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<br />Techniques for Estimating Specific Yield and <br />Specific Retention from Grain-Size Data and <br />Geophysical Logs from Clastic Bedrock Aquifers <br /> <br />ByS.G. Robson <br /> <br />Abstract <br /> <br />Specific yield and specific retention are <br />aquifer characteristics that are important in deter- <br />mining the volume of water in storage in an aqui- <br />fer. These characteristics can be determined by <br />laboratory analyses of undisturbed samples of <br />aquifer material. However, quicker, less costly <br />alternatives to these laboratory analyses can be <br />developed. This report presents techniques for <br />estimating specific yield and specific retention <br />based on grain-size analyses and OIl interpretation <br />of borehole geophysical logs. <br /> <br />Least-squares linear regression analysis of <br />specific yield and specific retention on grain-size <br />characteristics produced live regression equations <br />that can be used to estimate specific retention from <br />grain-size information. Specific yield can be cal- <br />culated from specific retention by use of porosity <br />data from geophysical logs. <br />Evaluation of various porosity logs indi- <br />cates that the density porosity log is well suited to <br />measuring porosity in aquifer materials. Effects of <br />errors in density porosity logs can be minimized <br />by calculation of mean porosity for borehole <br />intervals rather than relying on porosity values at <br />specific depths. <br /> <br />Effective-porosity logs and apparent grain- <br />density logs are produced by computer-assisted <br />well-log evaluation programs used by commercial <br />geophysical logging companies. Regression anal- <br />ysis of specific-yield data from core analyses on <br />effective porosity defined an equation useful for <br />estimating specific yield from effective porosity. <br />Regression analysis of specific yield divided by <br />porosity on apparent grain density produced <br />another equation for estimating specific yield. <br />Botltlog-interpretation techniques produce mean <br />specific-yield estim;ltes that are comparable to the <br />mean values obtained by laboratory analyses of <br />core samples. <br /> <br />INTRODUCTION <br /> <br />Increasing demands for potable water have <br />caused increased exploration aIld eva1uation of bedrock <br />aquifers as possible sources of water supply. The vol- <br />ume of water that can be potentially recovered :from a <br />deep aquifer can be difficult to detennine; yet, this vol- <br />ume is an important factor to be considered in planning <br />and in development of an aquifer. In some areas <br />(Colorado, for example), the rate of withdrawal from a <br />bedrock well is repIlated on the basis of the volume of <br />recoverable water in storage ill the aquifer under the <br />well owner's land. Such statutes were enacted to pre- <br />vent overutilization of a finite ground-water resource <br />and can limit pumping in areM where aquifers have <br />smaller volumes of recoverable water in storage. <br />Specific yield is the aquifer characteristic of <br />principal importance in calculating the volume of <br />recoverable water in storage in unconfined and con- <br />fined aquifers. In an unconfined aquifer, the water table <br />is within the porous material of the aquifer. Water is <br />released from storage as the water table declines, and <br />water drains by gravity from the pore spaces. Specific <br />yield is a measure of this volume of water released <br />from storage as the water table declines. In a confined <br />aquifer, the water level is above an impermeable con- <br />fining layer, aIld a water-level decline initially releases <br />water from storage by the elastic change in volume of <br />the aquifer (as measured by storage coefficient). How- <br />ever. once the water level declines below the base of <br />the confining layer, the aquifer becomes unconfined <br />and water is again released by gravity drainage (as <br />measured by specific yield). Specific yield typically is <br />about 1,000 times larger than storage coefficient for <br />most aquifers so the volume of water released from <br />elastic storage usuany is negligibly sma1l in compari- <br />son to the volume released from gravity drainage. <br />The Castle Pines Metropolitan District provides <br />water from bedrock aquifers of the Denver basin to res- <br />idential cOmnlunities and golf courses in an area about <br />20 mi south of Denver, Colo. In October 1987, me Dis- <br />trict completed a core drilling project that recovered <br />about 3, 100ft of drill COre from the bedrock formations <br />at Castle Pines. The U.S. Geological Survey and the <br />Colorado State University worldng in cooperation with <br /> <br />Abstract 1 <br />