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0016;~4 Inner Valley Alluvium As described previously, the channel and flood-plain deposits in the inner valley constitute the hydraulic connection between the Rio Grande surface-water system and the remainder of the Santa Fe Group aquifer system. Therefore, the lithologic composition of this alluvium and adjacent deposits is a major factor controlling the volume of water that can move between the surface-water system and the primary production intervals of wells in the Albuquerque area, which are completed in the Santa Fe Group. The hydraulic characteristics of these alluvial deposits can, in part, be estimated on the basis of their lithologic composition, Basin- and valley-fill aquifers are heterogeneous (hydraulic conductivity varies spacially within the aquifer) and anisotropic (hydraulic conductivity varies depending on direction). As a result, hydraulic conductivity is dependent on the size of a part of the aquifer under consideration. For example, channel deposits in the inner valley alluvium generally contain sand and gravel. A localized part of the aquifer containing only sand and gravel would have relatively large values of hydraulic conductivity. Values of horizontal hydraulic conductivity for these coarse deposits in the inner valley alluvium have been estimated to be about 90 to 350 feet per day by the Bureau of Reclamation (1994b; based on an auger-hole method described by Maasland and Haskew, 1957), and Willis (1993) reported the hydraulic conductivity of gravelly coarse sand to be 65 feet per day. The wide range in possible values (65 to 350 feet per day) for these coarse deposits can be explained by the degree of sorting--larger values of hydraulic conductivity are associated with more uniform grain sizes in the coarser deposits, Flood-plain deposits containing silt and clay have relatively small values of hydraulic conductivity: Willis (1993) listed the conductivity of silty clay to be 0.2 foot per day. A larger part of the aquifer is likely to contain layers or lenses of deposits with different grain sizes and varying degrees of sorting. In such a system, hydraulic conductivity parallel to the layers or lenses (typically horizontal) is in general equal to the weighted arithmetic average of the hydraulic conductivities of each of the deposits. Therefore, when considering a part of an aquifer large enough to be representative of the aquifer's heterogeneity, a middle range of values, rather than the extreme values of horizontal hydraulic conductivity is more representative. Hydraulic conductivity perpendicular to the layers (typically vertical), however, is controlled primarily by the layers having small values of hydraulic conductivity. Individual grains tend to be deposited in an alluvial system flat side down, which also contributes to reduced hydraulic conductivity in the vertical direction compared to the horizontal direction. Average vertical hydraulic conductivity in vertically anisotropic aquifer material, such as the inner valley alluvium, can often be about two or more orders of magnitude less than the average horizontal hydraulic conductivity (Freeze and Cherry, 1979, p. 34). As discussed above, layers of low-conductivity deposits control the vertical hydraulic conductivity and, therefore, the vertical movement of water in the inner valley alluvium. Flood- plain deposits consist primarily of silt and clay deposited by water moving at lower velocity than water in the active channel. Layers of silty clay exist within much of the inner valley. Many of these silty-clay layers are discontinuous and may be as much as about 15 feet thick locally (Anderholrn and Bullard, 1987). An extensive silty-clay layer at a depth of about 20 feet below land surface has been mapped based on about 470 borehole logs at a Super Fund site in the Albuquerque South Valley (Jacobs Engineering Group, 1995). However, the extent of these clay layers throughout the inner valley is not well known. The continuity of silty-clay layers can significantly effect the vertical hydraulic conductivity. A part of the aquifer with a continuous silty-clay layer will have a lower effective vertical hydraulic conductivity than a part with a discontinuous silty-clay layer of the same thickness and texture. Therefore, knowledge of the nature and distribution of the silts and clays in the inner valley alluvium is essential information to help provide a better understanding of the ability of the alluvium to transmit water vertically between the Santa Fe Group and the Rio Grande. 18