Deep-well injection zones

Geochemical Characteristics of Deep-Well-Injection Zones. 810... 

Water with a salinity of less than 10,000 mg/L is considered to be a potential underground source of drinking water. By regulatory definition, deep-well injection of hazardous waste can occur only in very saline waters or brines. Actual salinities of waters in currently used deep-well injection zones vary greatly.70 Normally, the term brine is used to refer to the natural waters in deep-well injection zones. As noted above, however, this term is not technically correct if TDS levels are less than 35,000 mg/L. 

With few, if any, exceptions, deep-well injection zones will be sedimentary rock, and the reactions that take place when hazardous wastes are injected are determined largely by the physical and... 

This section provides information on the range of environmental conditions that occur in deep-well-injection zones in different geologic regions of the U.S. The section on lithology discusses the types of sedimentary formations that are suitable for deep-well injection and confining layers and provides some information on geologic formations that are used for deep-well injection of wastes. The section on brine chemistry discusses the typical range of chemical characteristics of formation waters found in injection zones. 

Metals are divided into light (also called alkali-earth metals) and heavy. All toxic metals are heavy metals except for beryllium and barium. Additionally, other categories of elements that are or may be significant chemically as dissolved species in deep-well-injection zones include the following ... 

Oxidation-reduction reactions may affect the mobility of metal ions by changing the oxidation state. The environmental factors of pH and Eh (oxidation-reduction potential) strongly affect all the processes discussed above. For example, the type and number of molecular and ionic species of metals change with a change in pH (see Figures 20.5-20.7). A number of metals and nonmetals (As, Be, Cr, Cu, Fe, Ni, Se, V, Zn) are more mobile under anaerobic conditions than aerobic conditions, all other factors being equal.104 Additionally, the high salinity of deep-well injection zones increases the complexity of the equilibrium chemistry of heavy metals.106... 

Calcium-sodium-chloride-type brines (which typically occur in deep-well-injection zones) require sophisticated electrolyte models to calculate their thermodynamic properties. Many parameters for characterizing the partial molal properties of the dissolved constituents in such brines have not been determined. (Molality is a measure of the relative number of solute and solvent particles in a solution and is expressed as the number of gram-molecular weights of solute in 1000 g of solvent.) Precise modeling is limited to relatively low salinities (where many parameters are unnecessary) or to chemically simple systems operating near 25°C. 

In most instances, hydrodynamic dispersion is not great enough to require detailed consideration in hydrogeologic modeling for fate assessment of deep-well-injected wastes. Flowever, regional variations (such as the presence of an USDW in the same aquifer as the injection zone, as is the case in parts of Florida) should be evaluated before a decision is made to exclude it. 

Formations from all geologic periods have been used for deep-well injection, but Paleozoic rocks are used for most injection zones (53%), followed by Tertiary-age formations (39%). Older Paleozoic rocks have been more frequently used for injection primarily because they tend to be more deeply buried. However, the more recent Tertiary-age Gulf Coast sediments are also very thick, and most injection in rocks of this age takes place there. 

Table 20.19 summarizes information about each study, including the location of the well, the lithology of the injection zone, waste characteristics, and the major geochemical processes observed. Current commercial-hazardous-waste, deep-well-injection facilities can be found on the Environment, Health and Safety Online (EHSO) web site.163... 

The USEPA surveys identified 17 pesticide plants using deep well injection for the disposal of wastewater [7]. One plant used incinerators to remove pesticides as well as benzene and toluene from the wastewater before disposal by deep well injection. Using deep well injection to dispose of hazardous wastes is expected to decrease in the future because of more stringent regulatory requirements and increased concerns about the long-term fate of these wastes in the injection zone. 

Partition processes determine how a substance is distributed among the liquid, solid, and gas phases and determine the chemical form or species of a substance. Partitioning usually does not affect the toxic properties of the substance. Partitioning can, however, affect the mobility of the waste, its compatibility with the injection zone, or other factors that influence fate in the deep-well environment. The major partition processes are as follows ... 

The previous chapter examined the geochemical processes that can occur in the deep-well environment. The type and outcome of reactions that will actually occur when a waste is injected, however, depend on its chemical characteristics and on injection-zone conditions. This chapter examines six major environmental factors that must be taken into consideration. 

In extreme situations, incompatibility between injection fluids and reservoir components can be so great that deep-well disposal will not be the most cost-effective approach to waste disposal. In other situations, such remedial measures as pretreatment or controlling fluid concentrations or temperatures can permit injection even when incompatibilities exist. In addition to operational problems, waste-reservoir incompatibility can cause wastes to migrate out of the injection zone (casing/confining-layer failure) and even cause surface-water contamination (well blowout). 

A primary injection well and a standby well are situated about 460 m (1500 ft) apart. A shallow monitoring well is located near the primary injection well in the upper limestone Floridan aquifer that overlies the confining Bucatunna clay. Two deep monitoring wells in the injection zone are located 300 m (1000 ft) southwest and 2492 m (8170 ft) northeast of the primary injection well. 

At the time injection began, a shallow monitoring well was placed 23 m (75 ft) south of the injection well in the upper part of the Floridan aquifer above the confining layer. A downgradient, deep monitoring well was placed in the injection zone 300 m (1000 ft) southeast of the injection well. Another shallow well, located 3.2 km (2 miles) southeast of the injection site at the University of Florida s Everglades Experiment Station, has also been monitored for near-surface effects. 

From May 1968 to December 1972, the waste was injected at a rate of about 300,000 gal/d. The first injection well was completed to a depth of 259 to 313 m (850 to 1025 ft) (i.e., cased from the surface to 259 m with screens placed in the most permeable sections of the injection zone to a depth of 313 m). One shallow observation well was placed 15 m (50 ft) east of the injection site at a depth of 210 m (690 ft). Four deep monitoring wells were also placed in the injection zone, one at 15 m (50 ft) and three at 45 m (150 ft) from the injection well. 

Apps, J.A., Current Geochemical Models to Predict the Fate of Hazardous Wastes in the Injection Zones of Deep Disposal Wells, Lawrence Berkeley Laboratory, Report LBL-26007, 1988. 

Finally, the type of waste injected into the well is a determinant in how deep the injection will be made. The more toxic the waste, the farther down the disposal zone must usually be. Disposal zones have been classified into five different types ... 

Apps, J.A., 1992. Current geochemical models to predict the fate of hazardous wastes in the injection zones of deep disposal wells. Lawrence Berkeley Laboratory report, LBL-26007. 

Disposal of liquid hazardous waste has been undertaken by injection into deep weiis iocated in rock below freshwater aquifers, thereby ensuring that poiiution of groundwater suppiies does not occur. In such instances, the waste is generally injected into a permeable bed of rock several hundreds or even thousands of metres below the surface, which is confined by relatively impervious formations. However, even where geological conditions are favourable for deep well disposal, the space for waste disposal frequently is restricted and the potential injection zones are usually occupied by connate water. Accordingly, any potential formation into which waste can be injected must possess sufficient porosity, permeability, volume and confinement to guarantee safe injection. Also, the piezometric pressure in the injection zone influences the rate at which the reservoir can accept liquid waste. A further point to consider is that induced seismic activity has been associated with the disposal of fluids in deep wells (see the following text). 

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