Interpretation of artificial and environmental tracers in fissured rocks with a porous matrix

Tracer movement in fissured rocks with a porous matrix is considered. Solutions to the transport equation for the piston-flow and dispersion models in the case of instantaneous tracer injection are given. The great number of non-disposable parameters make a correct interpretation of tracer experiments impossible. However, an ordinary dispersion model yields the mean transit time of tracer (t-bar), which can be easily related to the mean transit time of water (tsub(0)) if the ratio of the matrix porosity (nsub(p)) to the fissure porosity is known, namely t-bar=(1+nsub(p)/nsub(f))tsub(0). Unfortunately, strong tailing effects make the interpretation difficult to perform for low values of tsub(0) (fast flow and/or short distance). In the case of environmental tracers it is suggested that the lumped-parameter approach may be applied in its present form, but to obtain the water age it is necessary to introduce a correction similar to that for artificial tracer experiments. However, this simple correction applies only if (lambda/Dsub(p))sup(1/2)L/2<=0.25 (lambda is the decay constant, Dsub(p) is diffusion in the matrix, L is fissure spacing). In other cases the correction must involve the above-given decay term. Fortunately, this term decreases the correction factor. However, both the fissure spacing (L) and the fissure porosity (nsub(f)) are very difficult to evaluate. Thus, in most cases, a tracer experiment will not yield the required information. (author)