A modelling approach to the design of in situ agricultural drainage filters

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Abstract
Agricultural drainage ditches provide a direct pathway for high nutrient loads such as phosphorus (P) from fields to surface waters. Most practices designed to reduce P loading from fields focus on overland flow and sediment‐bound P; however, P transport in these landscapes can be dominated by subsurface transport of dissolved P. Filters have been designed and are currently being field‐tested that utilize P sorbing materials to remove P directly from flow in ditches. Models have been developed to predict P removal and the time of filter effectiveness. While filter performance along with model development and validation is examined in separate papers, there is a need to translate these models into usable tools for watershed managers who have to decide on filter design. This study provides examples of how these models can be used to design P‐removal structures containing electric arc furnace slag (EAFS) or flue gas desulphurization gypsum (FGDG). Through the use of realistic parameters for retention time, inflow P concentration and material characteristics, the presented examples demonstrate that EAFS would outperform FGDG across a range of inflow P concentrations (0.5–10 mg/L) and removing 28–65% more P. With an average inflow P concentration of 1.62 mg/L (typical for ditches on the Delmarva Peninsula, United States), a 20‐Mg EAFS filter would remove 42% more P than a 31‐Mg FGDG filter. In addition, the EAFS filter would have a flow rate 10 times that of the FGDG filter. The predictive equations discussed in this paper can be used to determine the cost per unit of P removed of filters containing different materials.

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