FIELD-SCALE COLLOID MIGRATION EXPERIMENTS IN A GRANITE FRACTURE

An understanding of particle migration in fractured rock, required to assess the potential for colloid-facilitated transport of radionuclide s, can best be evaluated when the results of laboratory experiments ar e demonstrated in the field. Field-scale migration experiments with si lica colloids were carried out at AECL's Underground Research Laborato ry (URL), located in southern Manitoba, to develop the methodology for large-scale migration experiments and to determine whether colloid tr ansport is possible over distances up to 17 m. In addition, these expe riments were designed to evaluate the effects of flow rate and flow pa th geometry, and to determine whether colloid tracers could be used to provide additional information on subsurface transport to that provid ed by conservative tracers alone. The colloid migration studies were c arried out as part of AECL's Transport Properties in Highly Fractured Rock Experiment, the objective of which was to develop and demonstrate methods for evaluating the solute transport characteristics of zones of highly fractured rock. The experiments were carried out within frac ture zone 2 as two-well recirculating, two-well non-recirculating, and convergent flow tests, using injection rates of 5 and 101 min(-1). si lica colloids with a 20 nm size were used because they are potentially mobile due to their stability, small size and negative surface charge . The shapes of elution profiles for colloids and conservative tracers were similar, demonstrating that colloids can migrate over distances of 17 m. The local region of drawdown towards the URL shaft affected c olloid migration and, to a lesser extent, conservative tracer migratio n within the flow field established by the two-well tracer tests. Thes e results indicate that stable colloids, with sizes as small as 20 nm, have different migration properties from dissolved conservative trace rs. (C) 1997 Atomic Energy of Canada. Published by Elsevier Science B. V.