The field testing requirements of nuclide transport models are changing The existing methods of measuring hydraulic conductivity in-situ are reviewed and their relevance to modelling requirements assessed. Within the field of multiple borehole methods, some techniques, suitably adapted from the original oilfield environment, are suggested. The limited methods available to carry out three dimensional testing are supplemented with a new method based on propagating a sinusoidal pressure pulse. The distances over which testing could be accomplished are assessed together with the range of the rock parameters within which the test is practical It is concluded that the new method has interesting possibilities and could provide a series of hydraulic diffusivity vectors.
The last ten years have seen the increasing use of computer models and techniques within hydrogeology in general, and solute transport investigations in particular. The nuclide transport models produced so far are still in their infancy but show a trend towards increasing complexity which is likely to continue. The tendency is to add more and more effects, such as thermally induced flows, radioactive decay and rock stress effects, onto the basic groundwater flow model. As yet most groundwater flow models are of the porous medium type and assume that Darcys law broadly applies within the scale of interest. Some work though, has been carried out on a model involving flow through a system of blocks bounded by shear zones, and the results (Axelsson and Carlsson, 1979) differ considerably from the homogenous case (Stokes, 1979). Whatever the particular type of model involved, all require field measurements of those factors which delimit the velocity and direction of groundwater movements within quite large volumes of rock. In the simpler models, where groundwater is assumed to flow horizontally in response to a regional hydraulic gradient, equivalent to the average dip of a water table, the field measurements required are straight forward. However, as the models become more complex, the requirements on field investigations become more exacting and, although the basic hydrogeological parameters of hydraulic conductivity, hydraulic potential, porosity and dispersivity are still relevant, the nature of their variation is coming under closer scrutiny. It is the intention here to examine the changing methods relating to the field measurements of hydraulic conductivity in fractured crystalline rocks and their likely relevance.
Average values of hydraulic conductivity are the usual basis of most porous medium models and can be obtained by either injecting or abstracting water from a completely open borehole. In an attempt to measure the variation of hydraulic conductivity with depth, straddle packer systems are currently being employed by many investigators (e.g. Davison, 1980, BRGM, 1979 and Carlsson,1979). These packer results tend to apply to individual fractures near the borehole, and only by averaging large numbers of results, is it possible to infer a depth-dependent permeability relationship. The use of short spacing straddle packer measurements does mean however, that the spatial variability of hydraulic conductivity can be deduced.