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Abstract--Yucca Mountain is being studied as a potential site in southern Nevada for an underground, high-level nuclear waste repository. A major consideration for selecting this site is the presence of abundant zeolites in Miocene ash-flow tufts underlying the region. Beneath Yucca Mountain four diagenetic mineral zones have been recognized that become progressively less hydrous with depth.Zone I, the shallowest zone, is 375-584 m thick in the central part of Yucca Mountain, but 170 m thick to the north. Zone I contains vitric tufts that consist of uualtered volcanic glass and minor smectite, opal, heulandite, and Ca-rich clinoptilolite. Zone II thins south to north from 700 to 480 m and is characterized by complete replacement of volcanic glass by clinoptilolite with and without mordenite, and by lesser amounts of opal, K-feldspar, quartz, and smectite. Zone III thins south to north from 400 to 98 m thick and consists of analcime, K-feldspar, quartz, and minor calcite and smectite. Heulandite occurs locally at the top of zone III in the eastern part of Yucca Mountain. Zone IV occurs in the deepest structural levels of the volcanic pile and is characterized by albite, K-feldspar, quartz, and minor calcite and smectite.Clinoptilolite and heulandites in zone I have uniform Ca-rich compositions (60-90 mole % Ca) and Si:AI ratios that are mainly between 4.0 and 4.6. In contrast, clinoptilolites deeper in the volcanic sequence have highly variable compositions that vary vertically and laterally. Deeper clinoptilolites in the eastern part of Yucca Mountain are calcic-potassic and tend to become more calcium-rich with depth. Clinoptilolites at equivalent stratigraphic levels on the western side of Yucca Mountain have sodic-potassic compositions and tend to become more sodium-rich with depth. Despite their differences in exchangeable cation compositions these two deeper compositional suites have similar Si:A1 ratios, generally between 4.4 and 5.0. Analcimes have nearly pure end-member compositions, typical of these minerals formed by diagenetic alteration of siliceous volcanic glass; however, K-feldspars are Si-rich compared to the ideal feldspar formula.Bulk-rock contents of Si, Na, K, Ca, and Mg of zeolitic tufts generally differ significantly from stratigraphically equivalent vitric tufts, suggesting that zeolite diagenesis took place in an open chemical system. Both the whole rock and the clinoptilolite are relatively rich in Ca and Mg in the eastern part of Yucca Mountain and rich in Na in the western part. The Ca-and Mg-rich compositions of the zeolitized tufts in the eastern part of Yucca Mountain may be due to cation exchange by the sorptive minerals with ground water partially derived from underlying Paleozoic carbonate aquifers.Diagenetic zones become thinner and occur at stratigraphically higher levels from south to north across Yucca Mountain, probably due to a higher geothermal gradient in the northern part of the area. The diagenetic zones were established when the geothermal gradient was greater than it ...

that includes the LANL and the adjacent communities of Los Alamos and White Rock, San Ildefonso Pueblo The Pajarito Plateau is an important source of abundant potable land west of the Rio Grande, and a northern outlying groundwater for Los Alamos National Laboratory (LANL) and the part of Bandelier National Monument (Fig. 3). Areas communities of Los Alamos and White Rock. Geologic investigations were undertaken as part of a plateau-wide hydrogeological investiga-of the southern Pajarito Plateau in Bandelier National tion to develop conceptual models of the groundwater system as a Monument are not discussed because there are no hyframework for numerical simulations of groundwater flow. The Pajadrogeologic data from wells in this area. The northern rito Plateau is located in the western part of the Españ ola basin where plateau is not discussed because hydrogeologic data for rocks of the Jemez and Cerros del Rio volcanic fields overlie and the Santa Clara Pueblo is not available to the public. interfinger with Neogene basin-fill sedimentary rocks. The vadose Groundwater drawn from beneath the plateau is the zone is about 200 m (600 ft) thick beneath mesas on the east side of primary source of municipal and industrial water used the plateau and more than 375 m (1245 ft) thick on the west side. by LANL and by adjacent communities. Water supply Groundwater occurs as shallow groundwater in canyon-floor alluvium, wells on the plateau are 600 to 950 m (2000-3110 ft) moderately deep groundwater perched in bedrock units of the vadose zone, and groundwater associated with the regional saturated zone.deep (Purtymun, 1995;Koch and Rogers, 2003), andThe most productive rocks of the regional aquifer occur in a westward-they tap the western-central part of the Españ ola basin thickening wedge of coarse-grained, Miocene and Pliocene volcaregional aquifer in Miocene and Pliocene sedimentary niclastic rocks derived from the Jemez volcanic field. Eastern aquifer and volcanic rocks. Although numerous wells penetrate rocks consist of fine-grained, somewhat less productive Miocene sedithe upper part of the regional saturated zone, none fully mentary deposits derived from highland sources to the east and north.penetrate the aquifer. Intermediate and mafic lavas interbedded with the Miocene and Plio-Water quality is typically good, but the effects of cene sedimentary deposits are components of the regional aquifer LANL operations can be detected in parts of the locally. The hydrogeology of the Pajarito Plateau is probably typical groundwater system. Locally, shallow and intermediate of groundwater systems along the margins of the Rio Grande rift where arid to semiarid, sediment-filled basins receive most of their perched groundwater systems of the vadose zone conrecharge from adjacent mountainous areas.

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