Effects of hydraulic borehole mining on ground water at a test site in northeast St Johns County, Florida

Abstract

An experimental mining project was conducted in northeast St. Johns County, Florida, to determine the feasibility of extracting deeply buried phosphate ore by hydraulic borehole mining techniques. The phosphate zone is 18 feet thick, between 232 and 250 feet below land surface, and consists of approximately equal proportions of fine-grained phosphate, sand, and clay Six wells were constructed at the test site to monitor the effects of the mining operation on the ground-water resources of the area. One well penetrated the Floridan aquifer beneath the phosphate zone, another was completed in the phosphate zone, and the other four were finished in water-bearing zones above the phosphate zone. The mining project consisted of three separate tests during wh:j.ch different mining methods were used. Changes in water levels, other than in the mined zone, were detected only in the overlying monitored zone. These changes occurred in the water-bearing zone immediately above the phosphate zone during the first two tests and were caused by roof failures in the cavities formed by the mining operation. Water-quality changes were observed only in the mined zone and were a natural consequence of the injection of water from the deeper Floridan aquifer as part of the mining operation. INTRODUCTION From April 17 to August 30, 1980, an experimental mining project was conducted in northeast St. Johns County, Florida, by Agrico Mining Company (fig. 1). The U.S. Geological Survey, in cooperation with the U.S. Bureau of Mines, assisted in evaluating the effects of the mining project on the ground-water resources. The project utilized hydraulic borehole mining equipment and techniques developed by the U.S. Bureau of Mines. In the past, hydraulic borehole mining has been used for the extraction of coal, uranium, and oil sand that could not be mined feasibly using conventional methods (U.S. Bureau of Mines, 1978). The purpose of the project was to develop and test hydraulic borehole methods for the extraction of deeply buried phosphate ore and to study the economic feasibility and environmental impact of such an operation. The phosphate ore was contained in a zone 18 feet thick located between 232 and 250 feet below land surface.