Comparing Ground-Water Recharge Estimates Using Advanced Monitoring Techniques and Models (NUREG/CR-6836)

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Publication Information

Manuscript Completed: July 2003
Date Published: September 2003

Prepared by:
D. Timlin, J. Starr
Agricultural Research Service
U.S. Department of Agriculture
Beltsville Agricultural Research Center
Beltsville, Maryland 20705-2350

R. Cady, T.J. Nicholson
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

T.J. Nicholson, NRC Project Manager

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

NRC Job Code Y6363

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Abstract

Risk due to contaminant release and transport, as estimated by many multimedia environmental models, is highly sensitive to infiltration and ground-water recharge. Previous ARS-NRC studies developed methodologies, and showed the value of high-frequency monitoring of unsaturated zone water contents and piezometric levels to estimate infiltration and ground-water recharge. This study tested those methodologies through comparison of estimated ground-water recharge in a closed system using a highly monitored lysimeter (i.e., 14 by 20 by 3 meters). Specifically, near-continuous water content, water-table elevation, and meteorological data were collected to estimate infiltration and ground-water recharge and their attendant uncertainties. These highly-detailed monitoring data were evaluated to capture individual recharge-event characteristics (i.e., infiltration, drainage and evapotranspiration) and to estimate hydraulic parameters. Advanced monitoring techniques and models were used to compare hierarchical levels of information for assessing sensitivities and identifying uncertainty. The study included numerical simulations of ground-water recharge using the HYDRUS-2D code and the PNNL Water Balance model. The variation in input parameters and resulting recharge estimates derived from the different methods provide a framework for assessing uncertainty. Advanced monitoring instruments proved valuable in providing (1) an understanding of the soil water dynamics; (2) input for estimating hydraulic parameters for the various models, and (3) a realistic database for evaluating the modeling results. Comparison of results indicated that there was considerable variability of soil water dynamics in the near surface.