Comparison of Newly Proposed and Existing Design Approach for Saturated Buffers

Highlights More diverted flow to the buffer does not necessarily mean more nitrate load removal. A design approach should incorporate a nitrate removal component to maximize nitrate load removal. The newly proposed design follows a process-based approach to estimate the annual site-specific nitrate removal. The newly proposed design provided more consistent nitrate load removal regardless of the site conditions. Neglecting exit head loss in the design process leads to an overestimation of diverted flow and nitrate load removal. Abstract. A saturated buffer (SB) is a conservation drainage practice that removes nitrate from subsurface drainage discharge. The reported wide range of nitrate load removal necessitates improvements in design approaches for more consistent performance. There are two SB design approaches: Illinois Natural Resources Conservation Service (Design 1) and McEachran et al. (2020) (Design 2). We proposed a new Design 3 that builds on the previous two designs. In Design 3, the nitrate load removal was simulated for buffer widths ranging from 3 to 30 m with a 0.3-m interval, and the buffer width that maximized the annual nitrate load reduction over the long term was chosen as the SB design. The objective of this study was to identify the best design approaches for maximizing nitrate load removal based on field data. Daily drainage discharge data from two field sites in Michigan were used to design a hypothetical SB length and width for each approach. The designs were compared by applying an identical method to estimate the nitrate load removal for each hypothetical SB system. The method extends Designs 1 and 2 by incorporating a hydrological and nitrate removal component. The results showed that using the minimum recommended buffer width of 9.1 m and the minimum 5% SB design capacity of Design 1 resulted in 25% to 35% of diverted flow to the buffer and 14% to 16% nitrate load removal at the two field sites. However, Design 1 resulted in the lowest nitrate removal compared to Designs 2 and 3 (i.e., 0.3% to 3.4% lower). Designs 2 and 3 consistently provided maximum nitrate load removal regardless of the site conditions, whereas the performance of Design 1 was inconsistent. In conclusion, Designs 2 and 3 were equally good and resulted in higher nitrate load removal compared to Design 1. Keywords: Conservation practice, Exit head loss, Nitrate, Subsurface drainage, Tile drainage, Water quality.