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Since 1963 more than 4 billion gal of acidic industrial waste has been injected into a limestone aquifer near Pensacola, Florida. This aquifer is overlain by an extensive clay confining layer, which at the injection site, is approximately 200 ft thick. The waste--an aqueous solution containing organic acids, nitric acid, inorganic salts, and numerous organic compounds--is injected into the aquifer through two wells 1,300 ft apart between depths of approximately 1,400 and 1,700 ft. The present pH of the injected solution is about 2.5; however, before April 1968 the pH was about 5.5.
Approximately 10 months after waste injection began, an increase in calcium-ion concentration and alkalinity was detected in a monitor well open only to the receiving aquifer and located 0.25 mi south of the injection wells. About 5 months after lowering the waste pH, an abrupt increase in calcium-ion concentration followed by an increase in nitrate-ion concentration was observed at this monitor well. Large quantities of nitrogen and methane gas were present in water from the monitor well. This information plus other available data indicate that a major part of the waste is entering the upper 30-50 ft of the aquifer and that waste has moved more than 1 mi from the injection site. Geochemical effects have not been detected in two monitor wells open only to the receiving aquifer and loc ted 1.5 mi south and 1.9 mi north of the injection site nor in a shallow monitor well at the injection site which is open only to the aquifer just above the clay confining layer.
Before the pH of the waste decreased, backflushing tests indicated that denitrification and neutralization of the waste occurred within a very short distance from the injection wells. Denitrification may have accounted for more than half the neutralization, the remainder being caused by solution of calcium carbonate. Denitrification has not been observed since the waste pH was lowered.
Wellhead injection pressures were about 200 psi before the decrease in waste pH but subsequently have decreased slightly to about 175 psi, even though injection rates have increased from about 1,600 to about 2,000 gpm. Wellhead pressures in the monitor wells south and north of the injection site are about 108 psi, which is approximately 90 psi above the preinjection pressure. Calculations indicate that pressure effects in the receiving aquifer may extend more than 30 mi from the injection site. No apparent change in pressure has been detected in the aquifer directly above the clay confining layer.
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