Filtering Dissolved Phosphorus From Drainage Waters Using Synthetic Gypsum: Water Quality Impacts.

Following decades of chicken litter applications to soils of the Delmarva Peninsula, legacy phosphorus (P) is a major source of dissolved P entering drainage ditches that eventually empty into the Chesapeake Bay. In April, 2007, flue gas desulfurization (FGD) gypsum, also called synthetic gypsum, was used to construct a ditch filter on the University of Maryland Eastern Shore Research and Teaching Farm at Princess Anne, MD. We measured a mean removal efficiency of 70% of the dissolved P in ditch flow that passed through the bed of FGD gypsum. This presentation evaluates environmental concerns surrounding the slightly higher levels of arsenic (As) and mercury (Hg) in FGD gypsum than in naturally occurring mined gypsum under conditions of extreme leaching. Arsenate, which is present in elevated levels in poultry litter amended soils, has similar chemical behavior as phosphate, and the gypsum filter effectively reduced 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 acts as a sediment trap for particulate-bound P, As, and Hg. Aqua regia digests of samples taken from the surface (0 to 2 cm) of the gypsum bed in December, 2010 showed elevated levels of As and Hg in comparison to the concentrations of these metals in the FGD gypsum used to construct the filter. When these accumulations are taken into consideration, the gypsum filter not only effectively removes soluble P, but is a net sink for both As and Hg as well.