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Draft North & East Metro Groundwater Management Area Plan 3-3 <br /> In most circumstances limits on groundwater pumping will be defined by the protection of surface- <br /> water resources and dependent ecosystems or by preventing water use conflicts ratherthan by safe <br /> yield for water-table aquifers. <br /> Water-table well Deep well <br /> Land surface <br /> Water table <br /> J A 100%available head <br /> Water level in E <br /> confined aquifer <br /> Water-table aquifer <br /> B 50%threshold <br /> C 25%threshold <br /> Confining layer <br /> ToA o` er <br /> cteh rJsdrteD <br /> 4asit!"9 a,* <br /> Com'meo imtesian)aqui cr <br /> Figure 3-1 Safe yield thresholds for artesian (confined) aquifers <br /> The available head (A) is the distance between the top of the confined aquifer (D) and the water level (E) <br /> in the deep well.The 50-percent threshold (B) is halfway between these same points;the 25-percent <br /> threshold (C) is one-fourth of the way. Water levels in a confined aquifer must not stabilize below the <br /> 25-percent safe yield threshold. <br /> P52 <br />