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92.Y ♦y1 j� Mr. Kyle Christie November 9, I988 Page 3 t 40 feet to 65 to 70 feet below ground surface. An examination of these contour maps shows a seasonal groundwater flow direction change from mostly southerly in winter months to mostly westerly in summer months. The extremes of this groundwater flow direction change are shown on Figure 1. The hydraulic gradient averages about 10 feet per mile. Dissolved Product. An examination of Figure 1 shows a reduction over time in benzene concentrations in groundwater from on-site monitoring wells W-2 and W-3. Factors that may have influenced this change include removal of free product, seasonal water level r i changes, an overall lowering of the water table of about 4 feet per year over the last 1.5 years, vertical and lateral dissemination, and natural chemical and biological degradation. Benzene concentrations in monitoring well W-1 have increased over time. Although Well -.� W-1 is mostly upgradient of the hydrocarbon source, the relatively flat gradien-. could allow hydrocarbons to migrate in an upgradient direction toward well W-1. No hydrocarbon constituents have been identified in groundwater from off-site monitoring wells to the west 1 o and southwest (W-6, W-7, and W-8). Benzene has been identified at low concentrations in groundwater from off-site monitoring wells to the south and southeast (W=4 and W-5). x Free Product, Between July and November 1987, free product accumulated in on-site monitoring wells W-2 and W-3. Armour Petroleum Company (Armour) removed a total of 30 gallons of free product from monitoring wells W-2 and W-3 during this time period. l Water level monitoring showed no accumulation of free product in any well between November 1987 and May 1988. Between May and July 1988, free product again accuniu- lated in monitoring wells W-2 and W-3. Armour removed a total of 22 gallons of free t product during this time period. All Armour data sheets are included as Exhibit D. f: J:_d The underground tanks and lines at Station 434 tested tight on March 15, 1988. in addition, no inventory discrepancies have been noted since the test date. These data indicate there has been no new release of product. Therefore, the source of the free product accumulation in monitoring wells W-2 and W-3. is probably from a reservoir of hydrocarbon-saturated soil in the vadose. zone. Sediments in the vadose zone are made up Of complexly interlayered and interfingered sands, silts, and clays. E. Sc' Contamination. Of 18 soil samples collected from monitoring well boreholes W-1 ' through W-8 and analyzed for total petroleum hydrocarbons (TPH) by U.S. Environmental F Protection Agency g y (EPA) Method 8015, Wane have shown evidence of TPH. However, l field evidence (soil samples screened with a photo-ionization detector) indicates hydrocarbon vapors in the vadose zone in borehole W-2 from 10 feet below ground surface ?r to the water table, and in borehole W-3 in the capillary fringe zone. Hydrocarbon vapors I_a in the area of monitoring well W-2 may indicate residual contamination from the - underground storage tanks formerly located in that area (see Figure 2). Samples collected from soil boreholes B-I and B-2 (Figure 2) have identified high concentrations of TPH. -- t -- - Laboratory-results-of. soil sample-analyses-from-these-two boreholes are summarized in �^ Table 1. W s 9ROWN AND CALDWELL r �` - 7236STREET SACRAMENTO,CALIFORNIA BSa11.7082•M6)444.0123