Beneficial Use of FGD-Gypsum In Agriculture: How Uncertainty In Risk Assessment and Modeling Affects Evaluation.

FGD-gypsum (FGDG) is manufactured during removal of SO₂ from stack exhausts of coal-fired power plants. With increasing production of FGDG, and because gypsum can provide many benefits when used in agriculture and horticulture, research has been conducted to evaluate benefits and risks of FGDG beneficial use. In order to produce FGDG with low levels of trace elements, the industry changed their exhaust treatment systems to separately remove fly ash before the exhaust is treated to generate FGDG. Research has been conducted at a number of locations to test the effects of the new low contaminant FGDG in both greenhouse and field experiments in several states and regions of the US. Beneficial uses studied include: Ca and S fertilizer; amendment to reduce phosphate leaching or runoff; aid to water infiltration to reduce erosion potential; correction of subsoil Al toxicity; and correction of sodicity. Data from research is used to examine the potential risks to the environment where FGDG is utilized. Analysis of total and soluble elements in FGDG confirmed that the new cleaner product contains low concentrations of trace elements, generally lower than background US soils. Plant uptake of trace elements could be altered due to application of the FGDG or changes in amended soil chemistry. Field studies of the new FGDG have not revealed significant increases in plant trace elements. But risk assessment is conducted within the U.S.-EPA guidance on allowable concentrations in environmental media. Thre are several issues, however, associated with risks from As that will need to be addressed. U.S.-EPA currently indicates that soil As should be no higher than 0.43 mg/kg soil (1 in 10⁶ cancer risk), which is at the 1st percentile of background soil As levels in the US. Possible changes in As limits would lower this by 17-fold. Uncertainty in As risk assessment is driving many environmental decisions, even though the proposed soil As limit is below the backround levels in most US soils. Rice is the staple food with the highest levels of inorganic As; rice As may also raise questions about As limits based on uncertain As risk assessment research. When use of FGDG does not increase soil, crop As or groundwater As concentrations, it is hard to understand why As limits should inhibit FGDG beneficial use.