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t Additional Subsurface Investigation And Well Installation Report 5 The Southland 2oMration,Store#19976,1399 N Main St Manteca California November 15 1998 The top of casing and nm of each well box was surveyed in October 1998 by Morrow Surveying for horizontal position and elevation relative to mean sea level datum using the City of Manteca benchmark #16 (Elevation 33 00) located at the northwest comer of Northgate Drive and North Main Street as a reference The resultant map is included in Appendix B Field notes from the well Installation are included In Appendix C M 35 Groundwater Monitoring and Sampling On September 16, 1998,depth to water in each well was measured to the nearest 0 01 foot from the top ' of the well casing using an interface probe Field data sheets are provided in Appendix C A potentiometnc surface map was prepared using the groundwater elevation data collected on September 16, 1998 (Figure 2) No separate-phase hydrocarbons (SP)were observed in the four groundwater monitoring wells during this monitoring event The groundwater monitoring results from the September 16, 1998 monitoring and sampling event are presented in Table 2 Pnor to groundwater purging, dissolved oxygen was measured in each well using a field instrument Dissolved oxygen data taken from four monitoring wells indicate depleted oxygen levels due to Intrinsic biodegradation Dissolved oxygen levels are higher in wells which have lower dissolved hydrocarbon concentrations Dissolved oxygen levels Indicate that natural attenuation through biodegradation is likely to continue at this site Dissolved oxygen readings taken ranged from 12 mg/L in MW-1A to 4 3 mg/L to MW-3(Table 2) Subsequent to mondonng,approximately four well volumes of water were removed from each monttonng well to ensure stabilization of pH, electrical conductivity, and temperature The purge water was stored in DOT-approved 55-gallon drums The groundwater samples were collected using disposable bailers, then transferred to 40-milliliter glass vials filled to zero headspace and acidified to a pH of 2 The ' samples were sealed using Teflon®-septum caps, labeled, placed on Ice in an insulated container, and transported under chain-of-custody to state-certified Core Laboratories in Anaheim, California The groundwater samples were analyzed for BTEX, and TPH-g using EPA Methods 8020/modified 8015, and for TBA, MTBE, DIPE, ETBE, and TAME by EPA Method 8260 All purge water generated during the monitoring and sampling event was stored in a Department of Transportation (DOT)-approved 55-gallon drum pending characterization and proper disposal 19976w rpt •• I FLUOR DANIEL GTI ' fry