Controlling groundwater at PCS Phosphate mining operations–Aurora, Beaufort County,North Carolina

摘要

PCS Phosphate and its predecessors have mined and processed phosphate ore at the Aurora Phosphate Mine in Aurora, North Carolina since 1965. The mine is located in the central coastal plain where the average surface elevation near the mining operations is approximately twelve feet above sea level. The mining sequence includes the removal of forty feet of unconsolidated overburden by using bucket wheel excavators followed by the removal of seventy-five feet of overburden and thirty-five feet of phosphate ore by large walking draglines. Controlling the influx of groundwater into the open surface mine is required in order to maintain safe, dry mining conditions to accommodate the extraction of the phosphate ore to depths that can exceed one hundred fifty feet below sea level. PCS Phosphate controls groundwater ahead of the mine excavation by using wellpoints and shallow wells in the unconfined aquifer near the surface (Shallow Aquifer System). Typical withdrawals are approximately one million gallons per day. The ore deposit is underlain by a regionally extensive confined aquifer that exerts a significant pressure surface above the level of the ore excavation (Castle Hayne Aquifer System). A series of large diameter wells around the perimeter of the mine are utilized to depressurize the confined aquifer. Typical withdrawals are approximately sixty million gallons per day. Groundwater withdrawals at the mine are regulated by the North Carolina Department of Environment and Natural Resources (NCDENR). Two water withdrawal permits are granted by the NCDENR for the purposes of controlling groundwater for mining operations. Impacts from the aquifer dewatering and depressurization are monitored as a condition of these permits. Hydrologic monitoring includes the collection and analysis of groundwater data. The collection of the data began prior to mining and is ongoing. Groundwater withdrawals, groundwater levels, and water quality data are monitored. Additional hydrologic data are obtained from the United States Geological Survey (USGS). The groundwater monitoring system consists of a network of approximately two hundred twenty-five water level and water quality monitoring wells used to obtain data from the Shallow Aquifer System and the Castle Hayne Aquifer System.