Deep geological placement of wastes

Monday, November 23, 1998

Maurice Dusseault, PEng 
Department of Earth Sciences

Over the last ten years, new options for permanent disposal of noxious or toxic wastes have been developed in the Earth Sciences Department at the University of Waterloo.

Slurry Fracture InjectionTM (SFITM) involves placement of a solid-liquid slurry deep in porous, permeable rock by high-pressure injection. This process involves the continuous creation of fractures in the rock. When injection is stopped, the high pressure water bleeds off into the porous rock, leaving the solids trapped permanently by the weight of the overburden. Proper stratum choice and the right hydrogeological conditions are vital to the success of this process.

Salt Cavern Placement (SCP) involves solutioning a cavity in a salt stratum and filling it with a thick slurry of waste solids and brine. Because salt deforms slowly under stress, the cavern will close around the wastes, entombing the wastes in low permeability salt. Additives such as granular salt, gypsum, shale chips, or other materials can be used to formulate a slurry that, when compacted by cavern closure, will be of extremely low permeability, so that the possibility of leachate expulsion is minimal.

SFITM has been successfully commercialized, with about ten projects in Alberta, Saskatchewan, and California. Most involve disposal of large volumes of oily sand produced along with heavy oil. In one case, over 50,000 tonnes of sand were disposed over a period of a year in a single well. These wastes are non-toxic, but expensive to treat by other means. SFITM is favorably regarded by the regulators; after all, the waste sand and oil are being put back in the same formations from which they came!

SCP using specially formulated slurries has not yet been implemented, but salt caverns in Alberta and Saskatchewan are now being used to dispose of non-hazardous solid oil field wastes (sand, oily soil, drilling mud wastes). In the future, toxic waste disposal in salt caverns is a distinct possibility, using the methods developed at Waterloo.

Deep entombment is exceptionally secure for a number of good geological reasons. In contrast to other methods of geological disposal (landfills, mines in igneous rocks), it seems reasonable to suggest that security periods of millions of years can be expected with SFITM or SCP. Let us examine the arguments that lead to this conclusion. As usual in geological processes, many of the conclusions are eminently logical, but cannot easily be assigned a quantitative risk factor.

Why are landfills a poor alternative for wastes of any toxicity level? Because a landfill is on the surface of the earth, it is thermodynamically unstable over a time span of hundreds to thousands of years. As geologists know, if something is possible, given geological time, it will become inevitable. Therefore, landfills will inevitably leak, no matter how well engineers design them.

Lithostratigraphy above potash mines in Saskatchewan
LEFT: Lithostratigraphy above potash mines in Saskatchewan

Look at just two issues, membrane security and clay behavior. Is it possible to expect that a two-hectare polymer membrane can be installed without even a pinhole? Is there a 100% assurance that no chemicals will be added to the landfill that will degrade the membrane? Is the membrane geochemically stable for thousands of years?

Clays are used to generate rolled-clay liners as "seals". Clays have water adsorbed on their surfaces, and this water can be stripped by the organic chemicals and concentrated ionic solutions often found in landfills. When the water layer is reduced, the clay liner shrinks and develops vertical fractures, allowing leachates into the groundwater.

Deep geological placement takes advantage of the huge time span of deep formation water flow and the natural features of stratified rock. Figure 1 shows the approximate lithostratigraphy for a SFI™ site in Saskatchewan. The favorable features for deep waste placement include:

  • Permeable, porous strata with huge storage and pressure bleed-off capacity.
  • No tectonics for the last >50 MY, therefore all geological seals are intact.
  • Horizontal formation water flow at slow velocities (tens of cm per year maximum).
  • Ductile shales over 100 m thick overlying the site provide excellent seal.
  • No interaction with surface waters (deep waters dated to 40 MY).
  • Long exit paths for any fluids (hundreds of kilometres).
  • Clays are present in all sandstones and siltstones, providing adsorptive sites for heavy metals and organic materials that might be liberated into the flow regime.

In the case of salt caverns, similar geological security arguments can be made. Also, because brines are used (r = 1.2), there is no possibility of upward flow to displace fresh water (r = 1.0) because a density graded system in porous rocks is exceptionally secure. Also, remember that salt heals itself under stress, and once the salt cavern is closed, the compacted solid wastes are surrounded by material of permeability less than 10-18 m/s (10-10 Darcy).

For both methods, any generated flow is exceedingly slow, and the strata will adsorb most toxic species. If leachates do interact with the biosphere after millions of years, the concentration will be exceedingly low because of dispersion at depth and massive dilution near the surface because of rainwater. It is hard to be quantitative, but reduction of specie concentration for a geological-time process such as this one will undoubtedly be many orders of magnitude.

Fortunately, almost ideal geological conditions are found in most sedimentary basins. Consider for example the approximately 32,000 tonnes of wastes (~20,000 m3) generated by the Uniroyal waste management problem at Elmira, just a few kilometres from the University. Clearly, placing the wastes in a concrete floored warehouse on the crest of a river valley is perhaps the worst of all solutions. Groundwater flow is downwards at this site, concrete will inevitably crack with time, and the social and economic cost of maintaining such a facility for generations is unacceptable.

What could be done with these wastes? One option is to drill a well on site and use SFITM to inject the wastes into strata at a depth of about 800 m, substantially deeper than in the Alberta cases, and in excellent strata from the point of hydrogeological security (lateral flow, sealing shales, no active tectonics, etc.). The wastes could be disposed in about 150-200 days of carefully controlled and monitored injection. The monitoring data would demonstrate that the solid wastes are staying near the injection point, and the fluids are dissipating harmlessly into surrounding porous rocks.

Alternatively, Elmira wastes could be transported to a salt cavern specially developed to accept them. Only a single small cavern would be necessary, and suitable strata can be found about 150 km to the west or the south-west. The waste would be slurried with 40% granular salt, 40% shale, and make-up brine, to generate a fluid of density r = 1.65 to place in the cavern. Displaced cavern brine, which might have some toxic traces, would be re-injected into deeper strata using a second well. When filled, the cavern would undergo slow closure and the solids would be permanently entombed in the low permeability salt.

The two important points about these alternatives is that they are not more costly than other options, but the degree of environmental security associated with them is tremendous, compared to alternative storage approaches.

Without a broad understanding of both geology and engineering, genuine long-term solutions to society’s waste disposal problems will evolve only slowly, with many tragedies. Deep geological disposal makes a great deal of sense when various geological processes are understood. Also, geology helps explain why engineered landfills are inevitably doomed to failure, given enough time. The Geomechanics Group of the Department of Earth Sciences at Waterloo has been instrumental in developing deep geological placement approaches for permanent waste entombment.