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T ! Offs 109-affiivar YL9ged ufl ardl 0490 Ms Margaret Lagorio # San Joaquin County Public Health Services February 4, 2000 Page -2- Task 2—Vertical Delineation of Petroleum Hydrocarbons in Groundwater The CVRWQCB requested that the vertical extent of petroleum hydrocarbons in groundwater be evaluated at the Stockton Yard site To delineate the vertical distribution of petroleum hydrocarbons in groundwater, wells MW-6 (screened from approximately 45 to 75 feet bgs) and VW-2 (screened from approximately 45 to 70 feet bgs) were sampled using a Kabis discrete sampler Both wells are 4-inch diameter PVC and are located in areas where petroleum hydrocarbons have been historically detected in groundwater Groundwater samples were collected 1n two wells at the following locations (1) less than 2 feet from the top of the water table, (2) approximately 10 feet below the top of the water table, and (3) approximately 15 to 20 feet below the top of the water table Groundwater was not to be purged from the wells prior to sample collection Rather, the objective was to collect undisturbed samples (to the extent possible) at up to three sample depths . A State-certified mobile laboratory owned and operated by ONSITE Environmental Laboratories (ONSI"I E) was contracted to analyze collected samples for TPH-gasoline range (TPH-G) and volatile compounds (VOCs) in accordance with EPA Methods 8015 modified and 8260, respectively Nitrate and sulfate analyses were also conducted TPH and VOC analytical data are shown graphically on draft Figures 5-1 and 5-3, ONSITE's analytical report is included as an attachment The estimated groundwater gradient is shown on draft Figure 5-2 These draft figures are from the draft report, titled "Additional Groundwater Study, Caltrans Stockton Yard " Task 3 —Estimation of Hydraulic Conductivity To estimate the hydraulic conductivity beneath Stockton Yard, slug tests were conducted in three monitoring wells The slug tests entailed measuring the instantaneous response within the respective wells after introducing and removing a mandrel (slug) of specified weight and volume Water elevation data were collected until static conditions were achieved Slug test data were evaluated using Aqtesolv groundwater software to estimate the hydraulic conductivity for the water-bearing zone screened in each respective monitoring well Hydraulic conductivity estimates were then used to estimate the groundwater velocity The results will be included in the report 41SBA11S HAREDIPROJ141040100041SYYARDISJ CPHSLTR STKTNYD DOC 242000