Influence of Soil Capping Depth On Moisture Dynamics In Phosphogypsum Stack Reclamation.

Phosphogypsum (PG) is an acidic byproduct from the manufacturing of phosphate based fertilizers. Current estimates suggest over 100 million tonnes of PG have been produced in Canada and stored in stockpiles called PG stacks. Reclamation standards for PG stacks however are not universal and therefore site-specific criteria need to be implemented. Reclamation strategies for PG stack reclamation involve minimizing water infiltration into and percolation through the PG material, which can cause leaching of trace metals and radionuclides to the groundwater.  The main objective of this research is to evaluate the influence of soil capping depth (0, 8, 15, 30, 46, and 91 cm) on infiltration processes and the potential drainage of water from the topsoil to underlying PG material. In the fall of 2010 a conservative tracer (Br2) was applied to the various soil depth treatments across three repetitions (18 plots total). For real-time evaluation of moisture dynamics 23 Time Domain Reflectometry (TDR) probes were installed at varying depths of the 15, 30 and 46 cm plots of one replicate. Matric potential sensors were also installed at the PG/soil interface, as well as 7.5 cm above and below the interface. Preliminary results from the TDR and matric potential sensors suggest an accumulation of moisture at and immediately below the PG/soil interface. The temporal soil water data in combination with the measured Br transport will be used to model the infiltration of water over a given temporal scale and predict future water dynamics under different capping depths and climatic variability.