VEGETATION AND OVERBURDEN COVER ON PHOSPHOGYPSUM: EFFECTS ON RADON EMISSION, RUNOFF WATER QUALITY, AND PLANT UPTAKE OF FLUORIDE AND RADIUM 1 by

Phosphogypsum is a byproduct of phosphate fertilizer production, and more than 700 million metric tons have accumulated on 2,500 ha in Florida. Field research was conducted to compare the benefits of capping phosphogypsum with overburden (up to 15 cm in depth) from mined sites versus treatment of the phosphogypsum with minimal amendments. After four growing seasons, vegetation cover was excellent (no bare ground) on plots amended with dolomitic limestone or capped with overburden. However, more species became established with an overburden cap. Fluoride uptake by bermudagrass (Cynodon daclylon) was high when grown directly on phosphogypsum (895 mg kg-l in leaf tissue) and was reduced slightly by a 15 cm overburden cap (670 mg kg-l). Unexpectedly, radium 6cRa) uptake in bermudagrass grown directly on phosphogypsum (0.6 pCi g-l) was less than when grown on the overburden cap (1.8 pCi g-l). The presence of grass cut the radon ~2Rn) efflux from phosphogypsum in half (from 24 pCi m-z s-l to 11 pCi m“2S-l), while 15 cm of overburden, in addition to grass cover, halved it again (down to 5 pCi m“2s-l). Vegetation cover on phosphogypsum resulted in a 30-fold decrease in electrical conductivity and a 5-fold decrease in the fluoride concentration of surface runoff water. Runoff water quality from vegetated plots was equally good with or without a 15 cm overburden cap on top of the phosphogypsum.