Submitted to: Nureg Series
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 18, 2003
Publication Date: December 13, 2003
Citation: Timlin, D.J., Starr, J.L., Cady, R., Nicholson, T. 2003. Comparing ground-water recharge estimates using advanced monitoring techniques and models. Nureg Series. NUREG/CR-6836, U.S. Nuclear Regulatory Commission, Washington, DC.
Interpretive Summary: Knowledge of ground-water recharge is important for many reasons. These include estimation of potential for ground-water contamination by chemicals and fertilizers applied to the soil surface. Furthermore, 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. These methods may give reasonable averages over many years but can be highly inaccurate for within year estimates. We compared ground-water recharge estimates from a number of different measurement methods. These methods included water table height, infrequent (weekly) and frequently (10 minutes) measured soil water contents, simulation modeling, and use of weather data (rainfall and evaporation). Actual ground-water recharge for comparison purposes was measured in a vegetated outdoor soil plot measuring 14 x 20 x 3 m (35 x 50 x 7.5 ft) and closed to drainage. We found that uncertainty in knowledge of evaporation from soil and plant surfaces led to the greatest errors in estimation of ground-water recharge. Simulation modeling worked well if there were good estimates of evaporation from plant and soil surfaces. Measurements of soil water content underestimated ground-water recharge since they did could not accurately measure infiltrating water when the soil was very wet.
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.