Hydrologic monitoring of waste-injection wells near Pensacola, Florida, March 1970 - December 1977

Abstract

This report presents hydraulic and chemical data collected from March 1970 to December 1977 at a deep-well waste-injection system at the Monsanto Company's plant near Pensacola, Florida. The injection system presently consists of Injection Well A, Injection Well B, and two deep monitor wells all completed open hole in the lower limestone of the Floridan aquifer and one shallow monitor well completed in the upper limestone of the Floridan aquifer. The two deep monitor wells are used to observe hydraulic and geochemical effects of waste injection in the injection zone at locations 1.5 miles south-southeast and 1.9 miles north-northwest of the center of the injection site. The shallow monitor well, used to observe any effects in the first permeable zone above the 220-foot-thick confining bed, is 100 feet northeast of Injection Well A. Since injection began in July 1963, about 14.2 billion gallons of acidic industrial waste consisting of an aqueous solution of nitric acid, inorganic salts, and numerous organic compounds have been injected into a limestone aquifer containing saline water. From July 1963 to April 1968, the pH of the waste was raised to about 5.5 with aqueous ammonia prior to injection. Since 1968, no aqueous ammonia has been added and the waste has been injected at a pH of about 2.3. Wellhead injection pressures at both injection wells in December 1977 averaged 170 pounds per square inch and the hydraulic pressure gradient was 0.52 pound per square inch per foot of depth to the top of the injection zone. Increases in pressures since 1970 at the north and south monitor wells ranged from 21 to 31 pounds per square inch. The pressure in the shallow monitor well declined about 6 pounds per square inch. No changes were detected in the chemical characteristics of water from the shallow monitor and the north monitor wells. Since late 1973, concentrations of bicarbonate and dissolved organic carbon in water from the south monitor well have increased. These increases in bicarbonate and dissolved organic carbon have been accompanied by increases in gas content of water at the point of discharge from the well and a distinctive odor like that of the waste. In samples of water from the south monitor well, concentrations of nitrogen gas ranged from 5.1 to 12.7 milligrams per liter, methane from 24 to 70 milligrams per liter, and carbon dioxide from 14 to 56 milligrams per liter.