Modeling runoff pollutant dynamics through vegetative filter strips: A flexible numerical approach

Methods to estimate surface runoff pollutant removal using vegetative filter strips usually consider a limited number of factors (i.e. filter length, slope) and are in general based on empirical relationships. When an empirical approach is used, the application of the model is limited to those conditions of the data used for the regression equations.;The objective of this work is to provide a flexible numerical mechanistic tool to simulate dynamics of a wide range of surface runoff pollutants through dense vegetation and their physical, chemical and biological interactions based on equations defined by the user. This is the first time this approach has been used in vegetative filter strips. The ultimate goal of the model's flexibility is to help researchers and decision-makers estimate optimal filter characteristics (length, slope, vegetation) to achieve targeted runoff pollutant removal efficiency, while still considering the complex driving processes.;A flexible water quality model based on the Transport and Reaction Simulation Engine program (James, 2008b; Jawitz et al., 2008; James et al., 2009) is coupled to a transport module based on the traditional finite element method to solve the advection-dispersion-reaction equation using the alternating split operator technique. This coupled model is linked to the VFSMOD-W program (Munoz-Caperna, 1993; Munoz-Carpena et al., 1993a; Munoz-Carpena et al., 1993b; Munoz-Carpena et al., 1999; Munoz-Carpena and Parsons, 2004; 2010) to mechanistically simulate mobile and stabile pollutants through vegetative filter strips based on user-defined conceptual model where the pollutants and elements within the filter are expressed in terms of differential equations.;The numerical transport model was evaluated using analytical solutions and laboratory and field scale experiments. For the analytical testing, the water quality submodel performed well (Nash-Sutcliffe model efficiency coefficient > 0.99). The laboratory scale testing involved the simulation of bromide transport based on the exchange layer concept and the raindrop-induced chemical release theory (Gao et al., 2004). The model was able to explain the removal of bromide in runoff, but the incorporation of more processes is needed to give a more physically based explanation to the simulated results.;Field scale experimental testing involved the simulation of dissolved phosphorus in surface runoff, coming from phosphate mining tailing areas, through vegetative filter strips. The model helped to explain the dynamics of runoff dissolved phosphorus concentration through vegetative filter strips due to the dissolution of apatite (Kuo, 2007; Kuo and Munoz-Carpena, 2009).The difference between the field data and the simulated data total mass of dissolved phosphorus concentration was less than 1%.;The incorporation of physically based theories to the flexible water quality module provides a useful tool to explain and predict the removal of runoff pollutants in vegetative filter strips. Laboratory and field scale testing showed that the model can be applied to different conditions and scenarios.;One of the major advantages when using this tool is that the pollutant transport and removal thought vegetative filter strips is related to physically based process occurring within the filter. This mechanistic approach increases the range of use of the model when compared to the simplistic and limited empirical approach.;The use of more than one theory might be needed sometimes to simulate some pollutants. However, it is important to point out that the different theories to be used might not have additive properties, which could lead to the under or over estimation of the outflow concentration. The creation of a conceptual model that links how the pollutant interacts with the surrounding elements in the filter strip (i.e. grass, soil, sediments) is the key factor to consider in the flexible model. A poor conceptual model can lead to misunderstanding of the simulated results.;The potential uses and application of the model for different scenarios and pollutant models can help researchers, consultants, agencies or individual to predict the amount of runoff pollutants that can be removed when vegetative filter strips are used. The strength of the model relies on the mechanistic approach used for simulating the removal of pollutants, and the flexibility of the model allows the user to apply current and future theories when analyzing the dynamics of pollutants in vegetative filter strips.