Well Completion Report for Well ER-20-11, Corrective Action Units 101 and 102: Central and Western Pahute Mesa

Abstract

Well ER-20-11 was constructed to further investigate the nature and extent of radionuclidecontaminated groundwater encountered in two nearby UGTA wells, to help define hydraulic and transport parameters for the contaminated Benham aquifer, and to provide data for the UGTA hydrostratigraphic framework model.The 44.5-centimeter (cm) surface hole was drilled to a depth of 520.0 meters (m) and cased with 34.0-cm casing to 511.5 m. The hole diameter was then decreased to 31.1 cm, and the borehole was drilled to a total depth of 915.6 m. The hole was completed to allow access for hydrologic testing and sampling in the target aquifer, which is a lava-flow aquifer known as the Benham aquifer.The completion casing string, set to the depth of 904.3 m, consists of a string of 6⅝-inch (in.) stainless-steel casing hanging from a string of 7⅝-in. carbon-steel casing. The stainless-steel casing has one slotted interval at 796.3 to 903.6 m. One piezometer string was installed, which consists of 2⅞-in. stainless-steel tubing that hangs from 2⅜-in. carbon-steel tubing via a crossover sub. This string was landed at 903.8 m and is slotted in the interval 795.3 to 903.1 m.Data collected during and shortly after hole construction include composite drill cuttings samples collected every 3.0 m, various geophysical logs, fluid samples (for groundwater chemistry analysis and tritium measurements), and water-level measurements. The well penetrated 915.6 m of Tertiary volcanic rock, including one saturated lava flow aquifer.Measurements on samples taken from the undeveloped well indicated elevated tritium levels within the Benham aquifer. The maximum tritium level measured with field equipment was 146,131 picocuries per liter from a sample obtained at the depth of 912.0 m. The fluid level was measured in the piezometer string at a depth of 504.5 m on September 26, 2012.All Fluid Management Plan (FMP) requirements for Well ER-20-11 were met. Analysis of monitoring samples and FMP confirmatory samples indicated that fluids generated during drilling at Well ER-20-11 met the FMP criteria for discharge to an unlined sump or designated infiltration area.Well development, hydrologic testing, and sampling will be conducted at a later date. The Central and Western Pahute Mesa CAUs and the associated well drilling program are part of the UGTA Activity at the NNSS. Two goals of the UGTA Activity are to evaluate the nature and extent of contamination in groundwater due to underground nuclear testing and to establish a long-term groundwater monitoring network. As part of the UGTA Activity, scientists are developing computer models to forecast groundwater flow and contaminant migration within and near the NNSS. To build and test these models, it is necessary to collect geologic, geophysical, and hydrologic data from new and existing wells to define groundwater quality, migration pathways, and migration rates. Data from these wells will allow for more reliable modeling of groundwater flow and radionuclide migration in the region. Some wells may be use...