Laboratory bentonite colloid migration experiments to support the Äspö Colloid Project

One of the objectives of the Äspö Colloid Project is to evaluate the potential of bentonite colloids to facilitate radionuclide transport. Since bentonite colloids are not stable in Äspö groundwaters, the field-scale experiments performed at Äspö needed to be limited to using latex colloids. To support the Äspö Colloid Project, laboratory scale colloid migration experiments, using both bentonite and latex colloids, were performed in the Quarried Block (QB) sample, a 1 m × 1 m × 0.7 m block of granite containing a single, well characterized, through-going, sub-horizontal, variable aperture fracture. The main purpose of this laboratory program was to provide additional information that cannot be obtained on the field-scale regarding bentonite versus latex sphere colloid transport, particularly at low flow rates. Tracer tests were performed as dipole experiments within the fracture plane using a borehole pair with a separation distance of 380 mm. Flow rates and ionic strength were varied between tests.With low ionic strength water, results showed that at high flow velocities typical of forced-gradient field-scale tracer tests (10’s cm/h), bentonite colloids and 100 nm latex colloids were mobile and had similar transport behaviour. Differences in transport behaviour between the two colloid types became evident as flow velocity was reduced to centimeter per hour and then millimeter per hour, the latter being more typical of certain natural flow conditions. Bentonite colloid recoveries were reduced by up to a factor of three and eluted colloid sizes were predominately in the smallest particle size range (4–15 nm), particularly at low flow velocities. With high ionic strength, Äspö-type, water, bentonite colloids were not mobile in the QB fracture and could not readily be re-mobilized by subsequent injection of low ionic strength water.