Effect of soil capping depth on phosphogypsum stack revegetation

Phosphogypsum is a by-product of the phosphorus fertilizer production process and is typically stacked at the production sites. These stacks can potentially pose environmental hazards, which can be substantially reduced through reclamation by capping with soil and revegetation upon decommissioning. We conducted a study on a phosphogypsum stack using five soil capping depths (8, 15, 30, 46, 91 cm), an uncapped treatment, and five vegetation treatments (monocultures of four grass species Agrostis stolonifera L., Festuca ovina L., Deschampsia caespitosa (L.) Beauv., Agropyron trachycaulum (Link) Malte ex H.F. Lewis and one mix of the four species with Trifolium hybridum L.) to assess plant growth, health, rooting characteristics, and trace element uptake. Cobalt and nickel concentrations in plant tissue from plots with ≥ 15 cm soil capping were within ranges found at reference sites, whereas fluorine was three times elevated. Vegetation cover was significantly greater on capped than uncapped plots, being greatest for Agropyron trachycaulum (26%) and Festuca ovina (26%). Capping depths ≥ 15 cm had greater cover, biomass, and healthy plants than the 8 cm cover. Soil water content was similar in the 15-46 cm capping depth, with the lowest in the 91-cm caps. Fluorine, cobalt, and nickel were elevated in select plant tissue samples on the research plots relative to references, and cap depth affected tissue fluorine and cobalt concentrations but not nickel. Concentrations of these trace elements were lower than maximum tolerable levels for animal consumption. From this 5-year study, Agropyron trachycaulum and Festuca ovina and a soil cover depth of ≥ 15 cm are recommended for reclamation of phosphogypsum stacks.