Geohydrology of the eastern part of Pahute Mesa, Nevada Test Site, Nye County, Nevada

Pahute Mesa Well Development and Testing Analyses for Wells ER-20-8 and ER-20-4, Nevada National Security Site, Nye County, Nevada, Revision 0

“…This is consistent with higher recharge in the eastern portion of the Mesa compared with the western, and downward gradients in the eastern area (Blankennagel and Weir, 1973). It therefore appears that there may be some long-term climatic influences on the stable isotope data.…”

“…The zeolitized bedded tuff had the consistently lowest hydraulic conductivity (relative specific capacity), followed by the welded tuff, and then rhyolitic lava, which had considerable scatter in values. The presence of aquitards between aquifers would conceptually restrict vertical communication resulting in vertical head changes through the geologic section, a feature noted by Blankennagel and Weir (1973). However, one of the striking features of this area is the presence of faults and other large structures.…”

“…This results in the interleaved sequence of aquifers and aquitards seen in cross section (BN, 2002;Fenelon et al, 2010). Blankennagel and Weir (1973) summarized results of 297 hydraulic, mostly drill stem (slug) tests, performed on various volcanic rocks on Pahute Mesa. The zeolitized bedded tuff had the consistently lowest hydraulic conductivity (relative specific capacity), followed by the welded tuff, and then rhyolitic lava, which had considerable scatter in values.…”

“…Prothro et al (2009) also studied faults at the NNSS and observed the following: (1) faults often form discrete zones; (2) more recently active faults probably form permeable fault zones where they cut stronger rocks such as welded tuff and lava; (3) faults that intersect TCU form zones of enhanced permeability, relative to TCU protolith, although of less absolute permeability than those in welded tuff and lava; (4) fault cores were observed to be relatively thin, and thus are unlikely to form a complete and continuous barrier everywhere along the fault; and (5) any enhanced fault-zone permeability will be generally controlled by fractures that will be subparallel to the strike of the fault resulting in anisotropic permeability. Blankennagel and Weir (1973) suggested that well yields could be enhanced in rocks otherwise unfavorable for pumping near large structures because of fault damage zone enhanced permeability.…”

“…Fracture density may increase with proximity to faults; however, the hydrologic properties, if any, of faults themselves in the area are not well known. Limited data suggest that the full spectrum of hydraulic properties, from barrier to conduit, may be possible (Blankennagel and Weir, 1973;Faunt, 1997). In the area of interest, it may be that the major influence of faults is to juxtapose formations creating complex flow paths, as generally suggested by Faunt (1997).…”