WORK PLANS - Laserfiche WebLink

Home Browse Search

Mr.M. Scott Mansholt—CEMC Soil Sampling Work Plan August 1,2006 Surland Homes Property Page 2 of 8 ' REGIONAL AND LOCAL HYDROGEOLOGY Regionally, the site is located within the San Joaquin Valley Groundwater Basin. This basin is divided into nine sub-basins. The site lies within the Tracy sub-basin(Groundwater Basin Number 5-22.15),which is defined by the areal extent of unconsolidated to semi-consolidated sedimentary deposits that are bounded by the Diablo Range to the west,the Mokelumne and San ' Joaquin rivers to the north,the San Joaquin River to the east, and the San Joaquin-Stanislaus county line to the south. The Tracy sub-basin is located adjacent to the eastern San Joaquin Sub- basin to the east and the Delta-Mendota Sub-basin to the south. ' The Tracy sub-basin is comprised of continental deposits of late Tertiary to Quaternary age, which includes the Tulare Formation, Older Alluvium, Flood Basin Deposits, and Younger Alluvium. The thickness of these formations ranges from a few hundred feet in the western ' foothills to about 3,000 feet near the eastern margin of the basin. The Tulare Formation consists of semi-consolidated,poorly sorted,discontinuous deposits of clay, silt, and gravel. The Corcoran Clay is situated near the top of the Tulare Formation, separating groundwater in the ' basin in what has been reported as two primary aquifers. The upper aquifer is reported at 15 to 250 feet below ground surface(bgs)and the lower aquifer is reported at 600 feet bgs. Given the location of the site within the sub-basin,the site is likely immediately underlain by Younger Alluvium, and Older Allluvium and/or Flood Basin Deposits. ' Previous subsurface investigations at the site have shown that the site is underlain by predominantly silt and clay,with isolated sandy lenses,to approximately 16 feet bgs. ' Groundwater is typically encountered at approximately 5 to 7 feet bgs. Monitoring at the site has shown a north-northwesterly groundwater flow. PREVIOUS ENVIRONMENTAL ACTIVITIES A soil and groundwater investigation was performed in 1998 by Geomatrix Consultants,Inc. (Geomatrix),who advanced eight soil borings, from which soil and groundwater samples were collected(nine soil samples and three groundwater samples). Following this investigation,two groundwater monitoring wells(TW-1 and TW-2)were installed at the request of San Joaquin County. A third well was installed at the request of the Regional Water Quality Control Board (RWQCB)in 1999. Quarterly groundwater monitoring activities were performed through April ' 2000. Well TW-2 was destroyed prior to April 2000 due to development in the area of the well. Geomatrix prepared and submitted a HRSE for the site in August 2002. The HRSE concluded that the theoretical lifetime excess cancer risk for an adult and child resident at the site is 1 x 10-6, equivalent to the acceptable target risk of 1 x 10-6. The report noted that the majority of the risk was from ingestion of groundwater. The Department of Toxic Substances Control(DTSC)issued comments on the HRSE in December 2003, and concluded that there was insufficient data for the ' site to reach a conclusion regarding the overall risk, and requested additional analytical data from the site. The DTSC letter also noted that a site conceptual model had not been developed for the site. ' hr February 2003,the RWQCB issued a letter requesting an additional quarter of groundwater monitoring and sampling to confirm that total petroleum hydrocarbons quantified as diesel (TPHd)concentrations were stable(or less than the water quality objective for the TPHd [without ' silica gel]taste and odor threshold),and/or attenuating in the two remaining monitoring wells. In response, CEMC forwarded a work plan to the RWQCB prepared by SAIC for the subject site (Work Plan for the Implementation of Quarterly Monitoring, Surland Homes[Laurelbrook] ' Property,April 30,2003). The work plan outlined the methods to conduct one round of quarterly