United States Nuclear Regulatory Commission - Protecting People and the Environment

Results of Field Studies at the Maricopa Environmental Monitoring Site, Arizona (NUREG/CR-5694)

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Publication Information

Manuscript Completed: May 1999
Date Published:
June 1999

Prepared by:
M.H. Young*, P.J. Wierenga, A.W. Warrick, L.L. Hofmann
S.A. Musil, M. Yao, C.J. Mai, Z. Zou
Department of Soil, Water and Environmental Science
University of Arizona
Tucson, Arizona 85721

B.R. Scanlon (Subcontractor)
Bureau of Economic Geology
University of Texas at Austin
Austin, Texas 78713

T.J. Nicholson, NRC Project Manager

* Currently at School of Civil and Environmental Engineering
Georgia Institute of Technology
Atlanta, GA 30332-0512

Prepared for:
Division of Risk Analysis and Applications
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code W6151

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Abstract

The purpose of this study was to evaluate issues related to alternative monitoring strategies for sites containing low level radioactive wastes. The study consisted of a theoretical evaluation of monitoring strategies and field studies. This NUREG reports on the field activities and the results of the field experiments.

A field site, located at the Maricopa Agricultural Center (Maricopa, AZ) was designed for conducting controlled water flow and solute transport studies, and for testing the strengths and weaknesses of four monitoring strategies designated as 1) Monitoring Trench, 2) Monitoring Island, 3) Borehole Monitoring, and 4) Geophysical Monitoring. Field instrumentation was extensive, and designed to support alternative monitoring strategies. Two experiments were conducted at the site covering the time frame from Spring 1997 through Summer 1998. During Experiment 1, water was applied at an average flux of 1.85 cm d-1 to the 50 m by 50 m field plot for 24 days, with bromide tracer added for the first 15 days. The water application period was followed by a redistribution period of 69 days. During Experiment 2, water was applied at an average rate of 1.97 cm d-1 for 33 days with a redistribution period of 177 days. Field experiments ended officially on July 1, 1998.

Water movement across the plot was spatially variable during Experiment 1, due mostly to variability in the initial water content, and thus, the soil's hydraulic properties. The results of intrusive and non-intrusive instruments showed that 1) water movement in the western portion of the site was faster than the eastern portion; 2) a zone of more rapid water flow was observed near the northern and central areas of the plot, as confirmed using several types of instruments; and 3) the variability of water movement, as measured using a neutron probe, decreased with increasing depth. During Experiment 2, spatial variability of water movement was significantly reduced in surface soils (< 1.5 m), from a CV ≈ 41.8 % to 4.7 % between Experiments 1 and 2, respectively. The reduced variability was observed because the flux-controlled water application led to more uniform hydraulic property fields, and thus, more uniform water movement. Most of the monitoring systems performed well during the field experiments. Though some data were lost (or considered unreliable) due to electrical problems with the AC power supply and corrosion of electrical connections, an extensive data set was compiled and found useful for comparing monitoring strategies.

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