Field Studies for Estimating Uncertainties in Ground-Water Recharge Using Near-Continuous Peizometer Data (NUREG/CR-6729)

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Publication Information

Manuscript Completed: June 2001
Date Published: July 2001

Prepared by:
D. Timlin, J. Starr, USDA
R. Cady, T. Nicholson, NRC

U.S. Department of Agriculture
Agricultural Research Service
Beltsville Agricultural Research Center
Beltsville, MD 20705-2350

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 K6892

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Abstract

This study investigated uncertainties in ground-water recharge estimates using real-time, near-continuous piezometer data [six piezometers instrumented with automated pressure transducers at 10-minute measurement intervals]. Analytical and numerical methods were used to compare ground-water recharge estimates. These methods included: (1) time-series analyses of hydrographs, (2) steady-state estimates using meteorological data only, and (3) the PNNL water budget model. The time-series analysis methods included: (1) sum of positive changes in piezometric heads; (2) piezometric head changes corrected for hydrograph recession; and (3) variations in rainfall input. The density of the data allowed for several estimates of recession coefficients and specific yields for each piezometer location. Higher estimates of ground-water recharge were obtained using realtime, near-continuous piezometer data than estimates derived strictly from meteorological data. This observation was also true for comparison with the water-balance method (PNNL model). Taking into account errors introduced by estimating the recession coefficients, the sum of positive changes in head is the most appropriate approach for estimating recharge at this site. For all the time-series methods, the variation in specific yield contributes to the uncertainty of ground-water recharge estimates on the order of 10 to 100%. The spatial variability in ground-water recharge was also a large contributor to uncertainty. The real-time, near-continuous piezometer data showed a very rapid response (e.g. < 20 minutes) in the shallow water table to rainfall events over short time intervals, and verified the occurrence of significant episodic recharge. Uncertainty in characterizing site behavior can be reduced by utilizing a network of measuring devices to capture the spatial variability inherent in this dynamic process.