Comparing Monitoring Strategies at the Maricopa Environmental Monitoring Site, Arizona (NUREG/CR-5698)

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Publication Information

Manuscript Completed: June 1999
Date Published: July 1999

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

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 document is to discuss the alternative monitoring strategies used during field experiments at the Maricopa Environmental Monitoring site, Maricopa, AZ. The strategies used at Maricopa were selected so that they could potentially be incorporated into monitoring programs at Low Level Waste disposal facilities. Although the evaluation of the strategies was mostly qualitative in nature, they were supported by data collected during two, field-scale infiltration experiments. The results of the field experiments with respect to water movement and tracer migration were presented in a companion NUREG report (NUREG/CR-5694). The emphasis was on monitoring in the vadose zone.

This document describes and compares four monitoring strategies that were implemented at the Maricopa site. They were designated as Monitoring Trenches, Monitoring Islands, Borehole Monitoring, and Geophysical Monitoring. The strengths and weaknesses of each strategy were described with respect to installation, maintenance and replacement of monitoring systems and instruments. Monitoring Trenches and Islands provide excellent opportunities for specific placement of monitoring instruments, with the possibility of direct observation of undisturbed soil material. Borehole Monitoring is more flexible with respect to depth of installation than the other three strategies. Maintenance of monitoring instruments in the Monitoring Trenches and Borehole Monitoring is not always possible because instruments are often backfilled in place. Some techniques used to support the Geophysical Monitoring strategy (e.g., electroresistive borehole tomography) suffer from the same infeasibility. Instrument maintenance in the Monitoring Islands is easier if the island is not backfilled. Limiting access to undisturbed soil, especially with respect to the Monitoring Trenches and Borehole Monitoring strategies, will also make instrument replacement more difficult. Portability of surface geophysical instruments used during the infiltration experiments removes several restrictions on maintenance and replacement.

The document also presents the concept of primary performance measures (e.g., water content, water tension and solute concentration), each of which directly influences water movement and contaminant migration from disposal sites. The majority of commercially available monitoring instruments measure secondary performance measures, which are soil water conditions that are converted to primary measures using calibration curves. Unfortunately, each instrument has different operational limitations and sensitivities, which depend on the soil water environment. Therefore, it is recommended to use multiple instruments whose data convert to the same primary performance measures. This should improve the confidence that changes in soil water conditions are real and not affected by the monitoring systems themselves.