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<br />C 16~~3 <br /> <br />As described previously, the most penneablepart of the aquifer system is composed of <br />fluvial sediments containing a high percentage of sand and gravel deposited by an ancestral Rio <br />Grande Uithofacies Ib of Hawley and Haase, 1992). The extent of these deposits is of particular <br />significance in the hydrologic interaction of the aquifer system and the surface-water system <br />because the deposits provide a preferential path for movement of water in the Santa Fe Group. <br />The extentcifthese deposits within Albuquerque is reasonably well known and is shown by <br />Hanebergand'Hawley (in press). However, the extent of these deposits north and south of <br />Albuquerque is not well known. Kernodle and others (l995,p. 37,104) assumed that the axial- <br />channel deposits were unsaturated just north of the Bernalillo-Sandoval County line, but <br />extended southward in the saturated zone to the southern end of the basin. <br /> <br />Faults within or bounding the Santa Fe Group are partial barriers to ground-water flow. <br />Juxtaposed lithologic units of different hydralilicconductivities reduce the hydraulic <br />conductivity across faults. In addition, many faults in the Albuquerque Basin are probably <br />cemented to some degree/further decreasing hydraulic conductivity across the faults. Kernodle <br />and others (1995, fig. 4 and p. 37) reduced the area-weighted mean horizontalhydralilic <br />conductivity to one-fifth for model cells that contained an interpreted location ofa major fault. <br />Haneberg and Hawley (in press) have revised the interpreted locations of major faults in the <br />Albuquerque area. Because many ciHhese faults are buried and do not have surface exposures <br />over most of theidengths, actuallocations can only be inferred. Additional evidence of the <br />location of these faults and the extent cif these and other faults beyond the Albuquerque area <br />would be useful for additional understanding 'iftheSarttaFe Group, <br /> <br />Being able to estimate the .effects of fa Its on ground-water flow involves more than <br />. determining the,positionof a fault. Without ad itional information, the hydraulic characteristics <br />ofa particularfaultcanorily be inferred onothe basis of observation or estimation of the degree of <br />cementation or estimated on the :basisof degree of offset of differing lithcilogicunits across the <br />fault. Where steady-state hydraulic, gradients on. either side of a fault are available, the relative <br />transmissivitiesof the fault .zone and the aquifer on either side ofthefault can.becakulated <br />(Haneberg,1995a). Knowledgeoffaultczonethickness andoneciHhe transmissivitiesis needed <br />to calClilate'the other transmissivities, This technique. is not llPplicable for areas where ground- <br />water development has significantly altered hydraulic gradients andpredevelopment gradients <br />are not available. Where ground.water development is. significant, hydrologic characteristics of <br />a fault maybe estimated on the basis of observation of aquifer. .stresses (particularly ground- <br />waterwithdrawals)and the resulting changes in hydraulic head in different parts of the aquifer <br />system. This can be done only if observation wells are available and the aquiferStressisofa <br />great enough magnitude forthe:partial barrier to ground-water flow, in this case a fault, to be <br />significant. Forthis.reason, faults and otherpartial.barriers to .ground-water flow such as <br />changes in lithology, and others yet unknown, wilLbecome more Significant in the study of <br />hydraulic relations between the aquifer system and the .surface.water system as the effects of <br />ground_water withdrawals in the Albuquerque' Basin continue to increase and expand. <br /> <br />As discussed above, hydraulic conductivity may be estimated based on the lithology of the <br />basin-fill material. Sophoc1eousandothers (1995, p. 585~588) found that aquifer heterogeneity <br />was one of the most significant factors contributing to overestimation of stream depletion based <br />on the assumption that the aquifer is homogeneous (hydraulic conductivity does not vary <br />spacially) and isdtropic(hydralilic conductivity is equal in all directions). Therefore, <br />determining values of horizontal and vertical hydraulic conductivity is essential and needs to be <br />calculated by other methods to provide a basis for extrapolating or estimating values based on <br />lithology. <br /> <br />17 <br /> <br />