Hydrologic Evaluation of Final Cover System Alternatives for Closure of Phosphogypsum Stacks. Volume I
The measured hydrologic performance of three phosphogypsum stack system top gradient final cover alternatives and six side slope final cover alternatives over a 3-year period are presented. Hydrologic measurements consisted of monitoring: (i) rainfall, air temperature and pan evaporation; (ii) infiltration and runoff volumes and water table levels; (iii) runoff water quality; (iv) changes in soil and gypsum moisture content; and (v) root depth penetration. Measured hydrologic performance was used to calibrate the HELP model (Schroeder et al. (1994), Version 3.07) for top gradient and side slope final covers. Recommended values for runoff curve number, evaporative zone depth and leaf area index are presented for the final cover alternatives investigated. The calibrated HELP model provides reliable estimates of long-term post-closure hydrologic performance.
Several side slope final cover alternatives were investigated: (i) 6″ soil over leached gypsum; (ii) sodded amended leached gypsum; (iii) seeded amended leached gypsum on 2.5H:1V and on 3.3H:1V slopes; (iv) seeded amended unleached gypsum; and(v) 6″ seeded amended leached gypsum over 18″ compacted leached gypsum. Bermudagrass was used for each alternative. Amended leached gypsum covered with bermudagrass sod performed the best of the investigated side slope final covers. This alternative has the advantage of producing runoff suitable for off-site discharge as soon as planted, is less sensitive to growing conditions immediately after planting, and is not susceptible to erosion. A 6″ soil cover has the advantage of producing runoff suitable for off-site discharge soon after the grass starts developing, but has the disadvantages of being susceptible to erosion prior to establishment of the grass, being relatively sensitive to growing conditions shortly after planting, and allowing somewhat greater infiltration. Near surface compaction of in-situ gypsum was not effective in reducing infiltration through amended gypsum side slope final covers. Seeded amended leached and unleached gypsum side slope final covers may require several years to produce runoff suitable for off-site discharge, but these alternatives can be attractive at sites with a negative water balance, or at sites where runoff can be diverted to another water system (e.g., a mine water circuit) prior to discharge.
Grass growth on unleached gypsum side slopes is possible, but dolomitic limestone application rates in excess of 10 tons/acre are required. The depth to the water table must also be depressed 5 feet or more to prevent capillary rise of acidic process water into the evaporative zone. Infiltration through an amended unleached gypsum side slope final cover is initially greater than occurs for amended leached gypsum, but as the unleached gypsum pH increases and conductivity decreases, the infiltration should approach that of an initially leached gypsum side slope final cover.
Three top gradient final cover alternatives, each seeded with bermudagrass, were investigated: (i) 24″ soil over 40-mil HDPE liner; (ii) 24″ gypsum (upper 6″ amended) over 40-mil HDPE liner; and (iii) 6″ soil/18″ gypsum over 40-mil HDPE liner. A 6″soil/18″gypsum cover atop a liner is a satisfactory cost-effective alternative for elevated portions of a top gradient where the water table above the liner does not rise to the surface for an extended period of time. The use of a 24″ gypsum cover over a liner is not a viable top gradient final cover alternative.