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Stockton Scavenger Association Facility ' November, 1997 Page 5 ' 4 0 GROUNDWATER MONITORING EVENT The following sections include descriptions of groundwater level measurements, groundwater ' sample collection, and groundwater analytical results from June 1997 4.1 Groundwater Levels ' ENSR collected depth to groundwater measurements from monitoring wells MW-1 through MW-4 on July 16, 1997 The groundwater monitoring event originally occurred on June 30, 1997, ' however, well MW-2 was not sampled due to access constraints ENSR returned to the site on July 16, 1997 to monitor well MW-2 and, for consistency, collected groundwater level measurements from the remaining wells Groundwater level measurements are summarized in ' Table 1 The groundwater level measurements were taken from the referenced wellhead elevations, consistent with the ENSR standard operating procedures included in Attachment B Figure 3 is a groundwater elevation hydrograph depicting the relationship between groundwater elevations measured in the monitoring wells on the vertical axis and time on the horizontal axis Figure 4 is a potentiometric surface map on July 16, 1997 The hydrograph on Figure 3 shows that the groundwater elevations to the monitoring wells increased, on average, approximately 6 feet between Apnl 1996 and July 1997 The hydrograph also shows that the groundwater elevations have increased over 10 feet since monitoring began in 1993 4.2 Groundwater Sampling and Chemical Analysis ENSR collected groundwater samples from wells MW-1, MW-3, and MW-4 on June 30, 1997 Well MW-2 was sampled on July 16, 1997 Groundwater samples were collected with disposable bailers following a purge of three casing volumes with a submersible pump ENSR collected ' dissolved oxygen measurements during purging of the groundwater monitoring wells Groundwater monitoring was performed consistent with the ENSR standard operating procedures included in Attachment B ' The samples were delivered under chain of custody to Columbia Analytical Services of San Jose, California, and analyzed for total petroleum hydrocarbons identified as gasoline (TPHg), total petroleum hydrocarbons identified as diesel (TPHd), benzene, toluene, ethylbenzene, and total ' xylenes (BTEX), and methyl tertiary butyl ether (MTBE) The groundwater analytical results and dissolved oxygen measurements are summarized in Table 2 Concentrations of TPHg, TPHd, and benzene for the.lune 30 and July 16, 1997, monitoring events are presented on Figure 5 ' Petroleum hydrocarbon concentrations were not reported above laboratory practical quantitation limits (PQLs) in groundwater samples collected from wells MW-1, MW-3, and MW-4 TPHg was also not reported above PQLs in the groundwater sample from well MW-2 Concentrations of BTEX in the groundwater sample from well MW-2 decreased by a factor of ten since the April 4, 1996 sampling event During the latest monitoring event, benzene is the only analysis parameter ' that exceeds the California Department of Health Services Primary Maximum Contaminant Level (MCL) As shown on Table 2, dissolved oxygen concentrations in groundwater purged from wells 5027002ART