Hydraulic conductivity of natural soils permeated with acid mine drainage

Abstract

Three different soils from Canadian mine sites were evaluated in the laboratory for their compatibility with acid mine drainage (AMD). The key parameter evaluated was the hydraulic conductivity, k, measured with a flexible wall triaxial permeameter at low hydraulic gradients ranging from 15 to 60. An undisturbed varved clay gave a k value of 1–1.5 × 10−7 cm/s when permeated with both simulated pore water and two pore volumes of AMD. The k values for compacted tills were of the order of 2 × 10−8 cm/s and 3 × 10−7 cm/s with both reference permeant (0.01N CaSO4) and AMD. The data indicated that, while AMD did not change 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 secondary minerals appeared to have maintained or preserved the soil void ratio, resulting in no net measurable volume change during k testing. The degree of AMD attack was found to be related to its contact time 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 by more than 50%. There was clear evidence that AMD permeation could eventually deplete soil buffering capacity to the extent that heavy metals 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 compatibility with the intended AMD. Such compatibility testing should be conducted over a long period to establish chemical equilibrium of key mobile contaminants such as Zn. This will improve design confidence and also avert or minimize detrimental, postconstruction changes requiring costly remedial actions. Key words : hydraulic barriers, hydraulic conductivity, acid mine drainage, mineral alteration, buffering capacity depletion.