Phosphogypsum Capping Depth Affects Revegetation and Hydrology in Western Canada

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Abstract
Phosphogypsum (PG), a byproduct of phosphate fertilizer manufacturing, is commonly stacked and capped with soil at decommissioning. Shallow (0, 8, 15, and 30 cm) and thick (46 and 91 cm) sandy loam caps on a PG stack near Fort Saskatchewan, Alberta, Canada, were studied in relation to vegetation establishment and hydrologic properties. Plant response was evaluated over two growing seasons for redtop (Agrostis stolonifera L.), slender wheatgrass (Agropyron trachycaulum (Link) Malte ex H.F. Lewis), tufted hairgrass (Deschampsia caespitosa (L.) P. Beauv.), and sheep fescue (Festuca ovina L.) and for a mix of these grasses with alsike clover (Trifolium hybridum L.). Water content below the soil–PG interface was monitored with time-domain reflectometry probes, and leachate water quantity and quality at a depth of 30 cm was measured using lysimeters. Vegetation responded positively to all cap depths relative to bare PG, with few significant differences among cap depths. Slender wheatgrass performed best, and tufted hairgrass performed poorly. Soil caps <1 m required by regulation were sufficient for early revegetation. Soil water fluctuated more in shallow than in thick caps, and water content was generally between field capacity and wilting point regardless of cap depth. Water quality was not affected by cap depths ≤30 cm. Leachate volumes at 30 cm from distinct rainfall events were independent of precipitation amount and cap depth. The study period had lower precipitation than normal, yet soil caps were hospitable for plant growth in the first 2 yr of establishment. Copyright © 2011. Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.

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