Submitted to: Nureg Series
Publication Type: Government Publication
Publication Acceptance Date: 7/20/2001
Publication Date: 12/15/2001
Citation: Timlin, D.J., Starr, J.L., Cady, R., Nicholson, T. 2001. Field studies for estimating uncertainties in groundwater recharge using near-continuous piezometer data. Nureg Series.NUREG/CR-6729, U.S. Nuclear Regulatory Commission, Washington, DC. Interpretive Summary: In order to assess the safety of decommissioning waste disposal facilities, ground-water recharge calculations need to be performed as part of site characterization and facility performance analysis. Ground-water recharge is often estimated from changes in water table height or from weather data such as rainfall and evaporation. Use of these kinds of data may lead to uncertainties in estimation of ground-water recharge. This study investigated the use of frequent, near- continuous (10 minute interval) measurements of water table height to estimate ground-water recharge. These measurements often showed a very rapid response (e.g. less than 20 minutes) in the shallow water table to rainfall events over short time intervals, and verified the occurrence of significant episodic recharge. This study found that the use of frequent measurements of water table height can reduce uncertainty in ground-water recharge estimates as compared to methods using meteorological data or infrequent (daily or weekly) measurements of water table elevation. The results of this research are useful to contractors licensed with the Nuclear Regulatory Commission and others involved in ground-water measurements.
Technical 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, including: (1) time-series analyses of hydrographs, (2) steady-state meteorological data only, and (3) the water-balance budget model (PNNL 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 real-time, near-continuous piezometer data than estimates derived strictly from meteorological data. This observation was also true for comparison with the 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. The real-time, near-continuous piezometer data showed a very rapid response (e.g. less than 20 minutes) in the shallow water table to rainfall events over short time intervals, and verified the occurrence of significant episodic recharge.