Roles and Significance of Water Conducting Features for Transport Models in Performance Assessment

摘要

The term water conducting features refers to zones of high hydraulic conductivity. In the context of waste disposal, it is further implied that they are narrow so that chances of sampling them are low. Yet, they may carry significant amounts of water. Moreover, their relatively small volumetric water content causes solutes to travel fast through them. We argue, first, that water-conducting features are a rather common feature of natural media. Hence, the "anomalous" effects they cause are so frequent that they can hardly be considered anomalous, a statement we sustain on the basis of several examples. The fact that they have become a source of concern in recent years, reflects more the increased level of testing and monitoring than any intrinsic property of low permeability media (at most, these may display a higher variance). Our second point, however, is that accurate simulations of solute transport require a realistic accounting for water conducting features. We outline methods to do so and show examples to illustrate these methods, which demand a significant volume of data. This leads to our final point. Since detailed accounting of WCF's will not be possible in actual performance assessments, efforts should be directed towards typification, so as to identify the essential effects of WCF's on solute transport through different types of rocks. Field evidence suggests that, although individual WCF's may be difficult to characterize, their effects are quite predictable.