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<br />SPECIFIC-YIELD AND SPECIFIC- <br />RETENTION ESTIMATES FROM <br />GRAIN-SIZE DATA <br /> <br />Although specific yield and specific retention <br />can be detennined by laboratory analyses of undis- <br />turbed samples of aquifer material, this technique is <br />costly and slow because of the difficulty of recovering <br />undisturbed core samples and the complexity of <br />laboratory techniques used to analyze the samples. <br />Estimating specific yield and specific retention by use <br />of grain-size data has the potential to greatly decrease <br />the cost and time involved in detennining specific yield <br />and specific retention. The aquifer samples used in <br />grain-size analyses can be disturbed samples, thus <br />eliminating the need for core drilling. Disturbed <br />samples can be collected by wireIine sidewall coring or <br />from drill cuttings that have not been significantly <br />contaminated by drilling mud or other cuttings. Also, <br />grain-size analyses are relatively simple detennina- <br />tions and are less costly and time consuming than <br />laboratory detenninations of specific yield and specific <br />retention. <br /> <br /> <br />Most studies of specific yield and specific reten- <br />tion of aquifer materials were done between about 1920 <br />and 1960 (Johnson, 1967) and predated standardized <br />procedures for laboratory determination of specific <br />yield and specific retention. Many investigators (for <br />example, Meinzer, 1923; Eckis, 1934; Preuss and <br />Todd, 1963; Johnson, 1967) have reported general rela- <br />tions between specific yield, specific retention, and <br />grain-size characteristics of aquifer materials (fig. 2). <br />These relations are informative but generally do not <br />enable accurate estimation of specific yield or specific <br />retention from grain-size data. Historical attempts to <br />develop such quantitative techniques have had limited <br />success primarily because of the large variance in data <br />relating grain size to specific yield and specific reten- <br />tion. Some of the variance can be attributed to non- <br />standardized or antiquated laboratory procedures for <br />measuring specific yield and porosity, such as testing of <br />disturbed and repacked aquifer materials, use of col- <br />umn drainage techniques that cannot accurately mea- <br />sure specific yield of heterogeneous or fine-grained <br />materials, and incomplete saturation of samples. Cur- <br />rent technology and standardized procedures can <br />decrease such variance and enable better correlation of <br /> <br /> <br /> <br />90th PERCENTILE GRAIN DIAMETER (Dso)' IN MILLIMETERS <br /> <br />Modified from Eckis, 1934 <br /> <br />Figure 2. Relations among specifIC yield. specific retention, porosity, and grain size. <br /> <br />4 Techniques lor Estimating Specific Yield end Specific Retention from Graln-S1ze Data and Geophyaical Logs lrom <br />ClasOc Bedrock Aqullars <br />