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1 <br /> REA <br /> October 14, 1993 Working To Restore Nature <br /> ARCO Station No 6020, Manteca, California <br /> 1 <br /> GROUNDWATER MONITORING <br /> EMCON of Sacramento, California, was contracted by ARCO to conduct the field work at <br /> I the site, EMCON subsequently forwarded their field notes and the laboratory reports to <br /> RESNA Field work included measuring depths to water and collecting groundwater <br /> samples for subjective and laboratory analyses RESNA used these data to evaluate the <br /> direction and gradient of groundwater flow beneath the site The locations of the <br /> monitoring wells are shown on Plate 2. <br /> I <br /> On August 18, 1993, a technician from EMCON measured the depth to water and collected <br /> groundwater samples from monitoring wells MW-1 through MW-4 for subjective and <br /> laboratory analyses According to their data, floating product or product sheen was not <br /> observed on samples collected from the wells (Table 1) <br /> I Tom O Morrow Inc (licensed land surveyors) of West Sacramento, California,surveyed the <br /> monitoring wells to provide the elevation of each wellhead relative to mean sea level <br /> RESNA used the surveyed elevations and the depth-to-groundwater measurements (Table <br /> 1) to evaluate the potentiometric surface. These data were used to construct the <br /> Potentiometnc Surface Map (Plate 3) The direction of groundwater flow was toward the <br /> north with a hydraulic gradient of 0.002 The direction of groundwater flow has generally <br /> been north since our investigation began Groundwater elevations fell approximately 115 <br /> feet since last quarter <br /> Groundwater samples from each well were analyzed for total petroleum hydrocarbons as <br /> gasoline (TPHg) and benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) <br /> using Environmental Protection Agency Methods 5030/DHS LUFT and 5030/8020 The <br /> samples were analyzed at Columbia Analytical Services, Inc in San Jose, California <br /> (Hazardous Waste Testing Laboratory Certificate No 1426) The results of these analyses <br /> are summarized in Table 1 Copies of the laboratory analysis reports are attached Results <br /> of analyses indicated TPI-Ig and BTEX concentrations were below laboratory detection <br /> limits in groundwater samples collected from wells MW-1 through MW-4 <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> Results of groundwater analyses from the past four quarterly monitoring events indicate <br /> dissolved hydrocarbons are not present in groundwater beneath the site, therefore, we <br /> recommend quarterly monitoring be discontinued It is our understanding that the <br /> underground storage tanks will be replaced in the future Upon completion of the tank <br /> replacement, ARCO will petition for site closure <br /> 0915kchr <br /> 30055-6 2 <br />