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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:creator>Tracie R. Jackson</dc:creator>
  <dc:date>2021</dc:date>
  <dc:description>&lt;p class="p1"&gt;The HANDLEY nuclear test was detonated at about 2,700 feet below the water table on March 26, 1970, in Pahute Mesa, south-central Nevada. Measured tritium concentrations in boreholes &lt;i&gt;ER-20-12 &lt;/i&gt;and &lt;i&gt;PM-3 &lt;/i&gt;indicate that a shallow tritium plume has migrated more than 1 mile (mi) downgradient from the HANDLEY test within a semi-perched aquifer and deeper tritium plumes have migrated 4.5 miles (mi) within underlying regional aquifers. Boreholes &lt;i&gt;ER-20-12 &lt;/i&gt;and &lt;i&gt;PM-3 &lt;/i&gt;are in an area of moderate-to-low transmissivity, but observation of tritium moving 4.5 mi within 40 years of the detonation indicates that high-transmissivity intervals exist. However, the location of these permeable pathways is unknown.&lt;/p&gt;&lt;p class="p1"&gt;This report integrates geologic, hydrologic, and tritium data to infer the location of permeable pathways near and downgradient from the HANDLEY test. Numerical groundwater-flow and tritium-transport models were developed to estimate hydraulic and transport properties between the HANDLEY test and boreholes &lt;i&gt;ER-20-12 &lt;/i&gt;and &lt;i&gt;PM-3&lt;/i&gt;. Recharge, hydraulic-conductivity, specific-yield, specific-storage, and effective-porosity distributions were estimated with the numerical models by fitting simulated water-level altitudes, vertical-head differences, aquifer-test transmissivities, tritium concentrations, and drawdowns in wells &lt;i&gt;PM-3-1 &lt;/i&gt;and &lt;i&gt;PM-3-2 &lt;/i&gt;to measured equivalents. Drawdowns were estimated in wells &lt;i&gt;PM-3-1 &lt;/i&gt;and &lt;i&gt;PM-3-2 &lt;/i&gt;in response to groundwater withdrawals during the drilling of borehole &lt;i&gt;ER-20-12&lt;/i&gt;. A modified hydrostratigraphic framework model (mHFM) was developed that incorporates hydrostratigraphic units (HSUs) from the Pahute Mesa–Oasis Valley hydrostratigraphic framework model (PMOV HFM). HSUs in the mHFM were modified from the PMOV HFM by grouping HSUs that, conceptually, are hydraulically similar and splitting HSUs based on water-level, aquifer-test, and tritium data.&lt;/p&gt;&lt;p class="p1"&gt;Shallow and deeper tritium plumes have migrated to borehole &lt;i&gt;ER-20-12 &lt;/i&gt;from the HANDLEY test. The shallow plume migrated from the HANDLEY test through the Timber Mountain welded tuff aquifer, whereas the deeper plumes moved through the Belted Range aquifer (BRA) and modified pre-Belted Range lava flow aquifer (mPBRLFA). Simulated tritium concentrations indicate that the leading edges of tritium plumes reached borehole &lt;i&gt;ER-20-12 &lt;/i&gt;by 1990. From 1970 to 2020, the simulated tritium load mostly occurs between borehole &lt;i&gt;ER-20-12 &lt;/i&gt;and the HANDLEY test.&lt;/p&gt;&lt;p class="p2"&gt;An unmapped permeable feature was simulated between borehole &lt;i&gt;ER-20-12 &lt;/i&gt;and the downgradient Ribbon Cliff structural zone. This permeable feature hydraulically connects the BRA and mPBRLFA with the Tiva Canyon aquifer (TCA). The TCA is the most transmissive unit in the study area. Simulated tritium from the deeper plumes moves through the permeable feature downgradient from borehole &lt;i&gt;ER-20-12 &lt;/i&gt;and then migrates toward well &lt;i&gt;PM-3-1 &lt;/i&gt;through the TCA. The leading edge of the deeper simulated tritium plumes reaches well &lt;i&gt;PM-3-1 &lt;/i&gt;by 2010.&lt;/p&gt;&lt;p class="p2"&gt;The mHFM and PMOV HFM do not include a permeable HSU at the water table near borehole &lt;i&gt;PM-3&lt;/i&gt;, which is necessary for numerical flow and transport models to match measured water levels, transmissivities, and tritium concentrations in well &lt;i&gt;PM-3-2&lt;/i&gt;. Consistently higher measured tritium concentrations in shallow well &lt;i&gt;PM-3-2&lt;/i&gt;, compared to deeper well &lt;i&gt;PM-3-1&lt;/i&gt;, and a downward vertical gradient between these wells indicate that a permeable feature exists near the water table that causes faster tritium migration toward the shallow well. Reevaluation of the PMOV HFM and geologic investigations, such as drilling another well, are needed to more precisely understand the shallow permeable pathway from the Handley test to well &lt;i&gt;PM-3-2&lt;/i&gt;.&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.3133/sir20215032</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>U.S. Geological Survey</dc:publisher>
  <dc:title>Permeable groundwater pathways and tritium migration patterns from the HANDLEY underground nuclear test, Pahute Mesa, Nevada</dc:title>
  <dc:type>reports</dc:type>
</oai_dc:dc>