HYDROGEOLOGIC ANALYSIS OF THE SATURATED-ZONE GROUNDWATER SYSTEM, UNDER YUCCA MOUNTAIN, NEVADA

The configuration of the southward-sloping water table under Yucca Mou ntain is dominated by an abrupt decline of 300 m over a distance of le ss than 2 km. This northeast-striking zone of large hydraulic gradient (of 0.15 or more) separates an area of moderate gradient (of about 0. 015) to the north from an area of very small gradient (0.0001) to the south. The position of the large gradient does not correlate well with any evident geologic feature in the upper 0.5 km of the mountain, but we suggest that buried geologic features are present that can explain all the geohydrologic observations. The three areas of differing hydr aulic gradient under Yucca Mountain are parts of hydrogeologic domains that extend more than 70 km to the northeast. On a regional basis, th e moderate and very small gradients generally correspond to areas unde rlain by exceptionally thick Tertiary volcanic sections and a highly t ransmissive Paleozoic carbonate aquifer, respectively. The regional la rge gradient and water-table decline are spatially associated with a c ontact in the Paleozoic rocks between clastic rocks and carbonates. Th is contact marks a large abrupt drop in the effective base of the hydr ologic system because it is the upgradient boundary of the deep carbon ate aquifer, which has a thickness of 5 km. An aeromagnetic high follo ws the regional-scale domain of large gradient under northern Yucca Mo untain from outcrops of a magnetite-bearing clastic confining unit to the east, indicating that the regional correlation of the steep water- table decline with the upgradient boundary of the deep carbonate aquif er may extend to Yucca Mountain. Five additional features may be relat ed to an explanation for the large hydraulic gradient: (1) anomalously low heat flow has been measured deep in the volcanic section south of the water-table decline, suggesting underflow of cool water in the de ep carbonate aquifer; (2) the lower tuff sequence, of 0.5-1 km in thic kness, which underlies most of Yucca Mountain, is largely replaced in the volcanic section by lavas in the area of the large gradient; (3) a n analysis of the hydrogeology of the tuff section under Yucca Mountai n indicates that transmissivity in the tuffs increases to the south; ( 4) a northeast-trending gravity low is present immediately south of th e water-table decline; (5) units in the lower part of the volcanic sec tion are 50-100% thicker in the area of the gravity low than to the no rth and south. The abrupt stratigraphic thickening into the area of th e gravity low indicates that the low represents a buried graben with i ts northern bounding fault centered beneath the abrupt water-table dec line. These geologic features of the zone of large gradient under Yucc a Mountain suggest two possible hydrogeologic models. First, the north ern bounding fault of the buried graben may provide a highly permeable pathway (a drain) through the brittle lavas in the lower part of the volcanic section under northern Yucca Mountain. The drain would allow flow from the tuff aquifer north of the decline to be captured by the deep carbonate aquifer, resulting in the heat-flow low, the abrupt wat er-table decline, and the transition to a very small hydraulic gradien t. Alternatively, the northern bounding fault of the buried graben may be the effective northern limit of the tuff aquifer under Yucca Mount ain because of the permeability in the tuffs north of the fault may ha ve been diminished by hydrothermal alteration. In this second model, t he large gradient marks the point where the small southward flow of wa ter through the altered volcanic rocks to the north abruptly drops int o the tuff aquifer. In either case, heads in the tuff aquifer in the a rea of very small gradient may be regulated partly by upward flow from the deep carbonate aquifer. This upward flow under southern Yucca Mou ntain is indicated by linear thermal highs along fault zones, by groun d-water isotopic data suggesting inmixing of waters from the deep carb onate aquifer into the tuff aquifer, and by the upward hydraulic gradi ent found in a drill hole that penetrates the deep carbonate aquifer u nder southern Yucca Mountain.