The MADE Site and Limitations of the Hydrologic Approach

Abstract:

The hydrologic approach to transport problems involves measuring hydraulic conductivity (K) using borehole-based methods such as pumping, slug and borehole flowmeter tests and then using the K data to parameterize ADE or non-ADE models of transport. Lack of fit in modeling forward-only plume movement is often attributed to limitations of the model, spurring conclusions about suitability of the ADE or of various non-ADE alternatives. In MADE studies, reliability of the measured K data has apparently seldom been questioned. In contrast, the geologic approach to transport problems involves mapping or simulating spatial distributions of geologic facies which are then used as a template for mapping the 3D K field based on not only the measured K values, but also on literature or core-based estimates of K’s of the fines, that are typically too low to be measured by the borehole methods. The geologic approach commonly leads to K fields with substantially higher variance in which, in the absence of spatially persistent geological unconformities, the upper 15 to 20 percent of the K distribution percolates. The high variance, together with the good connectivity of the high-K facies leads to plume characteristics very similar to those observed at MADE when modeled with the ADE. Recent work at MADE has indeed reflected both a growth in the estimated ln-K variance and, when using the geological approach and the ADE, better representation of the MADE plume characteristics. Importantly, the geologically based ADE model has the advantage that it can handle changing boundary conditions (e.g., plume evolution followed by remediation) and is more scalable. The MADE work will hopefully lead to work on another grand challenge that should eclipse the challenges of plume scale phenomena – the modeling of regional scale groundwater quality sustainability of systems subjected to 1,000’s of contaminant sources or to non-point sources.