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How Does Phosphogypsum Storage Affect Groundwaters?

05-042-142Final

Perspective

The Florida Institute of Phosphate Research, through its “Strategic Research Priorities,” currently is stressing six program areas, three that are technology oriented and three that are more environmental in nature. A major focus is on phosphogypsum because the issue is of high priority from both technical and environmental standpoints. While one question is what could be done with the material, another question is what are the consequences of doing nothing with it and simply leaving it stockpiled on the ground? A major concern about phosphogypsum storage is the potential for contamination of groundwater under and near the stacks. Data from previous studies have shown there is some influence on the aquifers from the presence of the gypsum stack in terms of several chemical parameters, but evidence for radionuclide contamination is much less clear. This study addresses the specific question of whether or not a phosphogypsum stack contributes any significant amount of radionuclides to underlying aquifers. The approach taken was production of a flow model for an inactive stack, followed by a characterization of stack solutions and groundwaters, and solid phase gypsum core samples.

Phosphogypsum is composed mostly of calcium sulfate and is a by-product of the reaction between sulfuric acid and phosphate rock in the manufacture of phosphoric acid. Currently almost one billion tons of the material are stockpiled, or stacked, on the ground in central and north Florida, and more than thirty million tons are being added each year. Phosphogypsum, its uses and its environmental impacts, has been a priority of the Institute since virtually its inception twenty years ago. Large-scale uses, such as in road construction or in agriculture, have been mostly banned by the Environmental Protection Agency due to elevated levels of radionuclides. Material from central Florida typically contains about twenty-five picoCuries per gram of radium-226, although north Florida gypsum is much lower. Thus most material continues to be stored on the ground.

This study suggests that most radionuclides present in groundwater under and near phosphogypsum stacks are there because of the natural geology of the region, and not because of the presence of the stack. Model results indicate that at most one per cent of infiltrating water ever reaches the aquifer, most of the rest being intercepted by ditch drains around the stack. Radium-226 levels in stack fluids are only slightly elevated above background groundwater values, and are less than those found in most area monitor wells. It appears that various removal mechanisms, including adsorption within the stack, precipitation just below the stack, and especially interception by drainage ditches, prevent large-scale migration of radionuclides to the underlying aquifers.

ABSTRACT

This study was initiated to investigate the processes responsible for controlling the interaction and release of radionuclides from phosphogypsum from a phosphogypsum stack at Piney Point Phosphates. Our approach consisted of: (i) flow modeling of the stack-aquifer system; (ii) chemical/radiochemical characterization of fluid samples from monitor wells placed around and directly into the stack; and (iii) geochemical modeling. The flux analysis showed that only a small amount (approx 1%) of the infiltrating stack fluids escapes capture by the drainage ditches. Analyses of the stack solutions showed them to be acidic with high ionic strength, containing high total dissolved solids (18,700 plus or minus 2300 ppm) with a pH of 2.43 plus or minus 0.10. The stack wells are exceptionally high in activities of uranium (generally 600-1000 dpm/L 238U) and 210Pb (generally 400-4000 dpm/L). Concentrations of 226Ra in the stack fluids, however, are only slightly elevated (range about 5-10 dpm/L) above normal groundwater values and are actually less than most of the monitor well concentrations measured around the Piney Point gypsum stack. Our observations suggest that various removal mechanisms including adsorption within the stack, precipitation within the soil horizons just below the stack, and interception of stack fluids by drainage ditches, prevent large scale migration of radionuclides to the underlying aquifer.