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Draft Environmental Impact Report Bernal Property Phase II Specific Plan 1 . Framework for Analysis This section assesses the hydrologic and water quality environment of the Bernal Property Phase II Specific Plan area based on an inspection of current site conditions, published and unpublished hydrologic reports, and topographic and floodplain maps. This section also assesses potential impacts on groundwater resources associated with grading, erosion and sedimentation, and introduction of new sources of pollutants to runoff that could result from implementation of the Specific Plan and the Bernal Community Park Master Plan. 2. Setting The setting of the Plan area was described in the Bernal Property Phase I Specific Plan El R, and that setting discussion covered all the Phase II area. The following discussion is based on the Phase I EIR, supplemented by additional references and a recent site reconnaissance. The 318-acre Phase II Plan area drains into the Arroyo de la Laguna, which forms the western boundary of the Plan area. The floodplain (loo-year FEMA flood hazard zone) associated with Arroyo de la Laguna in the vicinity of the Plan area is generally confined to a band about 200 feet wide along the active channel. According to Zone 7, the Arroyo de la Laguna north of the Bernal Avenue bridge has been improved as an open channel with capacity to accommodate the 1oo-year flood flow of 17,000 cubic feet per second (cfs). No floods have been recorded which have exceeded the 1 OO-year flood. Improvements to the onsite drainage system were described in the Bernal Property Phase I Specific Plan EIR and most of these improvements have either been completed or are currently under construction. Two detention basins have been constructed (one in the central portion of the Plan area and the other in the southwest corner of the Plan area). These basins are reported to contain water for much of the yearP During a site inspection in October, 2005, the central basin did not appear to contain any water. Some water was observed in the northern portion of the southwestern basin during a site reconnaissance in October, 2005. In addition, the realignment and enhancement of the drainage channel along the boundary of the Eastern Area is under way. Based on regional studies, the depth to groundwater at the Plan area ranges from about 20 to 25 feet below ground surface (bgs).28 In preparation for development of the Phase I residential areas, geotechnical borings were completed at the Plan area in 2000. The logs for the borings indicate that in the vicinity of the detention basins, the underlying sediments consist of a heterogeneous mixture of silty clay, silty sand and clayey silt. In the vicinity of the central detention basin, the depth to groundwater ranges from 18 feet bgs to greater 27 Rasmussen, Wayne, President, Rasmussen Planning, personal communication with Bruce Abelli-Amen of BASELINE, October 20, 2005. 28 United States Geological Survey, Water Quality Conditions and an Evaluation of Ground.and Surface Water Sampling Programs in the livermore-Amador Valley, California, Water Resources Investigations Report 84- 4352, 1985. 86