Technical Abstract: Decades of chicken litter applications has led to phosphorus (P) levels up to ten times the agronomic optimum in soils of the Delmarva Peninsula. This legacy P is a major source of P entering drainage ditches that eventually empty into the Chesapeake Bay. A Flue Gas Desulfurization (FGD) gypsum ditch filter, constructed on the University of Maryland Eastern Shore research farm at Princess Anne, MD in April, 2007, precipitates soluble P as calcium phosphate. Ditch flow that passed through a bed of FGD gypsum showed a mean removal efficiency of 75%. However, large flow events exceeded the maximum filtration rate, and P rich water bypassed the filter. Subsequently, research on P transport pathways in Coastal Plain soils of the Delmarva showed that lateral groundwater flow, during storm events when water tables are high, is the major pathway for soluble P delivery to ditches. In a phase two design, gypsum-filled trenches parallel to the drainage ditch, dubbed “gypsum curtains,” were installed and monitored. Lateral flow was not obstructed, and soluble P was reduced by 50 to 95% as groundwater passed through the “gypsum curtains.” Environmental concerns about higher levels of mercury (Hg) and arsenic (As) in FGD gypsum than in naturally occurring mined gypsum proved unfounded. Filtered water had no detectable Hg. Photo-reduction and volatilization of Hg is inhibited by burial of the gypsum. Arsenate, which is present in elevated levels in poultry litter - amended soils, behaves similarly to phosphate and is also removed by filtration.