Bulk techniques and very large underground caverns may enable low-cost ,permanent geological
Disposal of hazardous elemental wastes Charles W. Forsberg Oak Ridge National Laboratory Oak Ridge, Tenn. 37831
The permanent, safe disposal of toxic elements such as arsenic, cadmium, and lead in hazardous wastes presents technical, political, and economic difficulties. These hazardous elemental wastes are currently disposed of by shallow land burial. Since the wastes are not degradable, they remain toxic forever; hence, shallow land burial grounds remain toxic forever and may require perpetual maintenance. In the US., the annual total toxicity of elemental wastes disposed of each year exceeds the annual total toxicity of all radioactive wastes ready for disposal each year, as measured by the quantity of water needed to dilute such wastes to drinking water standards (1 ). The continued mining of such materials is slowly transferring toxic elements to the top few meters of the Earth-the biosphere. These problems have generated opposition to shallow land burial and have led to state regulations restricting shallow land burial of such long-lived wastes (2).
The RUMOD system A permanent (and preferable) solution to the disposal problem for wastes that cannot be detoxified or recycled would be the burial of such wastes in geological repositories 300-2000 m underground, in a manner similar to that proposed for radioactive wastes. In effect, the toxic elements would be recycled into the Earth’s crust. This approach has not been seriously considered in the past because of projected high costs ( 3 ) . But a series of recent technical developments in fields as diverse as radioactive waste disposal and bulk oil 56A
Environ. Sci. Technol., Vol. 18, No. 2, 1984
storage suggests that it may now be possible to use geological disposal of hazardous elemental wastes at costs approaching those of current shallow land burial methods. One such method is the RUMOD (regional underground monolith disposal) system. T o dispose of hazardous elemental wastes via RUMOD, the wastes are processed into a granular form, transported in bulk to a regional disposal site, mixed with special cementbased grouts, and pumped as a wet, waste-cement mixture into large caverns 300-2000 m underground. The final waste form would be a series of monoliths composed of waste and a cement-based grout, with dimensions up to 25 m wide, 60 m high, and several hundred meters long. The economic and engineering feasibility of R U M O D is based on four factors: large underground caverns, bulk disposal of waste-cement mixtures, minimal handling operations, and high-volume throughput. The RUMOD disposal method requires a granular waste form that can be handled in bulk and is compatible with cement grout. Extensive research ( 4 ) and a variety of commercially available solidification agents ( 5 ) have provided many methods of producing an acceptable granular waste form. Cement-based solidification agents are currently the most popular for solidifying wastes in barrels because of their low cost, ease of use, and final properties. Thus, these are the likely solidification agents to produce granular waste forms at generator sites for RUMOD. Production of granular products is a simple, low-cost operation, as demonstrated in processes used for fertilizers and taconite pellets. A single disposal facility would likely receive hundreds of types of granular waste forms, reflecting the differences in wastes from various waste generators. The chemical form of the toxic ele-
ments in the waste would not be specified; thus a hazardous elemental waste containing lead could have the lead in the form of lead oxide, lead sulfide, or a variety of other chemical compounds. The granular waste would be required to meet limited leach standards to avoid cement grout-granular waste incompatibility problems, to minimize the risk in transport, and to ensure long-term, safe disposal. The major public health risk of geological disposal of toxic elements is possible contamination of drinking water. By definition, toxic elements are those elements with which humans did not have significant contact during evolutionary processes and thus for which effective biological defense mechanisms were not developed. This implies elements that are either very rare or that combine in naturally stable, low-leach mineral forms. A disposal site can be chosen where groundwater does not exist, but this condition is uncommon and may not be permanent. In nature, however, toxic elements seldom contaminate the groundwater. As water moves through rock, minerals are continually dissolved and reprecipitated in a process that yields more thermodynamically stable minerals, usually with lower leach rates ( 6 ) . Natural geological processes ensure long-term, safe disposal provided there is time for the various mechanisms to operate. The RUMOD system provides this time with only limited dependence on the leach rate of the waste received at the disposal site. It also furnishes a high degree of protection by two other mechanisms: a low surface-to-volume monolith form, which minimizes the leach rate, and a cement-based grout mixture incorporating the granular waste, which has a chemical composition designed to minimize leaching of hazardous elemental wastes. Meeting the RUMOD waste spec-
0013-936X/84/0916-0056A$01.50/0
@ 1984 American Chemical Society
IGURE 1
;ranular-waste-receiving facility operations
G ~ ~ l ~ iwastes iiv
Bulk rail delivery
Refrigerated bulk storage Sized reduced wastes
Granular wastes Water
7 db R
Grindersized reduced waste storage
Refrigerated cementadditive bulk storage
Cement- additive mixlure
11
K4
e Refrigerated wastecement mixer
ifications of grout compatibility and granular form may be simple in some cases and complex in others. Some hazardous wastes. such as certain incineratorashes, will be acceptable for the R U M O D system as generated. Others, such as electroplating sludges and salts, must be treated for conversion into a granular waste form.
The disposal site Granular wu.rIes. At the R U M O D facility, granular solidified wastes would be received, mixed with cement-based grouts. and pumped to underground disposal caverns. Figure I shows the major operations involved. Some granular wastes would be reduced in size before mixing with the cement grout and additives. By this procedure, the size distribution of the granular wastes would be altered to maximize waste loading in the cement grout. A waste loading of 70% is readily attainable. based on the experience of the construction industry, which makes concrete that is 70%sand and gravel and 30% cement, by volume. In effect. the wastes will replace the sand and gravel in a concrete mixture. Higher waste loadings will reduce the cement and underground excavation costs per unit of waste. After the wet mix is transferred down the mine shaft, it may need to be remixed because vertical drops sometimes separate components in wet
Waste- cemenl mix transfer down borehole
A# Refrigeraled waste- cement remixer
concrete. Pumps would then deliver the mixture via pipeline to the appropriate underground cavern (7). I f sufficient volumes of specific liquid hazardous wastes exist, the R U M O D facility could accommodate them by mixing them with dry grout materials, partially substituting for the water in the cement-based grout mix. Purkuged wus1e.v. R U M O D is primarily designed as a bulk waste disposal system; however. some package-handling capability is required, since it is not feasible to convert all wastes into granular form. I n this system, the waste packages would be placed in underground caverns with the waste-cement grout mixture poured on top to produce a monolith containing packages of waste. Because of economic considerations, however. it is anticipated that >95% of all wastes would be handled in bulk. Underground cavern construction The final waste disposal site proposed for the R U M O D system is a series of large caverns designed to minimize geological disposal costs. Existing mines have clear spans as large as 100 m ( R ) , and many permanent facilities have widths in excess of 20 m and heights of more than 40 m. Experience in building such caverns has increased rapidly with the construction of large underground caverns for oil storage. For example, the
Concrete
Underground
wnp
monolith disposal Site
Brofjirden oil storage project in Sweden required excavation of more than 4 X10"m30fgranitcrock(9).Thecost of these caverns per unit of volume drops dramatically as the size increases s o t h a t a t s i z e s > 5 t o 1 0 X 104m3,the caverns provide cheaper oil storage space than do surface tanks (10). The construction method used for very low cost underground space is characterized by the precision use of explosives to minimize rock fracture, and thus, the need for expensive rock bolting; high-speed hydraulic drills with two to four drills per vehicle; rubber-tired. 4-6-yd' front-end loaders for rock removal: and up to 40-ton trucks for rock transport. The large trucks and the maneuvering room needed for the front-end loaders require IS-20-m-wide caverns for efficient use of equipment. The basic procedure is to excavate the top of the cavern (the gallery) at its full width and then to lower the floor by a process called benching. in steps of 5-15 m. Benching is the lowest cost step; it rcsults in large cavern height-to-width ratios that minimize costs ( 1 1 ) . The most recent U S . engineering cost estimates for large underground rooms have been made for studies of underground hydro-pumped storage (UHPS) facilities and compressed-air energy storage facilities. Both concepts are being studied by electrical utilities to provide storage of electricity genEnviron. Sci. Technol.. VoI. 18. NO. 2. (984
57A
crated at night for daytime use. Site studies are currently under way to determine whether l o commit to construction ( / 2 - / 4 ) . A U H P S facility consists o f two water reservoirs; one is a surface lake, and the second is a large set o f caverns, 500-2000 m underground, with an excavated volume as large as -7 X IO6 m3. The characteristics o f a U H P S underground reservoir are nearly identical to the requirements for a large R U M O D facility. Costsof the UHPS underground reservoir (including rock disposal) are estimated. i n June 1982 dollars, to be $37.75/m3 of cxcavatcd space in hard rock ( / 5 - / 7 ) . The geological requirements for a R U M O D facility can be divided into two closely related categorics. First, the geological formations must isolate the wastes from the biosphere and from underground water supplies. This subject has been investigated i n detail for more than 25 years i n geological studies conducted for high-level radioactive waste disposal (18-20). Second, to minimize construction costs, “competent” rock is required. This is rock in which large caverns can be constructed without an expensive ceiling support system. i.e.. few rock bolts arc needed. In practice. these two requirements are similar in that both arc met with solid rock having low permeability l o water and few fractures. A variety o f metamorphic and sedimentary rocks meet these requirements. Based on data from studies o f radioactive waste repositories, U H P S facilities, and other projects (15, 21, 22). sites arc available throughout the US., cxccpt in the Gulf Coast area. Sedimentary basins near the Gulf Coast arc not fully consolidated. Therefore. except for salt domes, large underground cavern construction i s not feasible in this area. Regional treatment facilities The economics of scale associated with R U M O D require that i t be implemented on a regional basis. The two most expensive aspects o f the system. licensing the facility and constructing the mine shaft system, cost approximately the same, regardless o f yearly waste throughput rates. Construction costs for mine shafts are determined by factors such as the depth o f operation, the underground processing equipment required, and the size o f waste packages (ifany) to be transported through the mine shaft. I n contrast, the rate o f rock removal i s determined by the numberof miners and their equipment, but i s not affected by shaft limitations 5811
Environ. Sci. TechmI.. Val. 18. No. 2. 1984
unless rock removal rates reach several hundred thousand tons/y. An example o f current mine shaft construction costs comes from the new production shaft being built for the Lucky Friday Silver Mine i n Idaho ( 2 3 ) .This 1700-m-deep. 5.5-m-widc, concrete-lined production shaft with headframo, hoist. and auxiliary equipment i s expected to cost $27 million when completed next year. The cost o f a R U M O D facility would be similar. A disposal site must handle >IO’ m3/y o f waste to achieve low disposal costs. I n 1980, a total of41 million wet metric tons o f hazardous wastes were generated i n the US. ( 2 4 ) . Only a small fraction (a few million tons at most) contain hamrdous clemcnts and thus would beamenablctoRUMOD. This indicates that 5-15 R U M O D facilities should be sufficient to disposc o f all hazardous elemental wastes for the entire nation. Selecting components Granular wasleforms. A granular solidified waste form (4. I - I .O-cm size) provides major cost savings in transport, storage, and operations by allowing bulk waste shipments with dry solids, bulk handling aboveground, and pumping o f wet waste-cement mixtures underground. Packaged
wastes should be minimized because of the higher transportation costs. higher underground handling costs. and major problems with mine shaft 10gistics. Hoist cars within a mine shaft arc designed to load and unload rock within seconds, but these operations may take minutes for packaged wastes. A second, more cxpensive mine shaft system would be required to handle predominantly packaged wastes. Bulk operations with pumpable waste-cement mixtures make low-cost geological disposal possible. All lowcost underground construction techniques result i n caverns with high ceilings (>20 m), and all o f the spare can be effectively filled only when bulk waste materials arc used. Unfortunately, it is not feasible to stack packaged wastes beyond I O m high. because the weight will crush the containers at the bottom ofthestack, and the resultant dust will create operational problems for the workers and equipment stacking the cont..iiners. ’ Low-cost geological disposal and package disposal are mutually incompatible goals. Of the solidified bulk waste forms. the granular form minimizes thc surPace-to-volume ratio o f the solid waste. thus reducing possible interactions between waste types and waste-cemen1 mixtures. Some wastes (such as
Advantages of cement grout as a disposal agent A liquid waste-cement mixture can fully use the cavern space. F w liquid fill, the caverns can be shaped to minimize construction costs (with large height-to-width dimensions), and caverns with nonstandarddimensions caused by local rock conditions can be used. The solidified strength of concrete monoliths will allow lower cost underground layouts. Close spacing of caverns will reduce the costs of transporting rock from the working face. as well as the costs of transporting the wastes underground, but the spacing is limited by the requirement to suppwt the ceiling. With cement-waste disposal, caverns can be spaced so that other caverns can be constructed between them after they are built and filled, since the cement monoliths will help support the overhead rock. This technique is sometimes used in underground mining of ores (26). The physical characteristics of cement-waste m i x b e s will minimize potential groundwater contamination. When caverns are completely filled.
the possibility of ceiling collapse. subsequent disruption of geology above the disposal site, and disturbance of existing aquifers is eliminated. The physical blocking of each cavern provides assurance that. if future geological changes bring water into contact with the wastes, ltw water flow will be very’ low compared to that in a cavern of packaged wastes, because of the low permeability of solid concrete. The chemical properties of cement-based grouts also are favwable for the permanent disposal of hazardous elemental wastes. The naturally stable geological mineral forms found at disposal sites are the hydrates of oxides (27):cement grouts with additives can form the same, or closely related, mineral forms. Cement grouts also have a basic pH. which ensures the fwmation of hydroxides of very low solubility for most metal cations that are not incorporated into some other mineral form. (The term “cement” is used here in its broadest definition. not just as Type 1 Portland Cement.)
FIGURE 2
arsenic, which slows the curing of cement) can cause problems with the cement-based grout. Such wastes, in a high-surface-area powdered or liquid form, have a greater potential for handling and curing problems. In granular form with limited surface areas, however, these wastes can be mixed with grouts, pumped, and set rapidly enough to prevent troublesome materials from diffusing into the wet grout. This characteristic of granular solidified wastes minimizes possible chemical reactions and allows a wider choice of solidification agents at waste generation sites. A second advantage of granular wastes over liquids and powders is the reduced risk of accidents from leakage or blowing dust. Various types of waste that have been solidified by different chemical agents and methods into granular forms usually can be stored in the same bulk facilities, since handling characteristics of most granular materials are somewhat similar. On the other hand, a variety of liquid wastes cannot be stored together, because of the possibility of reactions between the liquids. Similarly, many types of powdered wastes cannot be stored together because the surface effects often will cause powders to clump together, creating handling problems. For a facility that receives wastes from several thousand sources, waste incompatibility becomes a major concern. This can in part be eliminated by use of a granular waste form. For underground monolith disposal using cement grouts, granular waste forms will provide a thermal control mechanism during monolith construction. When normal cement cures, it releases as much as I O 9 J/m3 of heat over a 30-d period. Excessive temperatures can degrade the concrete quality. The necessary cooling is not a problem for thin (
