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4.6 WATER RESOURCES Mitigation 4.6-4 The elevation of the lowest finished floor for each homesite should be a minimum of one foot above the 1010-year water surface elevation. This could be accomplished by fill, or by placing the homes on piers or piles with grade beams supporting the finished floor above grade. Impact 4.6-5 Development of the project site will reduce the available flood water storage in the Mokelumne River floodplain, contributing to a cumulative increase in the water surface elevation during large floods. The impact of the project alone is not significant. Mitiggtion 4.6-5 The proposed project should minimize the extent of fill in the 100-year floodplain to reduce the cumulative impact of reduced flood storage in the Mokelumne River floodplain. No levees should be constructed which exclude large portions of the site from the 100-yearfloodplain. This means the project should not construct earth embankments or levees that exclude 100-yearflood waters from spilling into the site. Groundwater Nitrate Loading Impact 4.6-6 The percolation of water from on-site septic systems lake seepage and the irrigation of the golf course and agricultural areas will contribute to a rise in the nitrate concentration of local groundwater,but not to the extent of violating or threatening the violation of applicable water quality standards. This is not a significant impact. The potential for increasing the nitrate concentration in groundwater exists in agricultural areas and wherever on-site systems or other land-based wastewater disposal methods are used. Domestic sewage contains high concentrations of nitrogen (in ammonia and organic forms), as does nitrogen-based fertilizer, which converts rapidly and nearly completely to nitrate in passage through the soil. Whether or not the groundwater quality is adversely affected for water supply uses is dependent on the concentration of wastewater and fertilizer loading in a given area. An anlysis of the groundwater from the northern well at the project site revealed relatively low levels of nitrates, both as N and as NO,. 4.6-20