The effectiveness of a combination weep berm-grass filter riparian control system for reducing fecal bacteria and nutrients from grazed pastures

Much of the pollution in our lakes and streams has been attributed to agricultural practices, with bacteria, nutrients, and sediment being the primary pollutants. Runoff from grazed pastures and manure-applied lands can contain high concentrations of fecal coliforms and nutrients. Riparian grass filters have proven successful in reducing pollutants reaching streams and wetlands but effectiveness is dependent upon achieving shallow, uniform flow. Most landforms have undulations that will concentrate flow, reducing grass filter efficacy. In an attempt to enhance the effectiveness of the riparian zone as a pollution control area, a low-cost control system, consisting of a combination weep berm-grass filter, was developed and tested under simulated continuous grazing and rotational grazing practices. Three replicate tests were conducted on three field plots subjected to simulator-generated rainfall. Plots were instrumented to enable monitoring of surface runoff up-gradient of the weep beim and down-gradient of the grass filter. The system achieved average reductions in fecal coliform concentrations (99%), total nitrogen (87%), total phosphorus (44%), and total suspended solids (90%). The control system also reduced peak runoff rate from high intensity, short duration rainfall events by 92%. Based on these results, the weep berm-grass filter system affords the following advantages over simple grass filters: 1) peak flows are highly dampened, 2) due to short-term storage, some settling and infiltration occurs above the berm, and 3) flow is passively and uniformly released through the weep berm to the grass filter at a slower rate, thereby allowing the grass filter to perform more effectively.