Deep Well Injection: Chemical Wastes Disposed and Their Subusrface Reactions

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More than half of the liquid hazardous waste disposed of annually in the United States is disposed of by deep well injection. Little is known, however, about the chemical compositions of these wastes or about the subsurface reactions that could degrade hazardous compounds within them. This study presents the compositions of waste streams disposed of into Class I wells in Texas and their degradation processes. These Texas waste streams, which constitute 60 percent of the industrial waste disposed of annually by deep well injection in the United States, are probably representative of such injection nationwide. Phenols, chlorinated organics, cyanide, nickel, nitriles, and ketones-aldehydes compose 92 to 95 percent of the acute hazardous and toxic wastes disposed of annually.

Biodegradation, if it occurs in deep injection aquifers, is probably the most effective degradation process because it causes nearly complete consumption of the organic nutrient and can degrade the widest range of hazardous organic compounds. Hydrolysis, chemical interactions, sorption, and oxidation-reduction are other possible degradation processes. Degradation in the deep subsurface probably changes with distance from the wellbore. Oxidation and hydrolysis are likely near the wellbore, where solutions are oxidized and have extremely low or high pH values, whereas anaerobic microbial activity probably dominates in an outer zone where toxic compounds are more dilute, the solution is reduced, and pH conditions are near neutral.

All compounds in the waste solution must be considered when waste degradation processes are predicted. For example, generally nonhazardous carboxylic acids and their derivatives, present in 30 of the 98 waste streams studied, are highly reactive, and their presence in solution significantly affects the solution pH and hydrolysis and sorption reactions of hazardous compounds. Because carboxylic acids are generally favored as a primary substrate, their presence may either inhibit biodegradation of other primary substrates in the waste solution or enhance degradation of secondary substrates. Another common component in the organic waste streams, cyanide, is generally toxic to microbes and therefore inhibits biodegradation of other hazardous compounds. Field experiments backed up by laboratory experiments and numerical simulations are the best method for verification of waste degradation reactions. Waste-stream compositions could be altered before injection to enhance degradation and discourage unfavorable reactions using the relationships predicted from this type of experiment.

This report was submitted in fulfillment of Cooperative Agreement CA-814056-01-0 by the Bureau of Economic Geology under the sponsorship of the U.S. Environmental Protection Agency. This report covers a period from August 1, 1987, to July 31, 1988, and work was completed as of July 31, 1988.


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