ENVIRONMENTAL SIGNIFICANCE OF ACID SULFATE CLAYS AS WASTE COVERS

The current regulatory requirement for cover soils in landfills and su rface impoundments is that the soils attain, upon compaction, a very l ow hydraulic conductivity of 10(-7) cm/s or less. Although the influen ce of the interaction between waste chemicals and clay soil on waste m igration has been extensively studied, attempts to incorporate as desi gn components the effects of sulfidic (sulfide-bearing) clays on the i ntegrity of clay caps have largely been ignored. These influences may include increasing the permeability of the cover to percolating moistu re, enhancing erosion of clay covers, and killing of vegetation on dow nslopes of the cover. Consequently, it is suggested that clay cap desi gners test the acid-generating capabilities of potential clay cap mate rials before exploiting these earth formations. This can be done by in cubating a sample of the candidate capping material (with pH > 3.5) un der moist aerobic conditions (field capacity) at room temperature. The soil will be said to contain sulfidic materials if it shows a drop in pH (1 : 1 by weight in water) of 0.5 or more units to a pH value of 4 .0 or less within eight weeks. Decisions should then be made as to whe ther the soil should be avoided or used with amendments to the cap des ign.