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• Discharge of treated groundwater to the publicly owned treatment works f (POTW) • Groundwater monitoring - Conceptual Well Field Design To evaluate the potential costs of groundwater capture by pumping, a capture well field consisting of two wells located along the western edge of the Disco site along North Filbert Street was evaluated. With the groundwater model, various pumping well schemes were evaluated to-better understand the capture zone that could be established for a range of pumping rates. With relatively low yield in the A Zone soils, small pumping rates produced large capture zones. After several iterations, it became apparent that on-site pumping could capture groundwater flow from well beyond the identified extent of contamination. The model forecasts that two wells pumping 0.5 gpm screened in the shallow A Zone would capture much more than the entire target area, regardless of groundwater flow direction. Figure 4-1 presents the flow lines and groundwater contours expected from a 2-well field with pumping rates of 0.5 gpm each. In Figure 4-1, the target area is the known extent of contamination, or"the plume." The predicted drawdown for a well pumping at 0.5 gpm continuously Is 3 to 4 feet. For pumping rates greater than 1 or 2 gpm, a significant portion of the flow will be drawn from the B Zone, below. Contamination has only been observed in the A Zone, so it would not be productive to pump at rates greater than 0.5 to 1.0 gpm per well. The capture zone of a pair of 0.5 gpm wells, as shown in Figure 4-1, is far greater than the extent of the gasoline plume, so placement of wells near the centroid of the plume will produce an optimum flow pattern for capture of the plume. The initial concentration expected ffrom the Figure 4-1 pumping wells should be similar, initially, to the concentration of MW- DIS-2, with TPH(g) and BTEX (by species addition) concentrations as high as 10 and 8.8 mg/L, respectively. Groundwater pumping and treatment are expected to initially remove 0.11 pounds of combined BTEX per day at this concentration. Again, removal will decline asymptotically with concentration over time. Treatment Technology Selection. j A commonly applied treatment technology for high-strength gasoline-contaminated I wastewater is air stripping. Air stripping produces an off-gas stream that is commonly treated by either vapor phase GAC adsorption or incineration by internal combustion engine or catalytic oxidizer. Under the City of Stockton pretreatment limits, wastewater with a I combined BTEX concentration of less than 4.57 mg/L is acceptable at the local POTW. For wastewater with a lower gasoline concentration, liquid-phase GAC is more economical than stripping because of energy savings. A lower gasoline concentration will not be reached for 6 to 10 years. Most groundwater treatment equipment typically has a service life of . I . f SF010030983.DOC 4-6