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<br />("") <br />l"Y") <br />C'J The method of Bouwer and Rice (1976) produced conductivity values generally a factor or two lower <br />C\I than estimates produced using the method of Hvorslev (1951), The higher values estimaled from Hvorslev's <br />CJ technique probably are due to the assumption by Hvorslev of infinite vertical (upward) extent of the flow <br />(;:) system, which is not met when the well screen is immediately below the water table (Bouwer and Rice. <br />p. 427), Furthermore, the small screened interval assumed in the Hvorslev technique decreases Ihe effective <br />radius to the extent that vertical flow cannot be ignored. The estimates based on the method of Cooper and <br />others (1967) probably represent the upper limit of the range of acceptable horizontal hydraulic <br />conductivities. The values are larger than the estimates from the other methods because Cooper and others <br />assume, in their technique. that only horizontal flow occurs to the well screen. In other words. the length <br />of the screened interval represents the aquifer thickness. In reality, however, venical flow could be <br />significant and probably accounts for the larger estimated hydraulic conductivities produced using the <br />method of Cooper and others (1967). The method ofBouwer and Rice (1976) probably p(ovides reasonable <br />values; however, owing to the prevalence of clay-size particles (table I) and the use of bentonite to seal the <br />wells, perfect hydraulic connection between the bore hole and aquifer cannot be expected, even after well <br />development, as clays may clog, or partly clog. some of the screened openings in the casing. Clogging may <br />result in lower estimates of hydraulic conductivity, <br /> <br />Estimated horizontal hydraulic conductivities at cluster-well sites from the three methods described <br />above are listed in table 2. Even with the different assumptions inherent in the three methods, the estimated <br />hydraulic conductivities varied by a factor of only about six. Hydraulic conductivities estimated from <br />specific capacities (discharge rate of a well divided by measured drawdown of water level within the well) <br />in areas dominated by fine to coarse sand and gravel are more comparable to those calculated from the <br />method of Cooper and others (1967); in areas dominated by silt. they are more comparable to hydraulic <br />conductivities calculated from the method of Bouwer and Rice (1976). <br /> <br />Under optimal conditions in which the assumptions inherent in the development and application of <br />the methods are true, a vertical hydraulic conductivity can be estimated by comparing the method of <br />Hvorslev (1951). which assumes spherical, isotropic flow to a point (or small well screen relative to aquifer <br />thickness), to the method of Cooper and others (1967), which assumes only horizontal flow to the well <br />screen. This can be done using the relation given by Freeze and Cherry (1979, p. 177): <br /> <br />K = K'/K <br />. .' <br /> <br />(I) <br /> <br />where K = <br />. <br /> K = <br /> K. = <br /> <br />vertical hydraulic conductivity being estimated, in feet per day; <br /> <br />isotropic hydraulic conductivity calculated using the Hvorslev technique, in feet per day; <br />and <br /> <br />horizontal hydraulic conductivity calculated using the method of Cooper and others <br />(1967). in feet per day, <br /> <br />Vertical hydraulic conductivities estimated using eqn. I result in ratios of horizontal to vertical conductivity <br />(anisotropy) of about 10:1 (table 2), This ratio may reflect the depositional history of the Whitney area. <br />which is dominated by flood-plain deposits. This estimate. however, may be in error by as much as an <br />order of magnitude, because vertical differences in conductivity are difficult to quantify without aquifer tests <br />involving multiple wells screened at different depths, <br /> <br />-13- <br />