THE HYDROGEOLOGY OF SHALES AND THEIR EMERGING ROLE AS HOSTS FOR NUCLEAR WASTE

Based on findings from underground laboratories and instrumented boreholes, a number of nations are planning or considering repositories for spent nuclear fuel (SNF) and other high-level waste (HLW) in clay-rich sedimentary strata such as shales, claystones, and argillites. Research is providing a picture of how clay-rich formations function in groundwater systems and how they might isolate SNF and HLW. Before about 1990 few reliable data existed on shale transport properties and flow regimes, in large part because the measurements are challenging. As dedicated investigations of argillaceous formations were conducted in the intervening decades, a picture emerged implying many have very low permeability at spatial scales large enough to accommodate repositories. In particular, anomalous pressures suggesting very slowly evolving transient conditions are surprisingly common. Many aspects of shale hydrogeology – and some lingering questions - are illustrated by recent research at the Bruce nuclear complex in Ontario, Canada, where anomalous hydraulic heads 200 m below sea-level and vertical head gradients of ~ 4 have been observed in rocks with borehole-measured permeability as low as 10^-22 m² (~ 10^-15 ms^-1 hydraulic conductivity). Simulations of groundwater flow subjected to mechanical loading and unloading by past continental ice sheets can reproduce the pressures; they yield estimates of permeability comparable to the measured values, an indication water-transmitting fractures are locally absent. Modeled advective transport, which reversed with each advance and retreat of the ice sheet, was limited to tens of meters. Methane is present in some strata at the Bruce site, but because of the small porosities of the rock (often ~ 0.01) and high capillary pressures (tens of MPa) it is unclear whether a separate gas phase exists in situ. If it does, multiphase processes may be important, but transport would remain extremely limited.