Nitrogen transport and dynamics in grass filter strips

Field research was conducted to investigate the impact of vegetative filter strips (VFSs) on surface runoff water quality and to determine if this impact decreases with time. The field research provided information for the development and testing of a model to describe the dynamics and fate of nitrogen (N) in VFSs. The experiment had a completely randomized design, with 3 treatments and 2 replicates per treatment. The treatments were 3 VFS lengths: 0, 4.27, and 8.5 m VFSs.

The distribution free Kruskal-Wallis test indicated that the runoff, TSS, NO₃⁻-N, NH₄⁺-N, and TKN yields and concentrations from the 8.5 m VFSs were significantly less (α = 0.05) than the influent values. The TSS and NH₄⁺-N yields and concentrations and the TKN concentration from the 4.27 m VFSs were significantly less than the influent yields and concentrations. The Mann-Kendall test indicated that the yields of TSS, NO₃⁻-N, NH₄⁺-N, and TKN from the filters did not significantly increase from 1992 to 1993 and neither did the FTKN yield nor the FTKN concentration from the beginning to the end of 1993.

The mean percentage reductions in influent runoff, TSS, NO₃⁻-N, NH₄⁺-N,, and TKN yields from the 8.5 m filters were 73, 91, 79, 86, and 83%, respectively. The mean percentage reductions in influent TSS, NO₃⁻-N, NH₄⁺-N, and TKN concentrations from the 8.5 m filters were 88, 50, 66, and 75%. The mean percentage reductions in influent runoff, TSS, NO₃⁻-N, NH₄⁺-N, and TKN yields from the 4.27 m filters were 43, 83, 55, 40, and 56%, respectively. The mean percentage reductions in influent TSS, NO₃⁻-N, NH₄⁺-N, and TKN concentrations from the 4.27 m filters were 81, 45, 26, and 41%.

Based on the information gathered from the experiment results and the literature, a continuous, long-term, field scale model (Grass Filter Strip Model, GFSM) was developed to describe N transport and dynamics in VFSs. The model was based on GRAPH (GRAssed-strip-PHosphorus), a field scale, event-based model that describes sediment and P transport in runoff. The model simulates sediment, nitrate, sediment-bound and dissolved ammonium, and sediment-bound organic N transport during a runoff event. The model simulates the daily percolation and evapotranspiration and dynamics of nitrate, sediment-bound and dissolved ammonium, and sediment-bound organic N in the filter between runoff events. The model predicts the amount of N and sediment exiting the VFSs, and it can be used to estimate the site specific effectiveness and length of VFSs. The model can also be run for an event to assess the effectiveness of VFSs in reducing nonpoint source pollution loading from a single design storm.

The model was validated using runoff, sediment, NO₃⁻ and NH₄⁺ yield field data gathered from April to December, 1993. The model predicted reasonably well (within a factor of 2) the cumulative runoff volume and the yields of TSS. NO₃⁻ and NH₄⁺. The model was most sensitive to the runoff rate, depth of the EDI, soil water storage depth, field capacity, and the steady-state infiltration rates.

The model was used to determine the minimum length of VFS required for a 1.3 ha field in Georgia to achieve 75% and 40% sediment and nutrient reductions, respectively, over a 10-year period. The model results indicated that a buffer length of 6.3 m was sufficient to reduce sediment and nitrogen losses by the specified percentages.