ARCHIVED REPORTS XR0000234 - Laserfiche WebLink

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Between March 1992 and January 1996, twelve monitoring wells (MW-1 through MW-12) were 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) ' were installed in the vicinity of the former USTs in order to facilitate future hydrocarbon 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 wells VW-1 and VW-2 at concentrations of 54,000 micrograms per liter (µg/1) TPHg and 8,100 µg/l benzene Concentrations of dissolved phase hydrocarbons in groundwater decrease to below test method detection limits in perimeter wells MW- ' 9 and MW-10 TPHd has been detected in perimeter well MW-12 TPHg, TPHd, benzene, and methyl tert-butyl ether (MTBE) have been detected in the eastern most perimeter well, MW-11, at maximum concentrations of 600, 120, 5 1 and 170 µg11, respectively 1 FIELD ACTIVITIES Groundwater Level Measurements and Flow Direction Representatives of Geocon measured the depths to groundwater in fourteen groundwater monitoring wells on October 22, 1996 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 between 59 and 61 feet below the top of the well casings A ' summary of the top of well casing elevations, groundwater level measurements and groundwater elevations is presented on Table 1 Based on the October 22, 1996 groundwater elevation data, the groundwater flow is directed towards the east at an approximate gradient of 0 0008 ft/ft which is consistent with the previous monitoring events performed in April and July 1996 Groundwater elevation contours and gradient for the October 1996 monitoring event are depicted on the Site Plan, ' Figure 2 Well Purging and Sampling Well purging and groundwater sample collection was performed on October 22 and 23, 1996 Prior to groundwater sample collection, approximately three well volumes of groundwater were extracted from each well 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 into a water trailer pending discharge to the City of Stockton (City) sanitary sewer system A new wastewater discharge permit ' for the disposal of purge water to the sanitary sewer is currently being processed by the City Following well recovery, a water sample was collected from each well using polyethylene disposable bailers and decanted into 40-m1 volatile organic analyses vials and one-Mer amber bottles The ' samples were sealed, labeled and placed in an ice chest containing ice and were initially transported to Geocon's Sacramento office for cold storage at 4 degrees Celsius pending shipment to the analytical laboratory The samples were subsequently shipped to Advanced Technology Laboratories (ATL), using standard chain-of-custody protocol Project No S8100-06-62 -2- November 27, 1996 1