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The distance that the water table has declined will affect the negative <br /> pressure imparted to the remaining water Ier molecules in the aquifer and, <br /> thus, influence specific yield. Specific yield will increase as greater <br /> negative pressures are imported to the remaining water molecules, which <br /> coincides with further lowering of the water table. Specific yield values <br /> will increase as the water table is lowered until the forces nolding the <br /> remaining water molecules to the aquifer particles are equal to the force <br /> of gravity. <br /> One method for determining the weighted average of specific yield for a , <br /> well is to conduct an aquifer test under unconfined or water table <br /> conditions. However, many of the aquifers are under artesian pressure and <br /> are, therefore, confined aquifers. An aquifer test under those conditions <br /> would not provide a value for specific yield. For confined aquifers, the <br /> value for specific yield must be determined in some other fashion. <br /> A method,,ford- determining , tiie--specific._ yield-- nf�for---coined-,,aqui J fers.- s- to <br /> collect core ­-s'aa-us,le-s­ from formation outcrops or from a well and conduct <br /> laboratory tests. Tests must be done on undisturbed core samples. <br /> Laboratory-,procedures for testing of coarse and mediUM textured materials <br /> have been standardized and are reported--i n-_the--7 ASTM Standards,-,. which are <br /> incorporated by reference in Rule 8.D.2) . It is ' i mportant that <br /> standardized procedures -be, used in sample preparation and testing so that <br /> values of specific yield will be representative of what would occur in <br /> nature, and so that there will be confidence in the obtained values. <br /> Since the values of specific yield will vary as a function of the decline <br /> in water tables, it is important that the laboratory test proceoures test <br /> the sample - for a ran,9e­­in­---nega�t­ive --pressures.' - ",-Rule 8.D.2) requires <br /> testing at five negative pressures in order -to provide data that could be <br /> used to construct the moisture content-tension curve in anticipation of <br /> water level declines of up to 450 feet. These values were also selected <br /> knowing the general properties of sandstones, clays and shales so that <br /> sufficient data would be available to construct the curve. <br /> The appropriate specific yield for a thickness of aquifer immediately <br /> above the water table would be less than for the same aquifer where the <br /> water table was several hundred feet below the zone. The maximum value of <br /> specific yield will result when the water table has reached its 'lowest <br /> level . Rule 8.C.21 specifies that the appropriate value of the specific <br /> yield will be either the one corresponding to the negative pressure which <br /> represents the maximum expected water level declines in one hundred years <br /> or the one associated with the specific laboratory test with the greatest <br /> negative pressure, whichever is less. This benefits the applicant <br /> because, in reality, most o-If the aquifer that will be dewat-ered will have <br /> a lower negative pressure and, thus, have a lower specific yield value. <br /> _10- <br />