A field tracer experiment was conducted in a lateral flow field in the weathered and highly fractured upper 6 m of a 40-m-thick clay-rich till plain in southwestern Ontario. In the upper 3 m where fractures are closely spaced (5 m/d. Simulations with a discrete fracture/porous matrix flow and transport model, which used the cubic law for flow in fractures, showed that diffusion of the solutes, but not the much larger colloids, into the matrix pore water between fractures is sufficient to cause the observed difference in solute and colloid transport rates. Transport-derived and hydraulic conductivity-derived fracture aperture values were similar, within a factor of 3 and falling mainly within a range of 5-40 &mgr;m. In the upper 3 m the solute tracers were evenly distributed between pore water in the fractures and the matrix, and as a result, solute transport can be closely approximated with an equivalent porous medium (EPM) approach. Below this depth, fractures are more widely spaced (0.13 to >1m) with concentration peaks tending to occur near visible fractures, and solute transport cannot be adequately described with an EPM approach. ¿ American Geophysical Union 1993