Synopsis of moisture monitoring by neutron probe in the unsaturated zone at Area G

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Moisture profiles from neutron probe data provide valuable information in site characterization and to supplement ground water monitoring efforts. The neutron probe precision error (reproducibility) is found to be about 0.2 vol% under in situ field conditions where the slope in moisture content with depth is varying slowly. This error is about 2 times larger near moisture spikes (e.g., at the vapor phase notch), due to the sensitivity of the probe response to vertical position errors on the order of 0.5 inches. Calibrations were performed to correct the downhole probe response to the volumetric moisture content determined on core samples. ... continued below

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51 p.

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Vold, E. December 31, 1997.

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Description

Moisture profiles from neutron probe data provide valuable information in site characterization and to supplement ground water monitoring efforts. The neutron probe precision error (reproducibility) is found to be about 0.2 vol% under in situ field conditions where the slope in moisture content with depth is varying slowly. This error is about 2 times larger near moisture spikes (e.g., at the vapor phase notch), due to the sensitivity of the probe response to vertical position errors on the order of 0.5 inches. Calibrations were performed to correct the downhole probe response to the volumetric moisture content determined on core samples. Calibration is sensitive to borehole diameter and casing type, requiring 3 separate calibration relations for the boreholes surveyed here. Power law fits were used for calibration in this study to assure moisture content results greater than zero. Findings in the boreholes reported here confirm the broad features seen previously in moisture profiles at Area G, a near-surface region with large moisture variability, a very dry region at greater depths, and a moisture spike at the vapor phase notch (VPN). This feature is located near the interface between the vitrified and vitrified stratigraphic units and near the base of the mesa. This report describes the in-field calibration methods used for the neutron moisture probe measurements and summarizes preliminary results of the monitoring program in the in-situ monitoring network at Area G. Reported results include three main areas: calibration studies, profiles from each of the vertical boreholes at Area G, and time-dependent variations in a select subset of boreholes. Results are reported here for the vertical borehole network. Results from the horizontal borehole network will be described when available.

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51 p.

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INIS; OSTI as DE98004337

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  • Other Information: PBD: 1997

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  • Other: DE98004337
  • Report No.: LA-UR--97-5186
  • Grant Number: W-7405-ENG-36
  • DOI: 10.2172/650175 | External Link
  • Office of Scientific & Technical Information Report Number: 650175
  • Archival Resource Key: ark:/67531/metadc702524

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  • December 31, 1997

Added to The UNT Digital Library

  • Sept. 12, 2015, 6:31 a.m.

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  • June 14, 2016, 6:12 p.m.

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Vold, E. Synopsis of moisture monitoring by neutron probe in the unsaturated zone at Area G, report, December 31, 1997; New Mexico. (digital.library.unt.edu/ark:/67531/metadc702524/: accessed June 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.