Impact of High Rates of Phosphogypsum on Radon Emissions and on Radioactivity and Heavy Metals in Soils, Groundwater, and Bahiagrass Forage
The Florida Institute of Phosphate Research, through its “Strategic Research Priorities,” currently is stressing six program areas, three that are technology oriented and three that are more environmental in nature. A major focus is on phosphogypsum, a technology area but with significant environmental aspects. The objective is to reduce the accumulation of phosphogypsum produced by the industry. A significant approach toward that objective lies in developing and demonstrating environmentally acceptable uses for the material. This project addresses one aspect of potential use.
Phosphogypsum is composed mostly of calcium sulfate and is a by-product of the reaction between sulfuric acid and phosphate rock in the production of phosphoric acid. Currently almost one billion tons of the material are stockpiled on the ground in central and north Florida, and about thirty million tons are being added each year. A priority of the Institute since virtually its inception has been to find ways to use phosphogypsum. Any proposed use, however, must meet three criteria; it must be technically feasible, economical, and protective of the environment and the public health. Over the years, three main avenues of use have been explored. Recovery of the sulfur value is technically feasible but not currently economical. Use as a base and as fill in road construction is both feasible and economical, but that use is not permitted by the Environmental Protection Agency (EPA) at the present time because of the material’s radionuclide content. A third major use would be as a soil amendment. Phosphogypsum would be an excellent source of calcium and especially sulfur on agricultural lands, many of which, especially those in the Southeast, are deficient in sulfur. EPA has determined that phosphogypsum can be used in unlimited quantities in agriculture as long as its radium-226 content does not exceed ten picoCuries per gram (pCi/g). While this limit generally permits the use of gypsum from North Florida and elsewhere in the country, it does not allow the use of central Florida gypsum. Material from central Florida generally contains about twenty five pCi/g.
A first environmental field study of the impacts of using phosphogypsum on forage crops was done by the University of Florida in the late 1980’s at their Ona research and teaching center, and sponsored by the Institute (FIPR #89-01-085). Nominal rates of application were used, typical of what would be applied to a pasture by the rancher. Results showed that the gypsum application increased forage yield and made it more digestible. However, at the application rates used, data on radiological impacts to the environment or to the forage were not conclusive. No statistically valid differences in radium-226 or radon-222 levels could be found in soils, groundwater, air radon, above-ground gamma, or the forage. Thus a question remained: “Does the application of phosphogypsum to forage affect the radionuclide levels in the environment or crop?”
To answer question a second study was developed, funded, and conducted. This report outlines the findings of that second work. The objective of the present study was to establish relationships between gypsum application and environmental/forage quality. To do this, very high quantities of phosphogypsum were applied to the crops in the hope of finding measured differences that were statistically significant. Meaningful differences were found in this second study. Results are presented in light of such variables as environmental parameter, rate of application, soil depth, type of forage, elapsed time after application, crop sequence, and type of radionuclide, i. e. radium, lead, or polonium. Dose evaluations are also included. It would appear that, while differences are detectable, radiation levels are still too low to be of significance. It follows that normal rates of phosphogypsum application, which would be much lower than rates in this study, also would be too low to be of significance. It is suggested that a follow-up study of any long-term impacts of excessive application rates of gypsum be pursued.