A REVIEW OF THE SCALE PROBLEM AND APPLICATIONS OF STOCHASTIC METHODS TO DETERMINE GROUNDWATER TRAVEL TIME AND PATH

摘要

The groundwater travel time along the fastest path of likely radionuclideudtransport is a regulatory criterion used to assess the hydrogeologic quality of a high -udlevel radioactive waste repository. Hydrologists and engineers are limited in theirudability to define with confidence the fastest path, owing to the heterogeneous natureudof geologic materials. Field measurements of hydraulic properties such as in test orudobservation wells, are inherently averages of properties at scales smaller than theudscale of the field measurement. As a result of averaging, subscale information is lostudand there is uncertainty in defining the fastest trajectory of groundwater. This scaleudproblem is explained through a review of the continuum and REV concepts inudgroundwater hydrology. The application of hydrodynamic dispersion concepts isudrecommended as a means of incorporating the effect of subscale heterogeneity on theudfastest groundwater travel time.udSources of uncertainties in predicting groundwater travel time are discussedudin the report. The uncertainties are mainly attributed to the heterogeneous nature ofudgeologic formations. The heterogeneity of geologic materials can, however, beudcharacterized quantitatively using geostatistical methods. Important statisticaludparameters include mean and variance. as well as the spatial correlation structures ofudthe hydrologic properties within the hydrogeologic system. These parameters may heudobtained from limited data base. Stochastic methods, reviewed and explained in thisudreport, can take advantage of the geostatistical characterization to predict large -scaleudgroundwater flow and solute transport. Several examples from recent scientificudliterature are provided to illustrate the application of stochastic methods to theudgroundwater travel time analysis.udStochastic methods in subsurface hydrology have only recently beenudevaluated under field conditions for a few locations, and validation of the theories is incomplete, especially in unsaturated fractured rocks. Nevertheless, research effortsudshould continue to improve the state -of -the art. Geostatistics and stochastic methodsudwill be valuable tools in addressing the groundwater travel time objective