The tensorial nature of effective porosity and large-scale dispersion coefficients: Application to the Creston study area, Eastern Washington

To describe flow in a complex system of fractures requires an understanding of the effects of direction or orientation on several hydrologic characteristics, such as hydraulic conductivity, porosity, and dispersion coefficient. The theory for hydraulic conductivity is well understood; this report deals with the effects of fracture orientation on porosity and dispersion coefficient. The tensorial nature of effective porosity was examined and was found to be a second-rank tensor in fractured rock units. Porosity varies at a fixed point, depending on its orientation. A method to calculate a dispersion coefficient from field tracer tests is described. The components of the dispersion coefficient can be calculated from the concentration profiles observed in downgradient observation wells. The method provides a procedure for studying the dispersion effect in large-scale field testing. The application of this method was successfully demonstrated in a tracer test performed in the research wellfield at the Creston study area, Lincoln County, Washington.