FIELD DOCUMENTS - Laserfiche WebLink

Home Browse Search

FROM Ground Zero Analysis �', PHONE NO. : 209 838 9883 Sep. 15 2004 10:32AM P2 MEMORANDUM DATE: September 15, 2004 TO: Harlin Knoll, EHD FROM: John.Lane, Ground Zero Analysis SUBJECT: Knowles Site, 7835 Thornton Road,Stockton,CA Status of Contamination Investigation at the subject site. Past Remediation 3000 cu.yd. soil excavated in 1990. Excavated to 35-40 feet. Confirmation samples of wall samples indicated ND results laterally, but residual soil contamination remained on the excavation floor (75-336 ppm TPHG; 0.62-0.74 ppm benzene). Groundwater pump test conducted in December 1993 on well MW4 removed about 900 gallons and determined that an extraction well could sustain about 3 gpm. No large volume of groundwater extraction has occurred. Vertical investigation (DB1-DB3) conducted in July 1998 indicated that vertical soil contamination is limited. TPHG detected 70 ppm at 70 feet, 2.6 ppm at 85 feet, and <1.0 at 98 feet. BTEX constituents also attenuated with depth. No deep groundwater samples were collected_ Grab groundwater samples collected from the augers at the water table were impacted at levels similar to well MWS. (See cross section sketches and tables). A lateral investigation north of the site in the center of Hammer Lane in July 2000 (SB1X) indicated that the lateral extent of groundwater contamination is limited to the north and does not extend significantly past the center line of Hammer Lane. TPHG was detected in a hydropunch sample at 120 ppb; benzene was <0.5 ppb. No gasoline constituents were detected in soil samples collected at the capillary fringe (30 feet bgs) or the bottom of the hole (47 feet bgs). Well MW9 east of the site confirmed that groundwater contamination has not migrated significantly downgradient. Well MW8 was generally ND with a few scattered hits. Well MW9 has been nondetect. Although no sand was logged within the saturated zone in well MW9, there was no sample recovery in the interval between 45 and 50 feet bas, which implies the presence of sand in that interval. Purge logs from well MW9 show less than 0.70 feet of drawdown in the well at a pumping rate of 1.2 gpm, which supports the conclusion that sand is present in the well at 45-50 feet bgs. The depth to groundwater at the site has ranged from approximately 59 feet bgs in 1990 to less than 30 feet bgs in May 2000. The most recent measurements in May ranged from 36-37.7 feet bgs. Gasoline constituents in groundwater have been steadily declining, and the plume is stagnant, but benzene levels in well MW5 over the last year have averaged 3,150 ppb, down from historic high of 32,000 ppb in December 1996_