An insoluble liquid or gas will separate from water, resulting in immiscible-phase separation. The behavior of nonaqueous-phase liquids (NAPLs) that may be lighter (LNAPLs) or denser (DNAPLs)
than water is important in near-surface groundwater contamination studies.40 However, aqueous-phase separation is generally not an issue in the deep-well environment because injected hazardous wastes are usually dilute. Failure to remove immiscible oily fluids from injected wastes potentially may cause plugging in the injection zone. Density and viscosity differences between injected and reservoir fluids, however, may need to be considered in transport modeling. Generally, pressures are high enough in the deep-well environment to keep gases such as carbon dioxide, generated as products of waste-reservoir interactions, in solution. Under certain conditions of high temperature and high waste concentrations, however, injected hydrochloric acid can cause carbon dioxide to separate from the liquid and produce a well blowout.
Transformation processes change the chemical structure of a compound. Because not all transformation processes convert hazardous wastes to nonhazardous compounds, geochemical fate assessment must consider both the full range of transformation processes that may occur and the toxicity and mobility of the resulting products. For deep-well-injected wastes, transformation processes and subsequent reactions may lead to one or more of the following:
Detoxification is an irreversible change in a substance from toxic to nontoxic form. For example, when an organic substance breaks down into its inorganic constituents, detoxification has taken place. Transtoxification occurs when one toxic compound is converted into another toxic compound. Toxification is the conversion of a nontoxic compound to a toxic substance. Table 20.6 lists some examples of each.
Transformation processes that may be significant in deep-well-injection fate assessments are as follows:
7. Thermal degradation
Two other processes that may transform hazardous wastes are photolysis and volatilization, but they are not considered here because they do not occur in the deep-well environment.
Acidic wastes with a pH of <2.0 and alkaline wastes with a pH of >12.5 are defined as hazardous (40 CFR Part 261). To meet the regulatory definition of nonhazardous, acidic wastes must be neutralized to a pH of >2.0 by reducing the hydrogen ion concentration, and alkaline wastes must be neutralized to a pH of <12.5 by increasing the hydrogen ion concentration.
Carbonates (limestone and dolomite) will dissolve in and neutralize acidic wastes with the following process:
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