Technical Abstract: Decades of applying chicken litter to meet nitrogen demand has led to accumulation of phosphorus (P) in soils of the Delmarva Peninsula. This legacy P that now approaches levels up to ten times the agronomic optimum 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 in April, 2007, precipitates soluble P as calcium phosphate. Although the filter removed 75% of soluble P from water that passed through the filter, large flow events exceeded the maximum filtration rate, and P-rich water bypassed the filter. Subsequent research on 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 were installed and monitored. Lateral flow was not obstructed by these “gypsum curtains,” and soluble P was reduced by 50 to 95% as groundwater passed through the gypsum. Construction of a berm over the buried gypsum forces surface runoff to infiltrate and pass through the curtain, thereby treating 100% of the water entering the ditch. Surface application of gypsum to enhance infiltration and improve subsurface drainage is being investigated. Environmental concerns about slightly higher levels of mercury and arsenic in FGD gypsum than in naturally occurring mined gypsum have proved unfounded. Gypsum filters effectively remove soluble P and provide a beneficial use for an industrial byproduct.