The following sections briefly describe the hydrogeological setting by cor­rection action units (CAUs) of geographical areas used for underground testing on the NNSS (Fig. 26.3).

26.3.1 Frenchman Flat CAU

Ten underground detonations were conducted in Frenchman Flat (Figs 26.3 and 26.4), a strike-slip pull-apart basin formed at the northeastern termina­tion of the Rock Valley fault (Bechtel, Nevada (BN), 2005). Seven tests were detonated in the northern part of the basin in the lower part of the unsatu­rated zone in alluvium and distal facies of the volcanic rocks originating from eruptive centers in the volcanic highland to the northwest. Three tests were conducted in alluvium in central Frenchman Flat; two of the tests are in the unsaturated zone and one test was detonated below the water table (NNES, 2010a). Local directions of groundwater flow in the Frenchman Flat basin are difficult to establish because of low hydrologic gradients in the basin. Flow is inferred to be predominantly to the southeast driven by higher groundwater levels northwest of Frenchman Flat across the north­east trending, right-slip Cane Spring fault (NNES, 2010a) (Fig. 26.4). Groundwater velocities are very low (1 meter per year or less) down gradi­ent of nuclear tests conducted in alluvium (high porosity alluvial aquifer) in central and northern Frenchman Flat, but may be higher down gradient of two tests where flow is in fractured volcanic aquifers (welded tuff and basalt lava; SNJV, 2006; NNES, 2010a). Gradients in the alluvial and vol­canic aquifers are downward but flow from these sections into the underly­ing carbonate aquifer is limited across a basal confining unit of zeolitized volcanic rocks.