ARCHIVED REPORTS XR0006098 - Laserfiche WebLink

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0 The property previously contained two 1000 gallon gasoline Underground Storage Tanks (UST's), product dispensers, and associated underground piping (Figure 3) The UST's, dispensers, and • piping were removed from the site in 1989 The property owner originally installed five onsite monitoring wells Three additional cluster wells have been installed to aid in the investigation of soil and groundwater conditions at the site LOCAL GEOLOGY AND HYDROGEOLOGY The site is located on surface soils belonging to the Stockton Adobe Clay and Honeycut Clay series They are derived from Recent alluvial fan and floodplain deposits The 40 feet of explored section consisted of clay and silt overlying interbedded lenses of silty and clayey sands below 12 feet This first sandy formation will be designated the A zone aquifer The base of the A zone is at approximately 17 - 18 feet below ground surface (bgs) An aquitard consisting of silty clay approximately 3 to 4 feet thick separates the silty clayey sand A zone aquifer from the B zone aquifer that is found at approximately 21 feet bgs and is approximately 6 to 7 feet thick The B zone aquifer consists of cleaner moderately sorted fine to coarse sand that shows graded bedding (fine to coarse with depth) Beneath the B zone aquifer is stiff clay that extends from approximately 27 feet bgs to 40 feet bgs The original five monitor wells did not encounter groundwater in the A zone aquifer when initially drilled and sampled Depth to water measurements taken between 1993 and February 1996 indicate that the A zone aquifer was dry (depth to water greater than 17 feet bgs) MW 1 through MW5 are screened across both the A and B zone aquifers Sometime after February 1996 depth to groundwater measurements indicated that groundwater was in contact with the A zone aquifer, see Table 1 and Figures 8 and 9 The groundwater in the vicinity of the site is semiconfined Groundwater elevation at the site varies from a wet year high of 7 feet bgs (silty clay formation above the A zone) to about 22 - 23 feet bgs (B zone) during the previous drought period Groundwater flow direction in the A zone at the site has generally been to the west since December 1997 The B zone shows a similar flow direction, which would be expected due to the connection between zones at wells MW 1 through MW5 ADDENDUM PUMP TEST The aquifer pump test will be performed in 8 stages Stage 1 -Destruction of MW 1 and installation of four piezometers, two in the A zone and two in the B zone between wells RIA and well MW3, see Figure 1 The two A zone wells will be dnlled south of RIA PZIA will be drilled approximately 10 feet north of RIA and PZ2A will be drilled approximately 20 feet north of RIA Drilling will utilize 8 inch hollow stem augers with 3 foot core barrels The bonngs will be cored to the 18 foot depth base of Zone A sand Core examination will determine the proper placement of 5 foot length of 2 inch diameter, 0 01 inch slotted PVC screen Blank Schedule 40 threaded PVC will be used to compete the piezometers to surface #2112 sand will be used as the filter pack across the screen from bottom of the boring to 2 Sanchez ADD PT 10-2002