HYDRAULIC CONDUCTIVITY OF NATURAL SOILS PERMEATED WITH ACID-MINE DRAINAGE

Three different soils from Canadian mine sites were evaluated in the l aboratory for their compatibility with acid mine drainage (AMD). The k ey parameter evaluated was the hydraulic conductivity, k, measured wit h a flexible wall triaxial permeameter at low hydraulic gradients rang ing from 15 to 60. An undisturbed varved clay gave a k value of 1-1.5 x 10(-7) cm/s when permeated with both simulated pore water and two po re volumes of AMD. The k values for compacted tills were of the order of 2 x 10(-8) cm/s and 3 x 10(-7) cm/s with both reference permeant (0 .01 N CaSO4) and AMD. The data indicated that, while AMD did not chang e k, it may have dissolved primary minerals such as chlorite, smectite , plagioclase feldspars, illite, and K-feldspar, in a decreasing order of susceptibility. The dissolution and concomitant precipitation of s econdary minerals appeared to have maintained or preserved the soil vo id ratio, resulting in no net measurable volume change during k testin g. The degree of AMD attack was found to be related to its contact tim e with the soil; one of the tills produced acidic effluents after 5.5 pore volumes or 243 days of permeation, the longest testing time used. The AMD permeation also reduced the till's cation exchange capacity b y more than 50%. There was clear evidence that AMD permeation could ev entually deplete soil buffering capacity to the extent that heavy meta ls such as Zn would no longer be attenuated. Soils intended to be used in the design and construction of barriers (for example, slurry walls and dam cores) against AMD seepage should, therefore, be tested for c ompatibility with the intended AMD. Such compatibility testing should be conducted over a long period to establish chemical equilibrium of k ey mobile contaminants such as Zn. This will improve design confidence and also avert or minimize detrimental, postconstruction changes requ iring costly remedial actions.