PHOSPHOGYPSUM RECLAMATION: EVALUATING ALTERNATIVE AND TRADITIONAL COVER SYSTEMS

Abstract

Phosphogypsum (PG) is a by-product from phosphorus fertilizer production that is stored in large piles called stacks. Stacks are formed as PG particles settle to the bottom of large holding ponds and excess water is returned to the production facility resulting in piles of dry PG. PG is comprised of gypsum with low concentrations of fluoride, trace elements and naturally occurring radioactive material from parent ore. PG stack reclamation focuses on limiting exposure to the surrounding environment by capping with soil. Environmental risks have been controlled using topsoil caps vegetated with grass. Alternative methods such as short rotational forestry plantations may provide added benefits by utilizing rapidly growing woody species to produce biomass for wood products and renewable energy, and sequester atmospheric carbon for credits. Five soil amendments were used in a field experiment to determine effects on Picea glauca (white spruce) and Populus balsamifera (balsam poplar) growth. After two years amendments had little effect on either species for the parameters measured. The healthiest and most successful trees were from control treatments. Tree survival was low relative to other studies with significant mortality over the 2016 to 2017 winter in all treatments. Nutrients from amendments significantly affected some soil properties but did not impact trees. Despite efforts to control unwanted vegetation such as weeds and competitive grass, use of amendments may have inadvertently benefited competing vegetation more than the planted trees. Pure soil, PG and mixtures of both were below Canadian Council of Ministers of the Environment guidelines for all elements. There was a poor correlation between soil and plant concentrations for most elements which suggests that elements were not in a bioavailable form. Grass had lower tissue concentrations than trees for most elements analyzed. Washed and unwashed vegetation had few differences. Radium-226 activity in pure PG and highly mixed soils were above Canadian guidelines for naturally occurring radioactive material. All plant species were likely not taking up radioactive isotopes from PG. Vegetation covers differed significantly in radon-222 emissions, likely due to canopy structure, not uptake or changes from plants growing on the site. Pure PG is not suitable as a soil to support trees on its own; by using topsoil and amendments the limitations of PG can be ameliorated. Short rotational forestry plantations appear to be meeting reclamation objectives for PG stacks, similar to traditional grass covers.