WORK PLANS - Laserfiche WebLink

Home Browse Search

Ms. Lori Duncan Y, July 26, 2006 Page 3 of 6 Geology and Hydroeeolo2y Much of the geological and hydrogeological setting described below was taken from the Conceptual Site Model for Unocal Service Station #0187, 437 East Miner Avenue in Stockton, California prepared by ENSR in January 2006. The Unocal Service Station #0187 is located north of the subject property. Soil is comprised primarily of silt and clay to approximately 25 feet bgs. Interbedded layers of silt, clay, and sand extend to approximately 75 feet bgs. Silt and clay are again predominant between approximately 75 and 95 feet bgs. Three zones of higher sand content have been previously identified in the area between approximately 35 and 50 feet bgs, 60 and 75 feet bgs, and below 115 feet bgs. These zones are identified as Zone A, Zone B, and Zone C, respectively. However, there is no contiguous low- permeability unit between Zones A and B (ENSR, 2006). The depth and thickness of these zones are highly variable across the site. Depth to groundwater has ranged from 25 to 48 feet in the vicinity of the Site. The groundwater flow direction in all three zones is generally towards the east, away from the San Joaquin River. Hydraulic gradient ranges from 0.004 to 0.007 feet per foot(ft/ft). The vertical hydraulic gradient is downward near the Site. Potentiometric surface monitoring data for the Unocal site indicates that the vertical component to the hydraulic gradient between Zone A and B is comparable to the horizontal; however, the vertical component is greater between Zone B and Zone C. Therefore, the primary direction of groundwater migration is downward between these units (ENSR, 2006). v"> _, ; z, un%C;L; WV-t'(QA KV : FWw'ztn C K The nearest water supply well is located approximately 1,600 feet down gradient to the east-northeast of the site(ENSR,2006). PROPOSED SCOPE OF WORK Based on review of previous investigations by others, the following proposes the following scope of work to fill data gaps: 1. Complete a file review at the SJCEHD to obtain all available documentation for the site and the Unocal#0187 site located north of the subject property. 2. Survey all five existing monitoring well locations. 3. Conduct a geophysical survey to locate the four 550-gallon USTs reportedly abandoned in place at the site. 4. Install a monitoring well in Zone A to the east of MW-I to verify groundwater impacts are not migrating off-site. 5. Vertical delineation of soil impacts is complete for the area near the USTs closed in-place by the z - soil samples collected from CPT- Horizontal delineation of soil impacts near sample location MW-1 is completed to t e west-southwest with soil borings C, D, E, and F. Additional soil borings will be advanced to the north and east of MW-1/CPT-1 to further delineate the soil impacts in this area of the site. 6. The quarterly groundwater monitoring program will be re-started at the site. J:00006\9-8227_00261022_2006 Work Plan.doc BLASLAND, BOUCK& LEE, INC. engineers,scientists, economists