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r 1117 Lone Palm Avenue, Suite Modesto, CA 953511 www.atcassociates.com 209-579-2221 i� ASSOCIATES I N C . Fax 209-579-2225 Environmental,Geotechnical and Materials Engineers the shallow groundwater bearing zone, MW5, MW6, and MW12 extending to a depth of 90 feet bgs in the intermediate groundwater bearing zone, and MW7 extending to a depth of 121 feet bgs in the deep groundwater bearing zone. Monitoring wells MW5, MW6, MW8, MW9, MW10, and MW12 were developed on December 22 and 23, 2004. An obstruction encountered in MW7 at approximately 64 to 65 feet bgs prevented development of this well. MW 11 was purged dry after removing 0.5 gallons of water. Efforts to develop this well using a bailer revealed the presence of grout inside the well and the total depth of the well was measured to be approximately 58 feet. On December 22 and 23, 2004 monitoring wells MW 1 through MW12 were surveyed relative to NAD 83 and NGVD29 for position and elevation and groundwater samples were collected from MW 1 through MW6, MW8, MW9, MW 10, and MW12. Petroleum hydrocarbon constituents were detected in soil samples collected from the boreholes of MW5 through MW9 and MW12. Groundwater samples were not collected from MW7 and MW 11 due to an obstruction in MW7 and grout infiltration in MW 11. Petroleum hydrocarbons were detected in the groundwater samples collected from MW 1 through MW6, MW8 through MW 10, and MW 12. The results of the subsurface investigation activities are presented in ATC's Subsurface Investigation Report, dated July 25, 2005. On April 7, 2005, an ATC geologist supervised the destruction of MW 11 and the installation of replacement monitoring well MW11R to a depth of approximately 65 feet bgs using 2-inch diameter. schedule 40 PVC casing. Refer to ATC's First and Second Quarter 2005 Monitoring Report, dated October 10, 2005, for additional information. SAMPLING ACTIVITIES The depth to water was measured in MW I through MW 10, MW I 1 R, and MW 12 (Figure 3) on December 29, 2005. Prior to collection of groundwater samples, the depth to water was measured in the wells and the pH, electrical conductivity, and temperature were measured in groundwater purged from the monitoring wells and recorded. A minimum of three well casing volumes were purged from each monitoring well prior to sampling. The wells were allowed to recover and samples were collected from each well using disposable polyethylene bailers. The groundwater samples collected from each well were submitted to State-certified Argon Laboratories (Environmental Laboratory Accreditation Program Certification. No. 2359) in Ceres, California for chemical analyses of total petroleum hydrocarbons quantified as gasoline (TPHg) utilizing EPA Method 8015M; and benzene, toluene, ethylbenzene, and xylenes (BTEX), methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE) di-isopropyl ether (DIPE), tertiary amyl ether (TAME), tertiary- butyl ether (TBA), 1,2-dichloroethane (1,2-DCA); and 1,2- dibromoethane (EDB) utilizing EPA Method 8260B. Groundwater well purge and sample logs are included in Attachment 1. s:lenviromental1283491reports12005 4QMR.doe 2 ATC Associates Inc.