A series of ground-water flow and tracer experiments were performed on an undisturbed column of fractured clay-rich till, 0.5 m diameter by 0.5 m long, in a pressure-controlled cell. The measured hydraulic conductivity of the sample was 1.0 to 1.2 × 10-6 m/sec and the average hydraulic gradient during the tracer experiments ranged from 0.45 to 0.49. The experiments clearly show that ground-water flow and contaminant migration through the sample is primarily controlled by fractures and root holes. Tracer experiments using a solute (chloride), colloid-sized bacteriophage (PRD-1 and MS-2) and uncharged latex microspheres, indicated very fast transport rates of 4 to 360 m/day. These rates are similar to fracture flow velocities calculated on the basis of the measured bulk hydraulic conductivity of the column, and measured fracture spacing, using the cubic law for flow through parallel-walled fractures. Fracture aperture values calculated from the ground-water flow data (35 to 56 μm) are of the same magnitude as values calculated from the breakthrough of tracers (13 to 120 μm). Aperture values calculated for fractures (1 to 94 μm) and root holes (2 to 188 μm), on the basis of measured immiscible creosote entry pressures, are also comparable with these values. The injected creosote, a DNAPL, penetrated most of the visible and a few invisible fractures and root holes, indicating that, for this till, fractures and root holes are important conduits for the transport of DNAPL's.
|Number of pages||11|
|Publication status||Published - Nov 1996|
- Programme Area 2: Water Resources