In dry stacking, the top surface of the stack is continuously disturbed by the equipment used to place and spread the gypsum. Except in very high rainfall locations (such as Santos, Brazil which receives three meters of rain annually), where the surface of the stack never gets a chance to dry out, dust generated by construction equipment is a problem that can be controlled by limiting the size of the active gypsum placement area and/or haul roads and keeping their surfaces damp by spraying with water as needed.
With wet stacking, the containment starter dikes are best constructed with wet gypsum using draglines or hydraulic excavators, in which case dust is not generated during excavation and shaping. Only the maintenance and access roads, which are subjected to construction traffic, are susceptible to dusting, but they can readily be kept damp to control particulate emissions.
Fluorides
Fluoride emissions from phosphogypsum pond water surfaces are primarily HF and have been measured to be in the range of 0.01 to 0.10 kg/hectare/day , . Fluoride emissions from dry gypsum surfaces are associated with fugitive gypsum dust. The fluoride emissions can be significantly reduced if fluosilisic acid is extracted during the production of phosphoric acid when wet stacking is used and if fugitive dust is controlled on dry stacks.
Vegetation very close to a gypsum stack can contain elevated levels of fluorides, which if ingested by cattle for years, can cause fluorosis.
Radon and Radium
Radon flux has been measured from the surface of several existing phosphogypsum stacks in Central Florida , , . The mean flux from stack, side slopes and roads was essentially identical and averaged 23 picocuries per second per square meter. The flux level from areas that were periodically ponded averaged 2 picocuries per second per square meter. Radon, being a gas with a very short radioactive half life, is generally not an environmental issue.
Radium concentrations for phosphogypsum depend on the composition of the phosphate rock used in the process. Based on the same studies8,9,10, radium concentrations for Central Florida phosphogypsum of 20 to 30 picocuries per gram were measured. Phosphogypsums from other rock sources have even lower radium concentrations. For example, the radium concentration in gypsum from North Florida rock is below 10 picocuries per gram, the USEPA threshold for agricultural uses, and some of that gypsum is used in peanut farming. Radium impacts can be avoided by controlling fugitive particulate emissions.
Seepage and Other Process Water Discharges
Groundwater Contamination
Evaluation of groundwater impacts from unlined phosphogypsum gypsum disposal sites in Florida by Ardaman , the USEPA and U.S. Geological Survey have shown that heavy metals and radionuclide in the seepage from these facilities are effectively attenuated by the fine-grained soils and calcareous strata upon which most of these facilities are founded. Nevertheless, in 1993, the FDEP4 promulgated rules requiring that all new phosphogypsum stacks have an impervious bottom composite liner system, consisting of a 1.5-mm thick high density polyethylene (HDPE) geomembrane in combination with either a compacted clay layer beneath it, or a compacted gypsum layer over it . Fuleihan and Cameron15 discuss design details for a project in Florida (Figure 9) which established design criteria that were essentially adopted by the FDEP.
The FDEP rules require that, unless it is demonstrated that groundwater impacts will be contained indefinitely within the property line or within a designated zone of discharge, gypsum stacks and associated process water ponds at all phosphoric acid plants in Florida be retrofitted by March 2001 to contain any groundwater plume, or be closed and replaced by new lined facilities. The rules covering closure require that the top of the stacks be covered with a liner and the side slopes vegetated.
Ardaman is currently working on retrofitting several existing wet gypsum stacks in Florida to bring them into compliance with the FDEP rules. Several approaches are being taken, depending on site specific and operating conditions. In some situations, a vertical hydraulic barrier, consisting of soil-bentonite cutoff wall in combination with pressure relief wells, is being built around the perimeter of the site. In other cases, the top of the existing gypsum stack will be lined with an HDPE geomembrane (Figure 10), and then wet gypsum stacking will continue on top of the liner. To achieve the required side slope stability for the new stack on top of the liner, an underdrain system will be constructed on top of the liner around the perimeter, beneath the future outer slope of the stack.
In all but very arid climates, dry stacking does not eliminate acid water seepage out of the base and toe of the slope of the stack. ln desert climates, where dry stacking is sometimes used without an impervious bottom liner, the potential for groundwater contamination can also exist during relatively rare heavy rainfall events because vertical cracks frequently occur in such stacks as a result of self weight differential settlements and/or low stability factor of safety of the side slopes. The impact can be minimized by filling in with gypsum to prevent rainfall runoff from entering and filling the cracks, seeping deep into the stack, and reaching the underlying aquifer. This measure also avoids possible slope instability caused by hydraulic pressures generated by any water-filled cracks.
Discharges to Surface Waters
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