Commonly, shales are considered to be effective aquitards in the subsurface environment owing to their generally low hydraulic conductivity compared to materials such as limestones and sandstone that compose aquifer systems. However, recent laboratory and field studies indicate that clay minerals may behave as natural semipermeable membranes. A semipermeable membrane is capable of retarding the passage of charged species through its micropores when a driving force such as a hydraulic gradient is imposed across the membrane. Likewise, if a chemical, electrical, or thermal gradient is imposed across a semipermeable membrane, the result is a movement of H2O in response to the gradient in order to equalize the chemical potential of H2O on the two sides of the membrane.
If liquid wastes are emplaced in a subsurface aquifer system which was previously in a state of dynamic equilibrium, the emplacement will likely upset the dynamic equilibrium; it may cause (1) chemical reactions with the existing fluid and rocks, (2) thermal changes, and (3) increased pressure on the aqueous phase. In addition to these well-recognized effects, if a shale capable of behaving as a membrane is expected to serve as an aquitard, its membrane characteristics must be taken into account. For example, if the chemical concentration in the aquifer is greatly increased as a result of waste emplacement, an osmotic cell may be set up between a nearby aquifer and the emplacement aquifer with the intervening shale acMng as a membrane. A pressure increase beyond that anticipated could result. Likewise, thermal and electrical osmosis could occur across the shale membrane with attendant pressure changes.
If pressure is increased simply as a result of emplacement of waste, and if it exceeds the pressure required for osmotic balance, ultrafiltration can result. The effect would be to cause flow across the shale and increase the chemical concentration of the filtrate in the emplacement aquifer beyond the planned amount.
In any plan to emplace liquid waste in an aquifer system, the possible membrane behavior of shale in this system must be taken into account. Whenever feasible, laboratory membrane tests which simulate field conditions should be conducted on cores of the shale prior to full-scale operation; the entire system, not just the emplacement aquifer, should be tested with a computer simulation model prior to initiation of waste injection.