Reclamation of abandoned bentonite mine spoils with phosphogypsum and magnesium chloride amendments

Abstract

Abandoned bentonite mine spoils are scattered over southeast and north central Montana. The clayey, saline-sodic nature of these spoils creates adverse physicochemical properties. Invasion of native plant species is precluded and revegetation is difficult. Infiltration of surface water is severely limited due to surface crusting and shrink-swell processes. Chemical amendments have recently been shown to be effective in ameliorating adverse physical and chemical spoil properties. Chemical amendment use increases reclamation costs significantly, therefore effective, low cost amendments are needed. Phosphpgypsum and magnesium chloride brine are low cost industrial wastes that have not been tested for use in land reclamation. Experimental field plots were implemented to evaluate the effects of phosphogypsum (40.4 mt/ha) and magnesium chloride brine (36.2 mt/ha) incorporated to a 35 cm depth. Effects of nitrogen fertilizer (0, 67 kg/ha, and 134 kg/ha) on seedling emergence were also tested. Representative unamended spoil at the site had a sodium adsorption ratio (SAR) of 33.8 and an electrical conductivity (EC) of 8.0 mmhos/cm. Over a 14 month sampling period, SAR (0-5 cm) declined to 24.5 on phosphogypsum treated plots, and to 21.3 on magnesium chloride brine treated plots. Electrical conductivity (0-5 cm) increased to 10.1 mmhos/cm on phosphogypsum treated plots, and to 15.9 mmhos/cm on magnesium chloride brine treated plots. Following 30 minutes of simulated rainfall, minesoil infiltration rates were 2.8 cm/hr on phosphogypsum treated plots and 3.8 cm/hr on magnesium chloride brine treated plots, compared to .1 cm/hr on unamended spoil. Nitrogen fertilizer at 67 kg/ha resulted in significantly greater seedling density among fertilizer treatments, at 192 seedlings/m^2. Plant canopy cover of 39% estimated on magnesium chloride brine treated plots was significantly greater than 28% on phosphogypsum treated plots. Above ground plant production was 1753 kg/ha on phosphogypsum treated plots and 2717 kg/ha on magnesium chloride brine treated plots. Production of pioneering (non-seeded) annual forbs comprised 60% of total production on phosphogypsum treated plots and 83% on magnesium chloride brine treated plots.