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ATC Associates Inc. <br /> 1117 'one Palm Avenue, Suite 201 <br /> Modesto, California 95351 <br /> 209-579-2221 <br /> ASS O C I ATE S I N C . fax: 209-579-2225 <br /> rr <br /> locking, watertight well plugs will be installed on each well and a watertight wellhead labeled <br /> "monitoring well" will be set in concrete over each well. <br /> Two monitoring wells will be installed off-site to the west across Holly Drive. One well will be <br /> installed to approximately 46 feet bgs to evaluate potential off-site groundwater impact of Zone <br /> .r B, and the other well will be installed adjacent to this well at a depth of approximately 75 feet <br /> bgs to evaluate potential off-site groundwater impact of Zone C. The off-site Zone C well will be <br /> constructed as previously described. The Zone B monitoring well will be installed inside <br /> conductor casing extending to approximately 30 feet bgs in an effort to minimize potential <br /> vertical contaminant migration from shallower groundwater. The Zone B well will be constructed <br /> of 2-inch inside diameter, Schedule 40 PVC casing and five feet of 0.010-inch slot screen PVC. <br /> A sand pack will be placed in the annulus of the well extending from the bottom of the well <br /> screen to approximately two feet above the top of the screen. Approximately two feet of <br /> bentonite will be placed on top of the sand pack and hydrated with potable water. The remaining <br /> annulus will be completed to surface with cement grout. To protect the integrity of the well, a <br /> locking, watertight well plug will be installed on the well and a watertight wellhead labeled <br /> "monitoring well" will be set in concrete over the well. The proposed monitoring well locations <br /> `r are shown on Figure 11. Refer to Attachment 1 which depicts typical groundwater Zone B and <br /> Zone C monitoring well construction diagrams. <br /> V <br /> The off-site Zone B well is proposed as an efficiency measure and to minimize inconvenience to <br /> the off-site property owner by reducing the number of potential drilling mobilizations. No Zone <br /> B wells are proposed at this time for the site, since existing data indicate that the underlying Zone <br /> C is impacted and the objective of this phase of work is to evaluate the vertical extent of <br /> contamination. The three proposed Zone C wells should provide sufficient preliminary data <br /> .. regarding the lateral extent of Zone C impacted groundwater. Additional Zone B wells can be <br /> installed at the site in the future if warranted. <br /> Soil samples will be collected from each boring in an attempt to define the vertical extent of the <br /> contaminants of concern. Continuous sampling will be conducted from approximately 20 to 30 <br /> feet bgs in the Zone B monitoring well borehole. Soil sampling and lithological data associated <br /> with the proposed off-site Zone A well will be assumed to be applicable to the off-site Zone B <br /> well, as these two wells are proposed to be installed adjacent to each other. Selected soil samples <br /> will be submitted for laboratory analyses. One of the samples submitted for laboratory analyses <br /> will be from approximately 30 feet bgs within the silty clay unit that lies directly above the Zone <br /> B water unit. The silty clay unit may serve as an aquitard between the Zone A and Zone B <br /> groundwater units. The soil sample(s) collected for laboratory analyses from this silty clay unit <br /> will aid in determining if this unit is water bearing or dry and attempt to define the vertical extent <br /> of adsorbed phase hydrocarbon contamination above the Zone B water unit. <br /> bw <br /> 8 ATC Associates Inc. <br /> r <br /> S:\Environmental\25847.0037\Work Plan additional vert-hor delineationTH2.doc <br /> 600 <br />