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Well ER-EC-11 was drilled for the U.S. Department of Energy, National Nuclear Security Administration Nevada Site Office, in support of the Nevada Environmental Restoration Project at the Nevada National Security Site (formerly Nevada Test Site), Nye County, Nevada. The well was drilled in September and October 2009 as part of the Pahute Mesa Phase II drilling program. A main objective was to investigate radionuclide migration down-gradient from Well Cluster ER-20-5 and Well ER-20-7 and across the northern Timber Mountain moat structural zone into the area referred to as the Bench, between Pahute Mesa and the Timber Mountain caldera complex. A secondary purpose of the well was to provide detailed hydrogeologic information for the shallow- to intermediate-depth Tertiary volcanic section in the Bench area. This well also provided detailed hydrogeologic information in the Tertiary volcanic section to reduce uncertainties within the Pahute Mesa-Oasis Valley hydrostratigraphic framework model (Bechtel Nevada, 2002). The main 52.1-centimeter hole was drilled to a depth of 507.5 meters and then opened to a diameter of 66.0 centimeters. It was cased with 50.8-centimeter casing to 504.9 meters. The hole diameter was then decreased to 47.0 centimeters, and drilling continued to a total depth of 979.3 meters. It was then cased with 34.0-centimeter casing set at 965.5 meters. The hole diameter was then decreased to 31.1 centimeters and the borehole was drilled to a total depth of 1,264.3 meters. The completion casing string, set to the depth of 1,262.5 meters, consists of 19.4-centimeter stainless-steel casing hanging from 19.4-centimeter carbon-steel casing. The stainless-steel casing has two slotted intervals open to the Tiva Canyon and Topopah Spring aquifers. Four piezometer strings were installed in Well ER-EC-11. A string of carbon-steel 6.0-centimeter tubing with one slotted interval was inserted outside the 50.8-centimeter casing, within the 66.0-centimeter borehole for access to the Timber Mountain aquifer, and landed at 475.3 meters. A second string of 6.0-centimeter tubing with one slotted interval was inserted outside the 34.0-centimeter casing, within the 47.0-centimeter borehole for access to the Benham aquifer, and landed at 911.7 meters. A third piezometer string consists of 7.3-centimeter stainless-steel tubing that hangs from 6.0-centimeter carbon-steel tubing via a crossover sub. This string was landed at 1,029.5 meters to monitor the Tiva Canyon aquifer. The deepest string of 7.3-centimeter tubing was landed at 1,247.8 meters to monitor the Topopah Spring aquifer. Data collected during and shortly after hole construction include composite drill cuttings samples collected every 3.0 meters, 67 percussion gun and rotary sidewall core samples, various geophysical logs, fluid samples (for groundwater chemistry analysis and tritium measurements), and water-level measurements. The well penetrated 1,264.3 meters of Tertiary volcanic rock, including three saturated welded-tuff aquifers and one saturated lava-flow aquifer. A water level was measured in the Timber Mountain aquifer at 449.6 meters, during open-hole geophysical logging on September 20, 2009. The fluid level measured after the total depth was reached and the upper aquifer was cased off was 450.0 meters when measured in the open borehole on October 17, 2009. Measurements on samples taken from the undeveloped well indicated that tritium levels averaging approximately 12,430 picocuries per liter (less than Safe Drinking Water Act levels) were encountered within the Benham aquifer. Tritium was below the minimum detectable activity concentration for samples collected from the Tiva Canyon aquifer and the Topopah Spring aquifer

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