Agricultural Drainage Filters. I. Filter Hydro-Physical Properties and Tracer Transport

Drainage filters using porous granular material constitute new innovative technologies for remediating phosphorus (P) from agricultural tile drainage water. In drainage filters where convective velocities are often high, we hypothesize that intragranular diffusion may affect solute transport depending on filter characteristics and flow rate. This was investigated for six drainage filter materials (Leca, Filtralite-P®, granulated limestone, crushed seashells, calcined diatomite earth (CDE), and a poorly ordered Fe oxide aggregate (CFH)) conducting a tritium (³H₂O) tracer experiment at low (0.26 cm h⁻¹), medium (23 cm h⁻¹), and high (41 cm h⁻¹) flux densities. The filter materials differed widely with respect to grain-size distribution (D ₅₀ from 1.6 to 3.3 mm), uniformity coefficient (1.7 to 2.2), particle density (1.75 to 2.76 g cm⁻³), bulk density (0.34 to 1.46 g cm⁻³), and water-filled porosity (0.39 to 0.73 cm³ cm⁻³). Measurements of specific surface area (SSA) included both SSABET and SSAEGME to ensure inclusion of the intragranular microporosity, not accounted by N₂-BET. SSA varied widely across methods and allowed the differentiation of filters according to the significance of the intragranular porosity. Tritium transport varied from approximately equilibrium transport at all flow rates in Leca, Filtralite-P®, and limestone, to progressive non-equilibrium transport as flow rate increased in Seashells, CDE, and CFH. In general, the filter materials were highly variable in hydro-physical properties. Filters with (approximately) equilibrium transport were, however, all characterized by low specific surface areas. The non-equilibrium transport was explained by an intragranular diffusion in filters with larger specific surface area (Seashells, CDE, and CFH).