PNNL

Abstract

Model-based decision making is commonly used in performance assessments to assure water resource protection for both human health and the environment for hundreds of years into the future. To make decisions regarding aquifer protection against potential contamination, a conceptual site model (CSM) describing the hydrodynamic behavior needs to account for subsurface heterogeneities in sufficient detail. When site-specific data are sparse, larger-scale geologic descriptions are adopted with the consequence of losing small-scale features that can control contaminant transport. In this study, a multiple lines of evidence approach is used to construct CSMs based on an evaluation of several types of data, including geologic logs, borehole moisture content and concentration data, geophysical spectral gamma logging data, and groundwater concentration data. The resulting CSMs represent a synthesis of what is known about flow and transport processes at the site-scale and maintains consistency with knowledge that has been accumulated at the regional scale. Through a process of extensive data analyses, a systematic approach is described to create an evidence base that supports the evaluation and development of CSMs. Numerical models are then used to evaluate the impact that smaller-scale heterogeneities have on contaminant transport for a performance assessment on waste tank closure at the Hanford Site in southeastern Washington State. Together, the field data and the numerical experiments suggest that the small-scale features have a relatively minor impact on transport for the site under study as evaluated by peak concentrations and arrival times for potential releases resulting from waste tank closure. Overall, results of this study underscore the value of using multiple lines of evidence in CSM development to provide consistency in interpreting site characteristics relative to the quantity and quality of the available data.