Brine Chemistry

Brines are classified according to their chemical constituents. At least nine distinct types are recognized by petroleum geologists, but most brines encountered in injection operations are either Na-Cl or Na-Ca-Cl brines.78 None is similar to seawater, and the geochemical mechanisms by which such brines develop are not well-understood. Three mechanisms have been proposed to explain the high concentrations of dissolved solids and the chemical composition of brines, but at present there is no consensus on their relative importance in explaining brine chemistry.78 The dominant mechanism at work in a deep-well environment has important implications for the hydrodynamic conditions affecting the movement of injected wastes. The mechanisms and their implications are summarized in Table 20.13. The salinity, pH, and chemical composition of the very saline and briny waters into which hazardous wastes are injected can vary greatly, both among geologic basins and within a single formation.

The maximum salinities in the Tertiary section of the Gulf of Mexico basin (the most extensively used strata for deep-well injection) reach almost four times that of seawater. The Michigan basin has the highest salinity, reaching 400,000 mg/L TDS, more than 11 times that of seawater. In Florida, however, where seawater circulates through the Floridan aquifer, maximum salinities tend to be controlled by the salinity of the seawater.79

The Frio formation, in Texas, receives more hazardous waste by volume through deep-well injection than any other geologic formation in the U.S. The average salinity of this formation is about twice that of seawater (72,185 mg/L TDS), but individual samples range from a low of 10,528 mg/L TDS (barely above the salinity cutoff for potential USDWs) to a high of more than 118,000 mg/L TDS. Data from sites in Illinois and North Carolina indicate the presence of very saline water (around 20,000 mg/L TDS, but still less saline than seawater).

The pH of formation waters in the Frio formation varies widely from moderately acidic (5.7) to moderately alkaline (8.2), with nearly neutral averages (6.8). The pH of formation waters from other injection sites tends to be more alkaline, ranging from slightly alkaline (Belle Glade, Florida, pH 7.5) and moderately alkaline (Wilmington, North Carolina, pH 8.6), to very alkaline (Marshall, Illinois, pH 7.1 to 10.7).

20.4.3 Influence of Environmental Factors on Waste/Reservoir Compatibility

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

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