COLLOID AND SUSPENDED PARTICLE MIGRATION EXPERIMENTS IN A GRANITE FRACTURE

To determine the mobility of colloids (0.001-0.45 mu m) and suspended particles (> 0.45 mu m) in granite fractures, laboratory particle-migr ation and conservative tracer studies have been carried out in a natur al fracture within a large granite block, with overall dimensions of 8 3 X 90 X 60 cm. Flow fields within this horizontal fracture were contr olled through a set of 9 boreholes drilled orthogonally to the fractur e. Laboratory experiments were performed using a range of average wate r velocities which contained values low enough to closely approximate the natural flow velocities of (2 m yr(-1) in plutonic rocks of the Ca nadian Shield. The particles used had diameters between 0.02 and 22 mu m, and included latex spheres, glass spheres and colloidal silica. Mi gration experiments were carried out with a filtered groundwater, ioni c strength of 0.01 mol kg(-1), obtained from a granite fracture within the Whiteshell Research Area of Manitoba. Flushing experiments showed that suspended particles as large as 40 mu m could be mobilized from the fracture surface. The mobility of suspended particles was signific antly less than that of colloids. However, within the size range of co lloids used in these studies (0.022-0.090 mu m), colloid size did not affect colloid migration. Although, in general, colloids eluted ahead of the conservative tracer, colloid mobility was significantly reduced when the average groundwater velocity dropped below between 32 and 24 0 m yr(-1). Colloid transport was found to be very sensitive to flow p ath and flow direction.