Since 1963, more than 6 billion gal of acidic industrial waste has been injected into a limestone aquifer near Pensacola, Florida. The industrial waste, an aqueous solution containing nitric acid, inorganic salts, and numerous organic compounds, is injected through two wells into the aquifer between depths of 1,400 and 1,700 ft (425-520 m). The aquifer receiving the waste is overlain by an extensive clay confining layer which, at the injection site, is about 200 ft (60 m) thick.
Industrial waste is presently (late 1971) being injected at a rate of about 2,100 gal per minute. Wellhead injection pressures are about 175 psi. Calculations indicate that pressure effects in the receiving aquifer extend out more than 30 mi (48 km). No apparent change in pressure has been detected in the aquifer directly above the clay confining layer.
Geochemical effects were detected at a monitor well in the receiving aquifer 0.25 mi (0.4 km) from the injection wells about 10 months after injection began. The geochemical effects included increases in calcium-ion concentration and total alkalinity and formation of large quantities of nitrogen and methane gas. Geochemical ef-fects have not been detected at monitor wells in the receiving aquifer 1.9 mi (3.0 km) north and 1.5 mi (2.4 km) south of the injection wells, nor have effects been detected in a monitor well at the injection site open to the aquifer directly overlying the clay confining layer.
Tests made at the injection wells early in 1968 indicated that rapid denitrification and neutralization of the waste occurred near the injection wells. Denitrification may have accounted for more than half the neutralization, and solution of calcium carbonate accounted for the rest. Denitrification has not been observed since mid-1968, when the pH of the injected waste was lowered from 5.5 to 3.