Tracer breakthrough and fracture networks : a modelling study
In order to protect groundwater abstractions used for drinking water supplies, it is necessary to understand contaminant transport processes. In fractured rock aquifers, the geometric and hydraulic properties of the fractures influence transport. Tracer tests are commonly used to estimate directly the transport properties of a fractured rock mass. Unfortunately, several of the methods used to interpret tracer tests are either only appropriate for continuum scales, suffer from problems of non- uniqueness or do not adequately describe the transport processes. A novel approach is presented as a possible methodology for interpreting tracer breakthrough curves in terms of fracture geometric and hydraulic properties at sub-continuum scales. The approach was one of forward modelling, starting with a series of cases of prescribed fracture networks in a homogeneous permeable medium. The flow field and tracer transport were simulated for each case in turn using a 2D finite difference numerical code. The fracture angle to the direction of the general hydraulic gradient and fracture aperture, spacing and pattern were systematically altered from one case to the next. Linear and radial flow fields were simulated, allowing results to be compared with natural and forced gradient tracer tests conducted in field experiments. The linear study was fairly exhaustive for single set patterns and a significant exploration was made for two set patterns, but only a pilot study was conducted for the radial flow. Classification schemes were devised for breakthrough curves both qualitatively and using principal component analysis (PCA), a multivariate statistical technique. These classifications were linked to the fracture pattern properties. Trials showed that even under highly simplified and controlled conditions quite different fracture patterns produced very similar breakthrough curves, and also demonstrated that a single breakthrough curve cannot reveal with any precision the fracture geometry. However, since PCA shows that there is a broad correspondence between fracture-pattern type and the breakthrough curve, in principle it should be possible to identify those types from tracer tests.