Geochemical fate

Processes Affecting the Geochemical Fate of Deep-Well-Injected Wastes. 790... 

May be a significant consideration in well design and geochemical fate Not a significant consideration under injection conditions... 

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. 

PROCESSES AFFECTING THE GEOCHEMICAL FATE OF DEEP-WELL-INJECTED WASTES... 

At the simplest level, the processes that most influence geochemical fate can be divided into three groups partition, transformation, and transport ... 

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 ... 

If osmotic effects are possible, several other effects would need to be considered in a geochemical-fate assessment, depending on whether the solute concentration is increased or decreased. If solute concentrations are increased, pressures associated with injection would increase beyond those predicted without osmotic effects. Also, the movement of ions to the injection zone from the aquifer with lower salinity (above the clay confining layer) would increase the salinity above those levels predicted by simple mixing of the reservoir fluid and the injected wastes. This action could affect the results of any geochemical modeling. 

Major Environmental Factors Influencing Geochemical-Fate Processes... 

This section focuses on environmental conditions that may result in physical or chemical incompatibilities between wastes and reservoirs. Determining the potential for incompatibility is a part of the geochemical fate assessment that must be undertaken for any injection project because of possible... 

Major Processes and Environmental Factors Affecting Geochemical Fate of Hazardous Inorganics... 

The major processes affecting the geochemical fate of hazardous inorganics are acid-base adsorption-desorption, precipitation-dissolution, complexation, hydrolysis, oxidation-reduction, and catalytic reactions. The significance of these processes to inorganic wastes is discussed only briefly here additional information on individual elements is given in Table 20.16. 

Acid-base equilibrium is very important to inorganic chemical reactions. Adsorption-desorption and precipitation-dissolution reactions are also of major importance in assessing the geochemical fate of deep-well-injected inorganics. Interactions between and among metals in solution and solids in the deep-well environment can be grouped into four types1 2 3 4 ... 

The geochemical interactions possible between an injected waste and the reservoir rock and its associated fluids can be quite complex. Thus a combination of computer modeling, laboratory experimentation, and field observation will inevitably be necessary to satisfy current regulatory requirements for a geochemical no-migration deep-well injection. This section covers the computer methods and models available for predicting geochemical fate. 

The expression of chemical fate can be computerized using a code to perform the computations and predict the results when inputs simulating conditions of interest are provided. Two critical aspects of the use of computer codes for predicting geochemical fate are the verification and validation of the models on which the codes are based. 

This section discusses how held studies can be used in geochemical fate assessment and includes six cases of deep-well-injection facilities, documenting the geochemistry of the injected hazardous and other industrial wastes. Each case study is organized in the same format, with section headings as follows ... 

This case study is interesting in that one hazardous waste (acrylonitrile) was quickly rendered nonhazardous after injection, whereas another (sodium thiocyanate) showed no evidence of decomposition during the duration of the study. The implication for geochemical fate assessment is that research should focus on the compounds likely to be most resistant to decomposition or immobilization, as they will be the ones most critical in demonstrating containment in a no-migration petition. 

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