Investigation of the Potential for Arsenic Release From Agriculture Based Soils Amended With Flue Gas Desulfurization Gypsum.

This presentation is focused on risk assessment for arsenic (As) and other trace elements during beneficial use of Fluidized Bed Desulfurization Gypsum (FGDG) in agriculture. Before 2000, FGDG contained both fly ash and the CaSO₄•2H₂O formed during removql of SO₂ from power plant exhausts. Some "old" FGDG contained high levels of As, Se, B, and other elements compared to soils, dependent on the coal used to generate power. Beneficial use evaluations of "old" FGDG indicated some potential adverse effects (B toxicity, etc.). Industry changed the exhaust gas processing to remove the fly ash before removal of FGDG resulting in a "new" FGDG product containing mean As levels of only 3.62 mg kg^-1 dry FGDG compared to the mean As in US soils of 5.2 mg kg^-1. "New" FGDG may dilute soil As and annual application of 3 t ha^-1 as Ca or S fertilizer hardly alters soil chemical conditions except for Ca and S.

Applying FGDG to fields or grassed border strips reduces runoff of P and As runoff significantly. ARS researchers evaluated application of FGDG with poultry litter (a source of As) and found that As and P were significantly reduced in runoff even immediately after application, but only if the FGDG was applied on top of, or mixed with the litter. Thus FGDG application may reduce As runoff from other sources rather than being an As risk itself.

Consideration of risks of As from all sources is complex. Excessive inorganic As ingestion from drinking and cooking water can cause skin disease and internal cancers. It remains uncertain whether As is a threshold carcinogen acting through excess cell proliferation rather than through DNA alteration which is assumed to extrapolate to zero dose. Risk from As in FGDG, soils and crops is especially unlikely at the low As exposure provided by crop accumulation of As (other than rice). Greater concern about As was raised by a 2010 risk assessment and proposed increase in cancer slope factor by the US-EPA. Public review of the proposal resulted in initiation of a new EPA assessment that is expected to complete peer and public review in 2016. The 2010 proposed number suggested that soil As greater than 0.025 mg kg^-1 comprised risk to humans (thru soil ingestion). This is much lower than the As in background US soils (mean = 5.2 mg kg^-1 ; 95^th percentile, 8.02 mg kg^-1; N=4813; Smith et al., 2013). Crops other than rice accumulate little As due to As biochemistry. Storage tissues such as grain and fruits are especially low in As due to phosphate competition in arsenate transfer to phloem fed crop tissues.

Selenium (Se) in FGDG averages somewhat higher than soils, but plant uptake of SeO₄ by the the SO₄ transporter is inhibited by SO₄ in the FGDG such that crops supplied FGDG do not accumulate higher levels of Se. Another possible concern is the possibility of excessive intake of S by ruminant livestock exposed to FGDG on forage or stockpiled in pastures. But FGDG is used at low application rates and where S fertilizers are required; no evidence has been obtained that crop uptake of S from FGDG beneficial use is a practical risk to ruminant livestock. Direct ingestion of FGDG by ruminants should be avoided by fencing field stored product before application.

FGDG has several clear beneficial uses in agriculture. Making the FGDG after removal of fly ash contaminants provides a product with low concentrations of trace elements and xenobiotics. Although the formal risk assessment for FGDG beneficial use is continuing at US-EPA in cooperation with ARS and others, finding that nearly all trace elements are lower in the "new" FGDG than in US soils leads us to conclude that FGDG comprises very low risk in agriculture.