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i r Manteca Bean Facility, 229 Moffat Boulevard, Manteca, Caldomia October 22, 1992 Golden Grain Company Page 2 summarized in Table 1 The water table elevations for August 21, 1992, were used to prepare a potentiometnc surface map (Figure 1) Monitoring Well Gauging and Sampling Following well gauging, groundwater samples were collected from eight monitoring wells in I accordance with the LUFT guidelines The samples were submitted under a chain-of-custody manifest to Sequoia Analytical Laboratories in Sacramento, California Each sample was analyzed for benzene, toluene, ethylbenzene and xylenes (BTEX), and total petroleum hydrocarbons-as- gasoline (TPH-as-gasoline) by Environmental Protection Agency (EPA) methods 5030/8015/8020 Historical and recent analytical data are summarized in Table 2 The laboratory analyses reports, and the chain-of-custody manifest for the August 21, 1992, samples are included in Attachment I Maps of dissolved TPH-as-gasoline and benzene concentrations are included as Figures 2 and 3, respectively I The potentlometric surface elevations in the site-related groundwater monitoring wells decreased in 9 of the wells an average of 2 56 feet since the last monitoring event conducted on May 21, 1992 • Wells MW-1 through MW-6 were dry Groundwater beneath the site flows approximately north- northeast at an overall gradient of 0 0015 The gradient at the southern end of the site is more steep than the gradient in the central and northern portion of the site Monitoring data from the last quarter (5/21/92) reported a sheen of separate phase petroleum hydrocarbons in MW-4 During this quarter, no separate phase petroleum hydrocarbons were detected in MW-4, due to lower groundwater elevations resulting in wells 1 through 6 being dry Of the eight monitoring wells sampled on August 21, 1992, dissolved TPH-as-gasoline was detected only in groundwater samples from MW-7 and MW-14 at concentrations of 6,600 and 68 parts per billion (ppb), respectively Benzene concentrations detected in MW-7 were 2,100 ppb and in MW-14 they were 0 5 ppb Concentrations of hydrocarbons detected in MW-14 this quarter however are not consistent with past sampling results Past laboratory results from groundwater samples of MW-14 for the last three quarters have been below detection limits MW-14 is located across Moffat Boulevard from the site and lies upgradient from the original source area on site Trace concentrations of toluene and xylenes from laboratory analysis were detected in groundwater samples from MW-9 and in one field blank sample Levels in these two samples were very low and indicate no significant change to site conditions The effect of trace amounts of hydrocarbons detected in the field blank may indicate that sample results with detected hydrocarbons are actually lower by 18 ppb toluene and 12 ppb xylenes Two possible explanations for these trace amounts . of hydrocarbons in the field blank result from slight contamination during handling Concentration levels this low indicate that, trace amounts of hydrocarbons, from contact source during field work, �L � CROUNDWA'TER �n TECHNOLOGY INC I I