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Impact DW-6: Potential Increase in the 100-Year Floodplain Elevation As a Result of New Construction Within the Floodplain �f As discussed above, all areas planned for development northeast of the runways, with the r exception of the area in the vicinity of the old abandoned runways, are currently located outside the 100-year floodplain (Figure 5-1). The areas between the old abandoned runways are subject to flooding of up to I foot in a 100-year storm(Brandley pers. comm.). The Master Plan states that all development planned within the 100-year floodplain will be constructed so that the grade elevation of all buildings is above the 100-year floodplain elevation. Because this new construction will encroach on the 100-year floodplain,the potential exists for the floodplain elevation to increase. The �r degree to which this could occur cannot yet be determined because the specific location and size of z construction has not been determined at this time. This impact is considered significant because of the potential for construction within the floodplain to increase the elevation of the floodplain. ` Implementation of the following mitigation measure would reduce this impact to a less-than significant-level. �r Mitigation Measure DW-4: Evaluate the Effect of New Construction on the 100-Year Floodplain Elevation and Implement Measures to Ensure That No Substantial Floodplain Elevation Increase Occurs. As part of the Special Purpose Plan, the Department of Aviation shall conduct a detailed floodplain analysis to determine the effect of new construction within the 100-year floodplain on the 100-year floodplain elevation. In the event that proposed construction would result in a substantial increase in the maximum floodplain water-surface elevation, measures to reduce the anticipated increase in the floodplain water-surface elevation and to prevent new construction from causing the water level to encroach into the freeboard space required by FEMA regulations shall be developed E. and implemented. The Department of Public Works shall determine the threshold for a substantial increase in the floodplain elevation. Potential measures to be implemented include limiting the i amount of new construction within the floodplain or limiting fill within the floodplain. The floodplain analysis and resulting measures to avoid elevation of the maximum floodplain water- surface shall be reviewed and approved by the Department of Public Works before grading permits . are issued. Impact DW-7: Potential Increase in Contaminated Groundwater Migration and Salinity Intrusion into the Groundwater Aquifer 1. Increased migration of contaminated groundwater and saltwater intrusion resulting from the proposed project could occur ifsubstantial additional groundwater pumping were to be required 'T Stockton Metropolitan Airport Project Chapter 5. Drainage and Water Quality Draft Environmental Impact Report 5-11 March 20, 1998 Y r' �a