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Section 5.0 Engineering Analyses and Recommendations �. 5.1 Potential Soil and Groundwater Contamination We selected the environmental laboratory tests based on the compounds documented as being released on site. These test indicate only the presence of non-fecal coliform bacteria. Based on these results and the absence of any visual or odor indication of other contamination during our field investigation,we judge that the risk of encountering other compounds not tested for during the investigation is low. The soil stockpiled during our field investigation can be disposed of at a Class III landfill. The liquids in the 55-gallon drums can be disposed at a water recycling facility or, more conveniently, discharged to the sewage system for treatment at the City-operated facility. ;6- Groundwater collected during construction should be disinfected prior to treatment and discharge,or discharged into the sanitary wet well for treatment at the City L wastewater treatment facility. 5.2 Groundwater Control AP. Care must be exercised when relying on a single in situ falling head permeability test to predict groundwater inflows or design a dewatering system. The permeability measured by the test(23.7 feet/day or 8.4 x 10-3 cm/sec) does fall in the range of well- sorted sand to well-sorted gravel (Fetter, 1994),which is representative of the soils encountered in the borings between about 20 and 35 feet bgs. However, the actual permeability likely varies considerably,both vertically and laterally. The data does confirm that dewatering of an open excavation is not likely to be practical. The high groundwater table and relatively permeable soils near the base of the pump station suggest that dewatering would cause settlements that could deform and further damage the pump station,the wet pit,and the existing pipes. In addition, given the close proximity to the adjacent Calaveras River,the water-bearing sandy layer is likely hydraulically connected to the river,which could lead to large dewatering volumes. 5.3 Construction Alternatives We considered two basic alternatives for construction of the new wall. The first is to .. construct the new wall adjacent to the existing wall using a form of in situ construction. The second is to use temporary excavation support in combination with conventional formed concrete construction of the new wall. These alternatives are discussed below. 5 CDM Camp Dresser&McKee inc. 5-1 O;M=28248-GEO-74-Repo"=kside Pump Station.doc