Phosphogypsum Stack Reclamation: Analysis and Best Practices

Introduction and Problem Statement

The types of contaminants that emanate from anthropogenic sources are extremely varied and range from simple inorganic ions — such as nitrate from septic tanks, feedlot wastes, and fertilizer use; chloride from highway deicing salts, saltwater intrusion, and certain industrial processes; and heavy metal ions from industrial processes such as plating works — to complex synthetic organic compounds resulting from industrial and manufacturing processes and from the use of pesticides and household cleaning fluids (e.g., trichloroethylene). Some compounds of low solubility may yield solutions that are toxic or offensive. The chemical composition of wastes deposited in landfills or surface impoundments is frequently known; nevertheless, when the constituents of such wastes interact, new compounds may be created. A number of industrial waste-disposal practices now include the stabilization of wastes, thereby making them less chemically active; however, it is still possible that leachate production may transform some of the constituents (Patrick, Patrick & Pye, 1987). One such industrial waste is phosphogypsum, which is primarily composed of calcium sulfate and is a by-product of the reaction between sulfuric acid and phosphate rock in the manufacture of phosphoric acid (How Does Phosphogypsum Storage Affect Groundwaters?, 2002).

A major focus of this study 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 is what the consequences are of doing nothing — simply leaving it stockpiled on the ground. Today, a major concern about phosphogypsum storage is the potential for contamination of groundwater under and near the stacks. The growing body of evidence from past research has demonstrated that there is some influence on aquifers from the presence of gypsum stacks in terms of several chemical parameters, but evidence for radionuclide contamination remains much less clear (How Does Phosphogypsum Storage Affect Groundwaters?, 2002). The closure of a phosphogypsum stack with a plastic liner and soil cover on top is also very expensive (Patel, Ericson & Kelley, 2002). Therefore, identifying the best approach to remediating phosphogypsum stacks for reclamation purposes has assumed an increasing level of importance.

In the United States, federal standards for new, existing, or expanding municipal solid waste landfills (40 CFR Part 258) are generally followed by the states, including technical design requirements for membrane liners with compacted soil or clay and/or double liners, leachate collection systems, leak detection and groundwater monitoring systems, and final cap and cover provisions (Smith, 1998). Similarly, Canada has in place regulations that address phosphogypsum stack management, but no firm consensus has been developed as to the most appropriate methods to be used, given the unique factors associated with every phosphogypsum stack. The purpose of this study is therefore to identify the extent to which reclamation techniques are effective in preventing contamination of groundwater sources — including liner applications and other stack closure techniques in use throughout the United States and around the world — in order to arrive at recommendations for a best practices model to be followed in phosphogypsum stack reclamation efforts.

Most authorities agree that "toxic waste disposal" is a misnomer; a better phrase would be "toxic waste storage," since these materials have a lengthy half-life and must be regarded as potentially hazardous for many years — in some cases for centuries or more. To the extent that a best practices approach can be identified and implemented, the risks associated with phosphogypsum stacks can be meaningfully reduced.

All terms used in this study follow the definitions provided by the State of Florida for phosphogypsum stack management.

Aquifer. A geologic formation, group of formations, or part of a formation capable of yielding a significant amount of groundwater to wells, springs, or surface water.

Closing. The time at which a phosphogypsum stack system ceases to accept wastes, including those actions taken by the owner or operator of the facility to prepare the system for any necessary monitoring and maintenance after closing.

Closure. The cessation of operation of a phosphogypsum stack system and the act of securing such a system so that it will pose no significant threat to human health or the environment. This term also includes closing, long-term monitoring, maintenance, and financial responsibility.

Disposal. The discharge, deposit, injection, dumping, spilling, leaking, or placing of any solid waste into or upon any land or water so that such solid waste or any constituent thereof may enter other lands, be emitted into the air, or be discharged into any waters, including groundwaters, or otherwise enter the environment.

Facility. All contiguous land and structures, other appurtenances, and improvements on the phosphate fertilizer manufacturing complex.

Final cover. Materials used to cover the top and sides of a phosphogypsum stack upon closure.

Geomembrane. A low-permeability synthetic membrane used as an integral part of a system designed to limit the movement of liquid or gas in the system.

Regulatory Background and Hazardous Waste Policy

Lateral expansion. The expansion, horizontally, of phosphogypsum or process wastewater storage capacity beyond the permitted capacity and design dimensions of the phosphogypsum stack, or cooling ponds, surge ponds, and perimeter drainage conveyances at an existing facility. For the purposes of this study, this term means any phosphogypsum stack, cooling pond, surge pond, or perimeter drainage conveyance constructed within 2,000 feet of an existing phosphogypsum stack system, measured from the edge of the expansion nearest to the edge of the footprint of the existing phosphogypsum stack system.

Leachate. The liquid that has passed through or emerged from phosphogypsum.

Liner. A continuous layer of low-permeability natural or synthetic materials that controls the downward and lateral escape of waste constituents or leachate from a phosphogypsum stack system.

Phosphogypsum. A preparation of calcium sulfate and its byproducts produced by the reaction of sulfuric acid with phosphate rock to produce phosphoric acid. Phosphogypsum is a solid waste within the definition of Section 403.703(13), F.S.

Phosphogypsum stack. Any defined geographic area associated with a phosphoric acid production facility in which phosphogypsum is disposed of or stored, other than within a fully enclosed building, container, or tank.

Phosphogypsum stack system. For the purposes of this study, this term means the phosphogypsum stack (or pile, or landfill), together with all pumps, piping, ditches, drainage conveyances, water control structures, collection pools, cooling ponds, surge ponds, and any other collection or conveyance system associated with the transport of phosphogypsum from the plant to the phosphogypsum stack, its management at the stack, and the process wastewater return to the phosphoric acid production or other process. This definition specifically includes toe drain systems, ditches, and other leachate collection systems, but does not include conveyances within the confines of the fertilizer production plant or existing areas used in emergency circumstances caused by high-volume or extended rainfall events for the temporary storage of process wastewater to avoid discharges to surface waters.

While this study examines phosphogypsum stack management as practiced around the world, the focus is on facilities in North America in general and Canada in particular. This is a historical/case study analysis of what problems face companies in their toxic waste disposal practices — in general and for phosphogypsum stack systems in particular — and what steps policymakers can take to help them achieve a safe disposition of these potentially hazardous materials.

According to Frazer, Gilroy, and Rabe (1994), the domestic application of industries that require toxic materials as part of their operation has traditionally been characterized as a collective good worth any inherent risk in its development. For example, millions of North Americans enjoy relatively inexpensive energy from modern nuclear power plants: "Thousands benefit from the post-World War II application of nuclear technology to medicine. From the formation of national regulatory entities in both nations through the 1970s, few people in either nation challenged the conventional wisdom that massive federal government subsidies for the development of nuclear power and medicine were anything other than a worthy endeavor which served broad, collective goals" (p. 67). That consensus, however, has been increasingly challenged in recent years, in part because of the issue of radioactive waste disposal. Disasters such as Chernobyl and near-disasters such as Three Mile Island drew attention to the safety of facilities generating nuclear power, but the issue of waste disposal poses a separate set of challenges for both nations. While nuclear power and nuclear medicine are perceived as collective goods, Canadians and Americans alike recognize radioactive waste as a threat to public health, environmental protection, and the economic stability of any community that might become contaminated (Frazer et al., 1994).

These threats were recognized early throughout North America, but regulatory bureaucracies and powerful business interests delayed substantive legislation for a number of years. For instance, the 1976 Resource Conservation and Recovery Act (RCRA) emphasized safer, better methods of land disposal. These new rules called for standards for liners, leak detection systems, leachate collection, and other initiatives. A number of significant regulations from the Environmental Protection Agency (EPA) were delayed for four years, however, and then emerged only in response to growing public concern over the many source points releasing toxic wastes across the country.

According to Mazmanian and Morell (1992), by the time EPA's landfill regulations appeared in 1980, initiatives in several states had already begun to shift focus away from landfilling of the continuing flow of huge volumes of new hazardous wastes. As more information became available on the dangerous environmental consequences of relying on land disposal for these wastes, Congress took steps to redress the balance in favor of treatment. In late 1984, the Hazardous and Solid Waste Amendments (HSWA) to RCRA set a new tone for national hazardous waste policy. Strict congressional guidelines called on the EPA to ban land disposal nationwide of most hazardous wastes, including the entire "California list." Federal bans were to take effect between 1989 and 1992. Illinois adopted similar restrictions, though on a faster schedule; California accelerated its phase-out program.

Despite popular misconceptions, most prescribed hazardous waste treatment is neither exotic nor exorbitantly expensive. Perhaps 80 to 90 percent of all hazardous wastes generated in the United States could be treated by modern variants of 1930s-era wastewater treatment technologies (Mazmanian & Morell, 1992). Acids can be neutralized while held in steel tanks; metals can readily be removed from liquid waste streams. In both cases, the remaining liquids can then be discharged safely into sewers available in every industrial area. Waste oils and solvents can be recycled through traditional separation and distillation techniques, akin to small refinery operations. Another 10 percent of today's hazardous wastes probably require thermal destruction in a rotary-kiln or fluidized-bed incinerator, which can be equipped to function safely using the latest air pollution control technology (Mazmanian & Morell, 1992).

Radiation, Leachate, and Process Water Concerns

The problem of hazardous waste management does not end with treatment alone, however; all these treatment methods leave residues requiring land disposal: sludge, ash, and typical residuals from pollution control equipment. Heavy metals, in particular, are ultimately amenable only to land disposal. Dried and stabilized, they can be stored safely in a new kind of landfill — a "residuals repository" — that uses a permanent cover to protect them from rain or snow. Such facilities already exist in Europe but not yet in the United States. With no liquids in the waste residues allowed for disposal in the residuals repository, and none entering from precipitation, no leachate can be formed to threaten nearby groundwater. Several decades hence, it may even be possible to reclaim the metals economically from carefully managed hazardous waste repositories (Mazmanian & Morell, 1992).

Treating hazardous wastes instead of dumping them obviously requires new treatment facilities. Within the context of widespread fear of toxics, few communities will consider siting such facilities, even those employing the newest and most advanced treatment technologies. People do not want someone else's wastes brought into their community for treatment, irrespective of purported facility safety. Consequently, during the first toxics decade, siting new treatment facilities proved to be one of the major stumbling blocks to realizing the treatment "solution" to hazardous waste management. By the mid-1980s, a few communities were beginning to see treatment facilities as an integral part of their overall waste management strategy, but it would take considerable further effort to accomplish siting even in those communities; elsewhere, successful siting seemed a virtual impossibility (Mazmanian & Morell, 1992).

Both natural gypsum and phosphogypsum contain radioactivity, but phosphogypsum contains more. In the manufacture of phosphoric acid, the acid is filtered through cloth to remove solids, and radium is filtered out with those solids. The solid portion is known as phosphogypsum. Phosphogypsum produced in North Florida contains roughly 5–10 picocuries per gram (pCi/g) of radium, while phosphogypsum from Central Florida contains about 20–35 pCi/g radium.

The U.S. EPA prohibits the use of phosphogypsum, with one exception: phosphogypsum with an average concentration of less than 10 pCi/g radium may be used as an agricultural amendment. The EPA's ban was based on a single scenario that assumed the by-product was used in road building or as an agricultural amendment, and that 100 years later a house was built on the farm field or abandoned road, with the homeowner living in the house for 70 years at 18 hours per day. Under this scenario, the homeowner's risk of radon-related health concerns only slightly exceeded the EPA's acceptable limits.

Phosphogypsum is primarily calcium sulfate, and plants require the sulfur it contains. Since much of the North Florida phosphogypsum is below the EPA restriction level, it can be used as a crop amendment, but for no other use. The Central Florida phosphogypsum is restricted to storage on land in large piles called "stacks." The overall radioactivity in the stacked phosphogypsum is actually less than what was present in the original phosphate ore extracted from the ground (Phosphate Primer, 2005).

International initiatives for the control of phosphogypsum stacks have included those by the Helsinki Commission. Pending the ratification and entry into force of the 1992 Convention on the Protection of the Marine Environment of the Baltic Sea Area, the Helsinki Commission continued to prepare and adopt recommendations for priority sources and sectors on the basis of the 1974 Helsinki Convention. In March 1989, the Baltic Marine Environment Protection Commission (the "Helsinki Commission") held its 17th Meeting in Helsinki, Finland. At that time, it adopted nine further recommendations relevant to land-based sources, including HELCOM Recommendation 17/6 concerning reduction of pollution from discharges into water, emissions into the atmosphere, and phosphogypsum out of production of fertilizers (Brunnée, Handl & Lammers, 1990).

The wet-process manufacture of phosphoric acid practiced in the U.S. and elsewhere requires an enormous volume of water, commonly referred to as process water. Process water is used as a water source for the phosphoric acid, for gas scrubbing, to slurry the phosphogypsum produced and transport it to storage, to operate barometric condensers, and for a wide variety of other uses in the chemical complex. A major portion of the heat released in the process ends up in the process water and is lost to the atmosphere by evaporative cooling in ponds.

Process water is stored both in ponds maintained on top of the phosphogypsum stack and in a below-ground-level pond (cooling pond). These ponds provide the large surface area needed for evaporation and cooling. When average yearly rainfall and evaporation rates are approximately equal, it is possible through strict control of water inputs to operate the chemical complex with a negative water balance. However, during years when rainfall is significantly above average — due to multiple tropical storms, El Niño weather effects, and similar events — it may become necessary to treat the surplus water and release it to surface waters to avoid an uncontrolled discharge of untreated process water.

Furthermore, if one of the operating plants is shut down and it becomes necessary to close the phosphogypsum stack and pond water system, the water in inventory must be treated before it can be discharged. The volume of water requiring treatment may be as much as 2 to 3 billion gallons. The process water has a low pH of about 1 to 2 and contains a dilute mixture of phosphoric, sulfuric, and fluosilicic acids. It is saturated with calcium sulfate and contains numerous other ions found in the phosphate rock used as raw material, as well as ammonia from the solid fertilizer manufacturing process (Bencherifa & Swearingen, 1996). The current treatment practice is to lime the water to a pH of approximately 4.5, remove the solids formed, lime again to a pH of approximately 11, remove the solids formed, air-strip the water to remove ammonia, and then add acid to reduce the pH to approximately 6.5; any treatment procedure proposed should also eliminate the dissolved solids problem (Bencherifa & Swearingen, 1996).

Over the years, regulations on how phosphogypsum is stacked have strengthened to protect against groundwater seepage. There are also strict standards companies must meet before releasing any process water into the environment, but human error and natural occurrences such as heavy rains can cause acidic water to spill. Even absent further spills, there remains a critical need to process the billions of gallons of water stored in and around phosphogypsum stacks during rainy periods and when a stack is eventually closed. In addition, a number of harbors and marine habitats around the world have experienced serious deterioration as a result of phosphogypsum discharges laden with heavy metals (Cd, Zn, Cu, Pb, and others). According to Bencherifa and Swearingen (1996), this type of pollution originates primarily from port activities, particularly those related to loading and unloading in main harbors, many of which lack adequate devices to handle accidental pollution.

Process water became a public topic of concern in Florida after the Florida Department of Environmental Protection (DEP) had to take responsibility for three phosphogypsum stacks with full ponds when Mulberry Corporation declared bankruptcy in January 2001. The company had two stacks in Mulberry, Polk County, and one at Piney Point in Manatee County. A spill from the Mulberry stack would endanger the Alafia River, while a spill from Piney Point would endanger Bishops Harbor, a prized estuary. Other process water incidents include a 1997 spill into the Alafia River resulting from an improperly installed overflow pipe, and two spills in late 2004 due to the high winds and heavy rains associated with that year's hurricanes.