CORRESPONDENCE_2007 (1/07 - 6/07)

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1 111111, MIT 111!! 11111;1111111111 111;1111 11111 111 111� The County proposes an evaluation monitoring program that includes the installation of two permanent groundwater monitoring wells and groundwater sampling at nine temporary borings. The purpose of the permanent ground water monitoring wells is to assess the extent of groundwater impact along portions of the landfill perimeter that do not now contain groundwater monitoring wells. These wells will also better define the direction of groundwater flow in the shallow aquifer. Permanent groundwater monitoring well MW -8 is proposed along the perimeter road of the landfill adjacent to perimeter migration monitoring well GW -2 (Figure 2). We are certain that the shallow aquifer is present at that location because the lower probe of this migration monitoring well is submerged. Permanent groundwater monitoring well MW -9 is proposed along the southern property boundary in the general vicinity of perimeter migration monitoring well GW -6 (Figure 2). We are not certain how far the shallow aquifer extends west from groundwater monitoring well MW -1. Previous attempts to define the shallow aquifer in that area was unsuccessful. The location of this well will be determined by the field geologist based on the results of the HydroPunch temporary borings. Nine temporary soil borings (Figure 3 — tentative locations) will be drilled for the purpose of collecting vadose zone soil gas samples at three depths each and for the collection of groundwater (if present). Figure 3 also shows possible areas for temporary monitoring points (see arrows). Temporary soil borings HP -1 through HP -3 are about 200 feet away from the landfill. These samples will be collected using HydroPunch technology or equivalent. The purpose of the soil gas samples is to determine the lateral and vertical extent of VOC impact within the vadose zone to the north and east of the landfill. The purpose of the groundwater samples is to determine the lateral extent of groundwater impact downgradient from well MW -5 in the shallow aquifer, if any. The City of Tracy trash burn dump was located on one or more of the parcels of Area No. 1 (east if Interstate 580). Experience at Shaw indicates that the presence of burned refuse above an aquifer does not necessarily impact the aquifer below. However, boring through the burned material could allow burned material to enter the aquifer by falling down the bore, potentially Corral Hollow EMP WPlan Rev2Mar 22-07.DOC 2-1 March 20, 2007