Using FGD Gypsum to Remove Soluble Phosphorus From Agricultural Drainage Waters.

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 soluble P entering drainage ditches that empty to the Chesapeake Bay. In April, 2007, Flue Gas Desulfurization (FGD) gypsum was used to construct a ditch filter to precipitate soluble P as calcium phosphate. Filtration through a bed of FGD gypsum removed 35 to 90 % of the P from ditch flow that passed through the filter. Although chemically effective, large flow events topped the weir, and P rich waters flowed to the Manokin River. Simultaneously, research on groundwater hydrology in Coastal Plain soils of the Delmarva showed that overland flow accounts for <10 % of P export to ditches. Lateral groundwater flow, during storm events when water tables are high, is the major pathway for soluble P delivery to ditches. In August, 2009, gypsum “curtains,” consisting of FGD gypsum-filled trenches parallel to the ditch, were installed and monitored by piezometers. Lateral flow rates were not diminished by the gypsum curtains, and soluble P was reduced by 50 to 95% as groundwater passed through the high calcium environment of the buried gypsum. Environmental concerns due to higher levels of mercury (Hg) and arsenic (As) in FGD gypsum than in naturally occurring mined gypsum were determined to be unfounded. Filtered water had no detectable Hg. Arsenate, which is present in elevated levels in poultry litter- amended soils, behaves similarly to phosphate. Data show lower levels of soluble As after filtration through both the ditch filter and the curtain. The potential for applying “spent” gypsum at typical amendment rates was assessed. There may be potential to use the calcium phosphate rich gypsum as fertilizer if it is applied before crops need the nutrient.