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Phosphogypsum Stack Closure:Evaluation of Phosphogypsum as an Alternate Final Cover


Phosphogypsum (PG) stack closure rules require a cap to preclude rainwater percolation and groundwater impacts. Industry practice is to use polyethylene liner of  permeability 10-7 cm/sec or less, overlain with overburden. Alternate approaches using compacted PG, alone or with additives, have not been investigated. Evapotranspiration (ET) from a vegetated stack also needs consideration. Therefore, laboratory work on the effects of PG compaction with additives such as bentonite, phosphatic clay, cement, and lime sludges on the permeability, cost consideration, desiccation, and tension cracking were evaluated on PG stacks. The field component consisted of ET measurements using a chamber method on existing vegetated plots. Phosphogypsum mixed and compacted with 15% phosphatic clay or 10% bentonite appears effective in achieving the 10-7 cm/sec permeability desired. The laboratory tests demonstrated low potential for cracking of PG from desiccation. Tension cracks occurred at 3% volumetric shrinkage for compacted PG, which correlated with other research. Cracks likely occur near the transition from side slopes to top of stack, where change in slope is greatest. These areas do not provide significant infiltration and are typically not covered when using liners. Approximately $25,000 per acre savings is possible using compacted clay-PG mixtures with vegetation. Vegetated stacks contribute to additional water loss but may be impacted by mowing, fertilization, and double cropping with grass. Results from this research warrant further field study.