Phosphorus removal in vegetated filter strips.

Vegetated filter strips (VFS) are used recently for removal, at or near the source, of sediment and sediment-bound chemicals from cropland runoff. Vegetation within the flowpath increases water infiltration and decreases water turbulence, thus enhancing pollutant removal by sedimentation within filter media and infiltration through the filter surface. Field experiments have been conducted to examine the efficiency of vegetated filter strips for phosphorus removal from cropland runoff with 20 filters with varying length (2 to 15 m), slope (2.3 and 5%), and vegetated cover, including bare-soil plots as control. Artificial runoff used in this study had an average phosphorus concentration of 2.37 mg L(-1) and a sediment concentration of 2700 mg L(-1). The average phosphorus trapping efficiency of all vegetated filters was 61% and ranged from 31% in a 2-m filter to 89% in a 15-m filter. Filter length has been found to be the predominant factor affecting P trapping in VFS. The rate of inflow, type of vegetation, and density of vegetation coverage had secondary influences on P removal. Short filters (2 and 5 m), which are somewhat effective in sediment removal, are much less effective in P removal. Increasing the filter length beyond 15 m is ineffective in enhancing sediment removal but is expected to further enhance P removal. Sediment deposition, infiltration, and plant adsorption are the primary mechanisms for phosphorus trapping in VFS.