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<br />OC1626 <br /> <br />information on the aquifer response to stresses and the vertical hydraulic conductivity is <br />essential for quantifying the river / aquifer hydraulic interaction. Suggested general locations for <br />additional observation wells are shown in figure 5. These locations were selected because the <br />observation wells would be in areas between major production wells, and the hydraulic-head <br />measurements would represent aquifer responses to stresses in the system rather than be <br />overwhelmed by the withdrawal from'nearby wells. As the area of influence from ground-water <br />withdrawal in the Albuquerque area expands, so will the need for additional observation wells. <br />Areas north and south of Albuquerque on the east side of the Rio Grande have experienced <br />declines in hydraulic heads from the cone of depression in east Albuquerque propagating north <br />and south (Thorn and others, 1993, figs. 30, 33; Kernodle and others, 1995, figs. 26, 27). <br /> <br />Aquifer Storage <br /> <br />Knowing the storage characteristics of the aquifer is important to better understand <br />surface-water and ground-water interactions. All water withdrawn by wells must initially corne <br />from aquifer storage. For the withdrawal to influence the surface-water system, the resulting <br />cone of depression must propagate to the ground-water / surface-water interface, causing a <br />reduction in hydraulic head. Only then can part of the depletion of aquifer storage be <br />replenished by induced recharge from, or decreased discharge to, the surface-water system. <br />Specific yield and specific storage characterize the storage properties of an aquifer. <br /> <br />Specific yield (dimensionless) characterizes the proportion of the volume of water added to <br />or taken from storage relative to the total volume of aquifer filled ,or dewatered by gravity at the <br />water table. Specific yield generally averages between about 0.1 and 0.25 for the types of <br />materials composing the Santa Fe Group aquifer system, although values vary depending on <br />type of material (clay, silt, sand, or gravel) and the degree of particle-size sorting (Johnson, 1967). <br />Average values of specific yield in the aquifer system can be estimated on the basis of <br />distribution of these material types. Kernodle and others (1995, p. 110) assumed specific yield to <br />be 0.15 in their ground-water-flow model. No aquifer tests have been done in the basin from <br />which values of specific yield can be calculated. <br /> <br />Below the water table, the volume of water added to or taken from storage relative to a <br />change in hydraulic head is dependent on expansion or contraction of the water a.nd the <br />compressibility of the aquifer material. This is characterized by the specific storage (dimensions <br />are per unit of length), which is the volume of water added to or taken from storage in a volume <br />of aquifer material relative to a unit change in hydraulic head. In an unconfined aquifer, the <br />specific storage component of aquifer storage is negligible in comparison to the specific yield <br /> <br />component (about 10-6 per foot of aquifer thickness compared to about 0.15). However, it is <br />necessary to consider specific storage in evaluating a transient three-dimensional flow system <br />(Lohman and others, 1972), such as the Santa Fe Group aquifer system. <br /> <br />As described above, specific storage is dependent on expansion or contraction of the water <br />and the compressibility of the aquifer material. The expansion or contraction of water is elastic-- <br />that is, the portion of water taken from storage due to expansion of water as hydraulic head <br />declines, may be restored with a subsequent equivalent increase in hydraulic head. There are <br />two components to the compressibility of the aquifer material, an elastic component and an <br />inelastic component. As water is withdrawn from storage, the aquifer will react elastically if <br />hydraulic head is not lowered beyond the point that the aquifer has been previously stressed-- <br />that is, the water taken from storage can be restored with a subsequent increase in hydraulic <br />head. If hydraulic head is decreased beyond the point that the aquifer has been previously <br />stressed, the aquifer will react inelastically and permanent compaction will occur (Leake and <br />Prudic, 1988). The portion of water released by inelastic compaction cannot be restored to the <br />aquifer with an equivalent increase in hydraulic head; it is a one-time withdrawal of water from <br />storage, and the storage characteristics of the aquifer are permanently changed. Although all <br />types of basin-fill sediments (clay, silt, sand, and gravel) can respond inelastically, inelastic <br />response is most significant in clays, <br /> <br />20 <br />