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r -r § . BAW r E, H CHAPTER 5 J INTERPRETATIONS AND CONCLUSIONS The analytical results from all of the wells indicate that hydrocarbon-impacted groundwater remains primarily beneath or immediately adjacent to ARCO Products Company(ARCO)Station 434. Hydrocarbon-impacted groundwater is defined by upgradient well W-10, downg7adient wells W-6, W-7, W-8, and W-12, and across-gradient well W-11. Because of the lack of monitoring wells, the west boundary is very uncertain. Monitoring wells were not installed to the west of the station due to lack of landowner consent. Figure 5-1 shows graphs of water elevation and xylene concentration versus time for wells W-2, W-2A, W-2B, W-3, W-4, W-5, W-6, and W-7. Xylene was selected over the other benzene, ethylbenzene, toluene, and xylene (BETX) constituents because it is the most mobile in water and.was most often detected in analyses. The time interval shown on the x-axis of the graphs is the number of months since monitoring started in April 1987. As can be seen from these graphs, there is a positive correlation between xylene concentration trends and water-level trends. This indicates that some measurable and soluble concentration'of BETX constituents are migrating downward with the`decreasing.water table,rather than remaining in the capillary fringe zone which existed during the initial leak. However, the absence of BETX constituents in well W-2B, which is screened approximately 10 feet deeper than the other wells, indicates that the BETX constituents are not migrating downward in a soluble phase. If BETX constituents are { being left in the soil of older capillary fringe zones,it can be expected that these constituents will be amenable to vapor extraction. Furthermore, we would expect the vapor extraction to lower concentrations in future groundwater samples collected during times of higher water levels. The water elevation histories, borehole Iogs, and water-table gradient data indicate that the southeast portion of the ARCO station, which is the area of the original gasoline leak, is comprised of less transmissive material relative to the other areas. This causes an apparent mounding and steepening of the gradient over this portion of the Station. It also indicates that fluids may move slower in the area of the Ieak than in off-site areas. The gradient is gentle and therefore the groundwater flow direction is subject to change due "- to minor influences, such as increased recharge due to precipitation or irrigation or increased discharge due to pumping wells. Because of the shallow gradient and relatively low p.-mneability conditions,an extractior well with a low pumping rate can affect a large area. Based on Brown ` and Caldwell's short-term pumping test, it is estimated that a pumping rate of at least 1 gallon per minute can be sustained in well E-1. The capture zone resulting from this pumping will be --- -- - -' a-maximum-of-120-feet-wide-(,)n each-side of-E-1)-and have a stagnation-point approximately-----------------:------ 40 feet down gradient of E=1. "These numbers were calculated using the folluwing equations from _. 7avendel and Tsang (1986).