Stochastic simulation of radionuclide migration in discretely fractured rock near the Aspo Hard Rock Laboratory

We study the migration of sorbing tracers through crystalline rock by combining relatively simple transport measures with particle tracking in a discrete fracture network. The rock volume is on a 100 m scale and is a replica of a thoroughly characterized site at the Aspo Hard Rock Laboratory, Sweden. Flow is driven by generic boundary conditions consistent with the natural gradient in the region. The emphasis is on the global effect of fracture-to-fracture hydraulic variability where individual fractures are assumed to be of uniform aperture. The transport measures are conditioned on two random variables: the water residence time (tau) and a parameter which quantifies the hydrodynamic control of retention (beta). Results are illustrated for two radionuclides: technetium (strongly sorbing) and strontium (weakly sorbing). It is found that the assumption of streamline routing or full mixing at fracture intersections has comparatively little impact on transport. The choice of the cubic or quadratic hydraulic law (i.e., relation between transmissivity and aperture) strongly affects water residence times but has little impact on average transport since it does not affect the statistics of beta. If the statistics of beta are known, then the distribution of water residence time (tau) is of little importance for transport. We assess the applicability of a linearized model beta = tau/b(ret) using two different approaches to estimate the effective "retention'' aperture 2b(ret): from transmissivity data and from fracture density and flow porosity data. Under some conditions, these conventional estimates may provide acceptable representation of transport. The results stress the need for further studies on upscaling of tau, beta distributions as well as on estimating effective parameters for hydraulic control of retention.