Drainage Ditch Filter Composition Effects On Hydraulic Properties and P Sorption.

            Current phosphorus sorbing material (PSM) filters in drainage ditches have been shown to effectively remove P in drainage water during base flow levels, but large storms may result in large flow quantities by-passing the filter during peak flow events.  This study attempted to maximize the flow through filters by mixing gypsum (a primary candidate PSM) with sand to increase hydraulic conductivity without sacrificing P sorption.  An experiment was established that utilized 50-cm long columns of filter material encased in 10-cm dia. PVC pipes.  A system of pumps and tanks was constructed so as to feed a near-constant 2.4 mg P/L solution to the columns at three different levels of hydraulic head (8, 12, 16 cm). Two types of gypsum were used: synthetic gypsum derived from flue gas desulfurization (FGD) and mined, coarsely ground gypsum. In addition to the gypsum, sand was added at 0, 10 and 20% by volume. Columns were continuously leached for 14 days. Leachate volume was measured and samples were collected over time to determine P retention and hydraulic conductivity.  Adding sand to the silt-textured FGD gypsum did not significantly affect the hydraulic conductivity. Sand decreased the hydraulic conductivity of the mined gypsum.  FGD gypsum initially captured almost all P entering the columns, but a sharp drop-off was noted.  The addition of sand caused earlier and greater reductions.  The mined gypsum initially sorbed little P, but the rate of sorption increased over time.  However, in terms of total P sorbed, 100% coarse gypsum sorbed significantly less P, but there was found to be no differences between the other mixtures.  The mixture containing 90% mined gypsum and 10% sand had the highest hydraulic conductivity without a reduction in total P sorption, while the mixture containing 80% mined gypsum and 20% sand could have potential as an alternate mixture.