Characteristics of Chemical Processes That May Be Significant in the Deep Well Environment

Characteristic

Homogeneous Heterogeneous Reversible

Irreversible

Types of Reactions

Acid-base, hydrolysis, hydration, neutralization, oxidation-reduction, polymerization, thermal degradation Adsorption-desorption, precipitation-dissolution, immiscible-phase separation, biodegradation, complexation Acid-base, neutralization, oxidation-reduction (most inorganic and some biologically mediated), adsorption-desorption, precipitation-dissolution, complexation Hydrolysis, oxidation-reduction (biodegradation of anthropogenic inorganics), immiscible-phase separation

Source: U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste: A Reference Guide, EPA/625/ 6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.

Thus, a substance may be in a solid form or in solution (described by the precipitation-dissolution process), but its toxicity remains unaltered regardless of form. The form or state of a substance, however, influences the transformation and transport processes that can occur. For this reason, partition processes are important to define in a fate assessment.

2. Transformation processes alter the chemical structure of a substance. In the deep-well environment, the transformation processes that may occur are largely determined by the conditions created by partition processes and the prevalent environmental factors. Transport processes do not need to be considered if transformation processes irreversibly change a hazardous waste to a nontoxic form.

3. Transport processes carry wastes through the subsurface environment and must be considered in a fate assessment if the interaction of partition and transformation processes does not immobilize or alter the hazardous waste. Waste migration can take place either in solution or in solid form (particle migration).

Table 20.4 presents the partition and transformation processes known to occur in the near-surface environment along with the special factors that should be considered when evaluating data in the context of the deep-well environment. Geochemical processes affecting hazardous wastes in deep-well environments have been studied much less than those occurring in near-surface environments (such as soils and shallow aquifers). Consequently, laboratory data and field studies for a particular substance may be available for near-surface conditions, but not for deep-well conditions.

As Table 20.4 shows, several processes can occur in both the near-surface and deep-well environments. For example, neutralization of acidic or alkaline wastes is a straightforward process, and although temperature differences between the two environments may need to be considered, no other factors make the deep-well setting distinctly different. The same holds true for oxidation-reduction (redox) processes.

The remaining processes, although they occur under near-surface and deep-well conditions, are less applicable to the latter. Distinct differences between the two environments, however, can lead to significant differences in how the processes affect a specific hazardous substance. Compared with the near-surface environment, the deep-well environment is characterized by higher temperatures, pressures, and salinity, and lower organic matter content and Eh (oxidation-reduction potential).

Table 20.5 lists the partition and transformation processes applicable in the deep-well environment and indicates whether they significantly affect the toxicity or mobility of hazardous wastes. None of the partition processes results in detoxification (decomposition to harmless inorganic constituents), but all affect mobility in some way. All transformation processes except complexation can result in detoxification; however, because transformation processes can create new toxic substances, the mobility of the waste can be critical in all processes except neutralization.

TABLE 20.4

Near-Surface Geochemical Processes and Their Relevance to the Deep-Well Environment

Surface Data Applicability to Deep-Well Environment

Process

Partition Processes Acid-base equilibria

Adsorption-desorption

Precipitation-dissolution

Immiscible-phase separation

Transformation Processes Volatilization Photolysis Biodegradation

Complexation Hydrolysis

Neutralization Oxidation-reduction

Comments

Partly Near-surface studies tend to investigate fresh or moderately saline water, which creates quite different conditions for acid-base equilibria. Studies of ocean geochemistry come closest to approximating deep-well conditions.

Partly Mechanisms for adsorption on similar materials will be similar.

Soil adsorption data generally do not reflect the saturated conditions of the deep-well environment. Organic-matter content is a major factor affecting adsorption in the near-surface; its significance in the deep-well environment is less clear. Fate studies involving artificial recharge are probably useful, but differences between fresh waters and deep brines may reduce relevance.

Partly Higher temperatures, pressures, and salinity of the deep-well environment may result in significant differences between reactions in the two environments.

No Fluids (such as gasoline) that are immiscible in water are a significant consideration in near-surface contamination. Deep-well injection is limited to wastestreams that are soluble in water. Well blowout from gaseous carbon dioxide formation is an example of this process that is distinct to the deep-well environment.

No No atmosphere.

No No sunlight.

Partly Some near-surface bacteria appear capable of entering and surviving in the deep-well environment. However, in general, temperature and pressure conditions in the deep-well environment are unfavorable for microbiota that are adapted to near-surface conditions. Biological transformations are primarily anaerobic.

Partly Humic substances are very significant factors in near-surface complexation processes, probably less so in the deep-well environment. Data on complexation in saline waters are probably most relevant.

Partly Basic processes will be the same. Higher salinity of deep-well environment may affect rate constants.

Partly Basic process is the same, but some adjustments may be required for pressure/temperature effects.

Partly The deep-well environment tends to be more reducing than the near-reduction surface environment, but equally reducing conditions occur in the near-surface.

Some adjustments may be required for pressure/temperature effects.

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