ARCHIVED REPORTS XR0000236 - Laserfiche WebLink

Home Browse Search

1 Project No 58100-06-62 June 18, 1996 Page 2 ' Between March 1992 and January 1996,twelve monitoring wells (MW-1 through MW-12)have been installed at the site to evaluate groundwater quality and to attempt to delineate groundwater impacts In addition, two dual purpose vapor extraction/groundwater monitoring wells (VW-1 and VW-2) ' have been installed in the vicinity of the former USTs in order to facilitate future source removal through vapor extraction ' Historically, the highest TPHg and benzene concentrations in groundwater have been detected within and in the vicinity of the former UST excavation in VW-1 and VW-2 at concentrations of 16,000 micrograms per liter (ug/1) TPHg and 3,000 ug/1 benzene Concentrations of dissolved phase hydrocarbons in groundwater decrease to below test method detection limits in perimeter wells MW- 9, MW-10, and MW-12 However, TPHg, benzene, and methyl-tert butyl ether (MTBE) have been ' detected in the eastern most penmeter well (MW-11) ' FIELD ACTIVITIES Groundwater Level Measurements and Flow Direction A representative of Geocon measured the depths to groundwater in thirteen groundwater monitoring wells on April 22, 1996 Well MW-10 was boned under construction materials and subsequently was not measured or sampled during this monitoring event Depth to groundwater measurements were obtained using a battery operated water level meter with measurements obtained from the top of ' each well casing Groundwater was encountered at depths ranging from 15 64 to 18 78 meters (50 99 to 61 62 feet) ' below the top of the well casings A review of groundwater elevations indicates that the groundwater elevation has risen approximately 0 98 meter (3 21 feet) since May 1995 A summary of the top of well casing elevations, groundwater level measurements and groundwater elevations is presented on Table I Based on the April 22, 1996 groundwater elevation data, the groundwater flow is directed towards the east at an approximate gradient of 0 001 ft/ft Groundwater elevation contours and gradient for the April 1996 monitoring event are depicted on the Site Plan, Figure 2 ' Well Purging and Sam lm Well purging and groundwater sample collection was performed on April 22 and 23, 1996 Prior to groundwater sample collection, approximately three well volumes of groundwater were extracted from wells MW-1 through MW-9, MW-11, MW-12, VW-1, and VW-2 utilizing a submersible pump Groundwater extraction was performed to allow fresh formation water to infiltrate the wells ' During well purging activities, the groundwater was monitored for pH, electrical conductivity and temperature Groundwater recovery was monitored in order to ensure 80 percent well recovery prior to well sampling Monitoring well sampling data sheets are presented in Appendix A Purged ' groundwater was discharged to the sanitary sewer system in accordance with a City of Stockton industrial waste discharge authorization