ARS | Publication request: Using flue gas desulfurization gypsum to remove dissolved phosphorus from agricultural drainage waters

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2012
Publication Date: May 1, 2012
Citation: Bryant, R.B., Buda, A.R., Kleinman, P.J., Church, C., Saporito, L.S., Folmar, G.J., Bose, S., Allen, A.L. 2012. Using flue gas desulfurization gypsum to remove dissolved phosphorus from agricultural drainage waters. Journal of Environmental Quality. 41(3):664-671. DOI: 10.2134/jeq2011.0294.

Interpretive Summary: USDA-ARS scientists and scientists from the University of Maryland Eastern Shore designed, constructed, and monitored a filter within an agricultural drainage ditch to remove dissolved phosphorus (P), thereby protecting receiving water bodies against P losses from upstream areas. Flue gas desulfurization (FGD) gypsum, a readily-available, low-cost coal combustion product, was used as the reactive ingredient in the ditch filter. Although the hydraulic conductivity of FGD gypsum was too low to treat large flow events during which water bypassed the filter and flowed to the Manokin River, the ditch filter did remove approximately 22 % of the total dissolved P load over a 3.6 year period. Arsenate, which is present in elevated levels in poultry litter amended soils, has similar chemical behavior as phosphate, and the gypsum filter effectively removed dissolved Arsenic from ditch drainage waters. Mercury concentrations in the influent and the effluent were approximately the same, indicating that mercury, which is present as a minor contaminant in the FGD gypsum, does not leach from the gypsum at concentrations that might cause concern. The gypsum filter also acted as a sediment trap for particulate-bound P, arsenic, and mercury. At the end of the study, the gypsum was enriched with all three of these elements that would otherwise have had negative impacts on downstream water quality.

Technical Abstract: After several decades of applying chicken litter to meet crop demands for nitrogen, high levels of legacy phosphorus (P) in soils of the Delmarva Peninsula are a major source of dissolved P entering drainage ditches that empty to the Chesapeake Bay. The objective of this study was to design, construct, and monitor a filter within the ditch to remove dissolved P, thereby protecting against P losses from upstream areas. In April, 2007, flue gas desulfurization (FGD) gypsum, a readily-available, low-cost coal combustion product, was used as the reactive ingredient in the construction of a ditch filter to remove dissolved P. The ditch filter was monitored through 2010 during which time 29 storm-induced flow events were characterized. For storm-induced flow, the event mean concentration efficiency for dissolved P capture for water passing through the gypsum bed was 73% +/- 27% confidence interval (alpha = 0.05), and by the summation of load method we observed a 65% +/- 27% reduction (confidence interval alpha = 0.05). FGD gypsum contains slightly higher levels of arsenic (As) and mercury (Hg) than is found in naturally occurring mined gypsum. Arsenate, which is present in elevated levels in poultry litter amended soils, has similar chemical behavior as phosphate, and the gypsum filter effectively removed dissolved As in ditch drainage waters. Hg concentrations in filtered water were approximately the same as those in unfiltered water (2 ug L-1), indicating that Hg does not leach from the gypsum at concentrations that might cause concern. The gypsum filter also acted as a sediment trap for particulate-bound P, As, and Hg. Although chemically effective, the hydraulic conductivity of FGD gypsum was too low to treat large flow events during which water topped the weir, allowing unfiltered water to flow to the Manokin River. When bypass flow and base flow were taken into consideration, the ditch filter removed approximately 9 % of the total dissolved P load over the 3.6 year period.