Method of sealing permeable unconsolidated materials

Abstract: Methods of forming layers of material upon or below the earth's surface and above the water table to prevent or effectively inhibit the flow of groundwaters through such laters of initially unconsolidated materials. Chemical components are added to materials such as incinerator ash or other waste materials deposited in a landfill operation to create alkaline conditions which, when contacted by water, induce precipitation of calcite in sufficient quantities to produce a cementing action which retards leachate formation. In another aspect, a site is prepared for use as a landfill with an essentially hydraulically sealed sub-base by choosing a site with alkaline soils or carbonate bedrock and adding appropriate chemical components to cause calcite to precipitate when contacted by water, which may be applied directly or by naturally occurring precipitation. In yet another aspect, low level radioactive or other potentially hazardous wastes are disposed of by placing containers of such waste in underground chambers and filling the balance of the chambers with chemical components to create an alkaline buffer zone in which calcite is precipitated to provide a cementing action and retard incursion of groundwaters through the buffer zone.

Description: The present invention relates to a method of sealing permeable unconsolidated materials, on or in the ground. The cementation of rocks and sedimentation in nature is a well-known phenomenon. Under alkaline conditions (pH >7.5), the most common natural cements by far are calcium or calcium/magnesium carbonates (mainly as calcite - CaCO₃). These natural carbonate cements occur by chemical precipitation or growth from the pore water or groundwater solution over time and reflect a physiological equilibration between mineral constituents of the sediment or bedrock and pore waters, connate waters, or ambient groundwater. AT-B-378389 discloses a method of forming hydraulically-bound stabilised ground strata, in particular involving the use of alkaline incinerator ash, whereby a hydraulically-sealed layer or stratum is formed. This may be in the ground. The determination of stoichiometric concentrations for achieving a sealed layer in the ground and binding loose particles in a ground reinforcement process is discussed in SU-A-3793841. The invention involves novel methods of artificially inducing carbonate cementation or, more specifically, precipitation of calcite, as a sealant in soils, sediments, bedrock, alkaline materials (i.e. cement, waste incinerator or coal ashes, marbles, etc), or inert or hazardous materials where an alkaline system can be maintained thereinafter. The invention also involves a novel method of providing an alkaline buffer system to maintain sealing action as a safeguard against future leaks. The methodology is subject to certain requirements in order to assure stability, as well as to certain material-specific and site-specific conditions. This introduces an added control effect, in terms of a capacity to continue a sealing action via cementation in the event of a future breach of seal developing, for example, by fracture upon settling of earth materials, or rupture consequent to seismic action. The invention can be used to render radioactive wastes inert from the environment, by entombing containers of the waste in a subterranean chamber surrounded by an alkaline buffer zone capable of producing artificially-induced calcite, and sealing the chamber, wherein the containers are enclosed by an alkaline buffer zone, to maintain alkaline conditions sufficient to induce calcite precipitation, and/or sealing the fractures in the bedrock immediately surrounding the chamber (provided a neutral or greater pH can be maintained) by induced calcite precipitation. Further, the invention can be used in the sealing of waste where alkaline conditions can be maintained over a substantial period of time. This would include, but is not limited to, the entombment of discarded, hazardous uranium mine tailings, where it is desirable to prevent the encroachment of hazardous leachate waters, derived from meteoric precipitation on the tailings, into the environment. This invention can provide a compatible alkaline buffering method for certain applications, to ensure long-term stability of an existing or induced carbonate seal and/or provide a capable for continued or future carbonate sealing action in the present of leachate or groundwater, in the event of rupture or fracture after initial application. More specifically, in accordance with a first aspect of the present invention, a method of preparing a site located on alkaline soil or overburden or having a naturally-occurring layer of water-permeable carbonate bedrock for use as a landfill, comprises the steps of: (a) analytically determining the pH of the bedrock and any ground-water therein; (b) contacting the soil or bedrock with an aqueous solution of one or more components which, in the presence of the bedrock and any ground-water, produce precipitated calcite, and continuing to produce the precipitated calcite until at least the upper stratum of the bedrock is substantially fluid-permeable; and (c) constructing an alkaline sub-base soil buffer zone, about 60 mm thick, of dry chemical components to serve as a barrier against future leachate incursion. According to a second aspect of the present invention, a method of disposing of low-level radioactive waste, comprises the steps of: (a) placing the waste in an underground chamber; and (b) completely filling the balance of said chamber with a layer of material having physical and chemical characteristics such that, when the material is contacted by water, calcite is precipitated, thereby rendering the chamber essentially fluid-impervious. Artificially-induced carbonate cementation as a means of isolating or rendering hazardous or other materials inert should meet the following requirements or restriction for use: a) The carbonate precipitate must be stable, i.e. compatible with the ambient hydrochemistry of not only the material to be sealed, but the encapsulating environment of the eventual resting place of the sealed material to prevent breaching by dissolution. Realistically, the system should sustain a pH of 7.5 or greater. b) Added chemical components dispersed after formation of the carbonate precipitate, or during subsequent maintenance or stabilization, must not ultimately enter or reach an aquifer or other aspect of the environment in such quantity as to be unacceptable in terms of water quality. c) In cases such as coarse mine tailings or other similar materials, overall high permeabilities must be temporarily suspended by the creation of impoundments or similar structures. An overall limiting bulk hydraulic conductivity of K = 10⁻¹ cm/sec (vertical or horizontal) should be adequate control, as a general rule. d) Induced precipitation sealing by the methods of this invention requires adequate time commensurate with the hydraulic conductivity and voids of the material to be sealed. Induced precipitation must also allow time for over-growth crystallization, or recrystallization of initially precipitated unstable calcium carbonate phases (such as vaterite or aragonite) in pore and fracture spaces. If the calcite solubility product is exceeded by the ion activity product of interstitial ambient pore waters in the material(s) to be sealed, then precipitation analogous to natural cementation will be induced in this area, or zone, and a chemical sealing will occur. This effect can be produced in several ways, e.g. by increasing pH, by increasing Ca ion concentration, by increasing bicarbonate ion concentration or alkalinity, by degassing CO₂, or by use of unstable solid forms of calcium carbonate, as described later herein. The rate of precipitation, however, is a function of mass ion flux (Ca⁺⁺ and CO₃ = ) through the material, and thus can be controlled by direct additions of appropriate constituents into the material to be sealed. These additions may be as simple as surface applications of appropriate materials for subsequent leaching into the material to be sealed, or more controlled via well injection of solutions, or by virtue of simple physical mixing of the material to be sealed and the appropriate chemicals. e) Production, maintenance, and long-term stabilization of induced carbonate cementation will normally require an alkaline buffer system to be present, either natural or artificial, and such a system can also be used to provide a carbonate sealing capacity for the future, as described hereinafter. The invention is described in a number of aspects, each directed to different natural/artificial conditions which are present at a site where alkaline buffering, or buffering plus sealing is desired to take place. The site itself may be considered in terms of 1) near or at the ground surface (natural or artificial); 2) subsurface unconsolidated materials; 3) subsurface excavations in bedrock. Detailed Description Broadly speaking, the invention involves the addition of certain chemicals and natural rock products in controlled amounts to a preselected area or zone at the ground surface, or in adjacent subsurface uunconsolidated materials, or in underground excavations in bedrock or other materials. These additions are for the purpose of producing and maintaining an alkaline environment in which calcite (CaCO₃) is deposited by any penetrating ground or surface waters that would potentially extract leachates if allowed to flow through unimpeded. This artificial control of the chemical environment at an application site as to pH and a capacity to precipitate calcite is referred to as chemical buffering. A long-term chemical support system to maintain an induced carbonate seal in the applications noted also provides additional carbonate sealing capability into the future. In this manner the buffering system is similar to the methodology for dry chemical surface addition of reactants to allow for subsequent (surface) water infiltration to achieve a cementing action. This alkaline environment (pH > 9.0) is also desirable because the oxides, hydroxides, and carbonates of most metals are of very low solubility (in fact, essentially insoluble) under such conditions, and thus are inhibited from entering any leachate waters. In some ground surface applications such as landfills, a sealing action to inhibit the ingress of meteoric precipitation can be obtained by the methods of US-A-4869621, but are maintained by repeated application during landfill operation in another aspect of this invention. If bedrock or surficial materials are to be directly sealed by calcite on site, then some, or all, of the chemicals may be applied to water solution, or as a slurry, as in US-A-4869621. Chemicals and rock products for a surface or subsurface alkaline buffer medium will normally be applied as mixed solid particulates tilled into the surface soil or landfill materials, or otherwise packed or used as a matrix in subsurface excavations. This invention is intended for application to ground above the water table with an ambient pH of 7.0 or greater; subsurface sites below the water table should be of low permeability (K < 1x10⁻⁵ cm/sec) for long term maintenance of buffer capacity. The alkaline buffer assemblage and calcite precipitating/sealing reactions used in this invention have been described in US-A-4869621. A schematic reaction in the buffer assemblage, to express its function, is: Ca(OH)₂ + Na₂CO₃ (+ H₂O) = CaCO₃ + 2NaOH (solution) or, alternatively CaCl₂ + Na₂CO₃ (+ H₂O) = CaCO₃ +2NaCl (solution) Expressed more generally, the essential reaction, in water solution, is: Ca²⁺ + CO₃²⁻ = CaCO₃ (solid), with the pH (OH⁻ concentration) being controlled by use of varying amounts (proportions) of such compounds as Ca(OH)₂, CaO, CaCl₂, CO₂ and NH₄OH. This buffered reaction serves to seal cracks, fractures, or porosity should water "leak" into or traverse the site where the alkaline buffer assemblage has been applied. In a first aspect of the invention, chemical components plus limestone or dolostone fines are combined to form an alkaline buffer zone in the sub-base soils required in the construction of new solid waste landfills, where located upon neutral to alkaline soils or carbonate bedrock, or a combination thereof, or where an alkaline soil zone is desired. The landfill sub-base is a prepared and graded soil beneath the liner(s) and leachate collection system(s). The presence of an alkaline buffer zone in the sub-base soil in such cases safeguards against leachate leakage beneath the landfill in the event of liner failure or small scale base soil-bedrock rupture as commonly occurs with settling or loading-excavation stress, or incidental seismicity. Any such leachate introduction into the alkaline buffer zone produces calcite deposition and sealing as described previously herein. In yet another aspect, the method of the invention is implemented by the addition or packing of chemical components and/or limestone/dolostone fines as a solid filler, or matrix, or host, or shield around and during the deposition of low-level radioactive wastes, for burial storage in underground excavations or storage in constructed underground chambers or vaults. In such application the alkaline buffer zone completely surrounding the chamber provides two functions, viz. 1) sealing against groundwater intrusion via rupture in the containing soils, bedrocks, or constructed vault, and 2) provision of an alkaline medium in proximity to any hazardous waste to render metals to the chemically least soluble forms. Specific examples of the foregoing aspects of the invention will now be discussed. In each case it will be remembered a desired or suitable site of alkaline character is selected and the requirements for sealing or alkaline buffering identified. Samples of soil, waste materials, bedrock, and groundwater may be analyzed by conventional techniques to aid in this determination. All of the chemicals and rock materials utilized in the aspects of this invention are inexpensive and precise control of amounts applied is unnecessary as long as the quantity is sufficient to effect the desired zone of buffering and/or sealing, and constituents are mixed on a fine scale where so required. (This statement is qualified to the extent that certain proportions of chemical reactants may be specified in applications partly or wholly addressed by US-A-4869621. Example I A sealed buffer zone may be established in the soil sub- base during initial preparation of a landfill site in or upon carbonate bedrock, where the site contains such naturally - occurring formations. The buffer zone is an artificial soil layer preferably up to 610 mm (2 feet) thick and is provided by depositing one or more of Na₂CO₃, Ca(OH)₂, lime, (NH₄)₂CO₃, NH₄HCO₃, urea, CaCl₂ and CaSO₄ upon the carbonate bedrock or other landfill excavation base in dry form, to provide for subsequent sealing of any cracks, fissures, etc. in the bedrock or overburden by artificially induced calcite precipitation from any penetrating leachate waters. The resulting calcite precipitation produces an effective seal, preventing leachate waters from the subsequantly formed landfill to enter the ground water below the buffer zone. Some typical reactions are schematically represented as follows: Example II A safety zone (seal) is provided around cannisters of low-level radioactive waste which are buried or deposited in underground chambers or vaults, such as abandoned mine drifts, etc. One or more of the aforementioned chemical components are placed in the soil, rock, or other formation surrounding the chamber in sufficient quantity to establish the required alkaline conditions for calcite precipitation to a degree effecting a moisture-inhibiting layer around the waste. The deposited materials should also be capable of ensuring that solid precipitates form to fill any cracks or voids which may form by seismic activity or other subsurface earth movement, which otherwise might permit groundwater intrusion. The chemical reactions are the same as those previously schematically illustrated.

Claims: A method of preparing a site located on alkaline soil or overburden or having a naturally-occurring layer of water-permeable carbonate bedrock for use as a landfill, which comprises the steps of: (a) analytically determining the pH of the bedrock and any ground-water therein; (b) contacting the soil or bedrock with an aqueous solution of one or more components which, in the presence of the bedrock and any ground-water, produce precipitated calcite, and continuing to produce the precipitated calcite until at least the upper stratum of the bedrock is substantially fluid-permeable; and (c) constructing an alkaline sub-base soil buffer zone, about 60 mm thick, of dry chemical components to serve as a barrier against future leachate incursion. The method of claim 1, wherein the one or more components are selected from Na₂CO₃, Ca(OH)₂, lime, (NH₄)₂CO₃, NH₄HCO₃, urea, CaCl₂ and CaSO₄. The method of either preceding claim, wherein the one or more components include limestone and/or dolostone fines. A method of disposing of low-level radioactive waste, which comprises the steps of: (a) placing the waste in an underground chamber; and (b) completely filling the balance of said chamber with a layer of material having physical and chemical characteristics such that, when the material is contacted by water, calcite is precipitated, thereby rendering the chamber essentially fluid-impervious. The method of claim 4, wherein the chamber is a mine passageway. The method of claim 4 or claim 5, wherein the chamber is initially completely surrounded by a layer having naturally alkaline conditions, and including the further steps of chemically determining the nature of those conditions and thereafter contacting the layer with an aqueous solution which, in those conditions, produces the calcite precipitate. The method of any of claims 4 to 6, wherein the waste is placed in a metal container prior to being placed in the chamber.

Inventors: MCLAREN EUGENE H, PUTMAN GEORGES W, YOUNG JAMES R, MCLAREN, EUGENE H., PUTMAN, GEORGES W., YOUNG, JAMES R.
Applicants: TERRAN RESEARCH INC, TERRAN RESEARCH, INC.
Application year: 1990
Main classification: E02D3
IPC codes: B09C1, B09B3, C09K17, B09B1, G21F9