Monsanto operates one of the world's largest nylon plants on the Escambia River about 13 miles north of Pensacola, Florida. The construction, operations, and effects of the injection-well system at this site have been extensively documented by the U.S. Geological Survey in cooperation with the Florida Bureau of Geology. Pressure and geochemical effects are reported by Goolsby,67 Faulkner and Pascale,167 and Pascale and Martin.168 Additional microbiological data are reported by Willis and colleagues169 and Elkan and Horvath.170 Major chemical processes observed at the site include neutralization, dissolution, biological denitrification, and methanogenesis.
The waste is an aqueous solution of organic monobasic and dibasic acids, nitric acid, sodium and ammonium salts, adiponitrile, hexamethylenediamine, alcohols, ketones, and esters.67 The waste also contain cobalt, chromium, and copper, each in the range of 1 to 5 mg/L. Wastestreams with different characteristics, produced at various locations in the nylon plant, are collected in a large holding tank; this composite waste is acidic. The specific characteristics of the waste varied somewhat as a result of process changes (e.g., after 1968 more organic acids and nitric acid were added). Until mid-1968, wastes were partially neutralized by pretreatment. After that, unneutralized wastes were injected. No reason was reported for suspending treatment. Goolsby67 reports pH measurements ranging from a high of 5.6 in 1967 (at which time the pH was raised before injection by adding aqueous ammonia) to a low of 2.4 in 1971, and Eh ranging from +300 mV in 1967 to + 700 mV in 1971. The chemical oxygen demand in 1971 was 20,000 mg/L.
Monsanto began injecting wastes into the lower limestone of the Floridan aquifer in 1963. In mid-1964, a second well was drilled into the formation about 300 m (1000 ft) southwest of the first. A shallow monitoring well was placed in the aquifer above the confining layer about 30 m (100 ft) from the first injection well, and a deep monitoring well was placed in the injection zone about 400 m (1300 ft) south of both injection wells. The deep monitoring well (henceforth referred to as the near-deep monitoring well) was plugged with cement in 1969. In late 1969 and early 1970, two additional deep monitoring wells were placed in the injection formation, 2.4 km (1.5 miles) south-southeast (downgradient) and 3 km (1.9 miles) north-northwest (upgradient) of the site. From 1963 to 1977, about 50 billion liters (13.3 billion gal) of waste were injected. During the same period, injection pressures ranged from 8.8 to 16.5 kg/cm2 (125 to 235 psi). Since then, a third injection well has been added.
Ten months after injection of neutralized wastes began, chemical analyses indicated that dilute wastes had migrated 1300 ft to the nearest deep monitoring well. Injection of unneutralized wastes began in April 1968. Approximately 8 months later, unneutralized wastes reached the near-deep monitoring well, indicating that the neutralization capacity of the injection zone between the injection wells and the monitoring well had been exceeded. At this point, the monitoring well was plugged with cement from bottom to top because operators were concerned that the acidic wastes could corrode the steel casing and migrate upward.67 The rapid movement of the waste through the limestone indicated that most of it migrated through a more permeable section, which was about 20 m (65 ft) thick. By mid-1973, 10 years after injection began, a very dilute waste front arrived at the south monitoring well, 2.4 km (1.5 miles) away. As of early 1977, there was no evidence that wastes had reached the upgradient monitoring well. The shallow monitoring well remained unaffected during the same period.
Increases in permeability caused by limestone dissolution approximately doubled the injection index (the amount of waste that can be injected at a specified pressure). As of 1974, the effects of the pressure created by the injection were calculated to extend more than 40 miles radially from the injection site.167 An updip movement of the freshwater/saltwater interface in the injection-zone aquifer, which lies less than 32 km (20 miles) from the injection wells, was also observed.
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