Influence of Soil Cap Depth and Vegetation On Phosphogypsum Stack Reclamation In Alberta, Canada.

Phosphogypsum (CaSO₄∙H₂O) is an industrial byproduct created during phosphorus fertilizer production. At Agrium's Fort Saskatchewan plant, fertilizer production has ceased and phosphogypsum was pumped into settling ponds, dewatered and piled into solid stacks covering 35 ha. Phosphogypsum stacks can pose environmental hazards including residual acidity, small quantities of radium and uranium and elevated trace elements that can become mobile in water. This study quantifies environmental risks and will help to develop reclamation strategies for phosphogypsum stacks to support sustainable soil and vegetation systems. Experimental plots with soil cap depths (0, 8, 15, 30, 46, 91 cm) and vegetation treatments (four grass monocultures, a grass mix with Trifolium hybridum), established in 2006, are being studied.

In 1.25 m cores significant root mass accumulations occurred at the soil - phosphogypsum interface with 8, 15, 30 and 46 cm caps in 50% of the cores. Mean peak water content occurred at the interface with all cap depths. Maximum rooting depth increased with increasing cap depth to an mean maximum rooting depth of 70 cm. Above ground biomass increased with increasing cap depth to 30 cm where it plateaued. Substrate and above ground vegetation were analyzed via neutron activation for concentrations of 33 elements. On average experimental plots with cap depths ≥ 30 cm contained substrate with lower concentrations of fluorine, cobalt, cerium, european, lanthanum, nickel, samarium, and ytterbium and vegetation tissue with lower concentrations of nickel and cobalt than those with cap depths < 30 cm.