Sediment and chemical load reduction by grass and riparian filters.
Daniels R.B. | Gilliam J.W.
Vegetated filter strips help reduce non-point source pollution from agricultural areas. Even though they are an accepted and highly promoted practice, little quantitative data exist on their effectiveness under field conditions. The objective of this research was to determine the amount of nutrients and sediment removed by natural and planted filters. This was achieved by collecting and analyzing runoff at field edges and at various locations in vegetated buffers. Total weight of sediment and nutrients in runoff from North Carolina agricultural fields showed that the grass and riparian filter strips studied reduced runoff load by 50 to 80%. Total sediment decrease through the filters was about 80% for both grass and riparian vegetation. The reduction in the chemical load depended on the nutrient and its form. Filters reduced total P load by 50%, but 80% of the soluble PO4-P arriving at the field edge frequently passed through the filters. The filters retained 20 to 50% of the NH4 and approximately 50% of the total Kjeldahl N and NO3. High-volume flows commonly overwhelmed both grass and riparian filters next to culivated fields. Forested ephemeral channels had little vegetation and were effective sediment sinks during the dry season but were ineffective during large storm events because there was little resistance to flow. When possible, drainageways should be designed to hold sediment and to disperse the discharge into a riparian area.
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