in deeply screened wells MW-3 and MW-7, which are situated between the on-site water supply well and off-site wells MW-1, MW-12,MW-13, and MW-15, is problematic to this explanation. K:\Bp-Arco for URS\6100\Work P1ans\wco6I00-workplan 09-2005.doc