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<br />Mineralogy and Particle Size
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<br />X-ray diffraction and particle-size analyses were made on sediment-core samples collected during drilling
<br />of cluster-well sites to provide information about the mineral assemblages present in the alluvial deposits and
<br />permeability of the shallow aquifer, The information was used to understand the possible origin of the
<br />minerals, and to relate the chemical character of the ground water to the minerals, Particle-size analyses were
<br />made to determine the spatial heterogeneities in the sediments, both vertically and area\ly. Particle-size data
<br />were used to estimate variations in permeability and, combined with the mineralogy, to determine the
<br />depositional history of the alluvial sediments.
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<br />X-ray diffraction data indicate that salt content, particularly gypsum, varies markedly b?th with depth'and
<br />from site to site (Emme and Prudic, 1991). In general, near,surface samples haVe' a consideraply
<br />greater percentage of gypsum than do samples collected from greater depths, The greater percentage probably
<br />results from precipitation of salts in the shallow capillary fringe during summer months, due to
<br />evapotranspiration. A comprehensive description of the mineralogy at selected cluster-well sites is presented
<br />by Emme and Prudic (1991),
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<br />Analysis of particle-size data collected at cluster-well sites (table I). using the Wentwoith Classification
<br />System, indicates a wide range of sizes in most of the samples collected, Sizes range from medium gravel
<br />(coarse pebbles) to clay, with medium to fine sand and silt dominating. This size distribution is typical for
<br />alluvial-apron and flood-plain deposits (Morris and Johnson, 1967). Because of the diverse grain-size
<br />distribution. even qualitative relations between grain size and permeability are difficult to detennine. However,
<br />certain trends in sediment grain size have been identified, both with depth and from well site to well site. For
<br />example, the coarser sediments are in the middle to deeper parts of the aquifer. whereas finer particles dominate
<br />the top and deepest parts of the aquifer (table 1).
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<br />Thickness
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<br />An abrupt change in lithology occurs at a depth of approximately 30 ft in the Whitney area. Drillers'
<br />logs indiCate a sharp increase in drilling difficulty beneath the shallow alluvial deposiIs (at about 30 ft) at well
<br />sites WG035. WG044. WG13I, and WG062 (fig. 4), Sediments at this depth were more elastic and difficult
<br />to penetrate with a hollow-stem auger. A sediment core from the depth interval of 32-33 ft at cluster-well site
<br />WG062 contained a large percentage of clay (table I) and probably represented the top of the Muddy Creek
<br />Formation, a relatively thick, low-permeability unit in the lower part of the Valley (Bohannon, 1984; Smith,
<br />1985; D. Art Tuma, U.S. Bureau of Reclamation, oral commun,. 1986), Locally, the Muddy Creek Formation
<br />contains sand and gravel lenses, and may, as a result, be difficult to visually discern from the overlying alluvial
<br />deposiIs (Bohannon, 1984; Patrick A, Glancy, U.S. Geological Survey, oral commun;, 1987). In the Whitney
<br />area, however, evidence suggests that the Muddy Creek Formation is dominated by fine-grained sedimenIs and
<br />probably is relatively impermeable compared to the overlying alluvial deposiIs. On the basis of available
<br />information, the top of the formation probably ranges from 25 to 35 ft below land surface and thus an average
<br />thickness of 30 ft is indicated for the alluvial deposits.
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<br />Hydraulic Conductivity
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<br />Probably the most important property that influences the behavior of ground-water flow is hydraulic
<br />conductivity. If a porous medium is isotropic and the fluid is homogeneous, the hydraulic conductivity of the
<br />medium is the volume of water at the existing kinematic viscosity that will move in unit time under a unit
<br />hydraulic gradient through a unit area measured at right angles to the direction of flow (Lohman, 1972, p. 4).
<br />Estimates of hydraulic conductivity can vary widely depending on the technique used. Four general methods
<br />are commonly used to estimate hydraulic conductivity: (I) direct laboratory measurements including falling-
<br />head and constant-head permeameter tesIs on sediment core samples, (2) indirect laboratory methods based on
<br />particle-size or pore-structure distribution and consolidation tests on sediment cores. (3) slug tesIs at wells, and
<br />(4) aquifer tests at wells. The slug-test method, which provides an estimate of hydraulic conductivity near the
<br />well screen, was used in the Whitney study area.
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