Modeling Drainwater Selenium Removal in Wetlands

Prediction of selenium (Se) mass distribution and Se speciation in wetlands is desired for a comprehensive assessment of the capability of vegetated wetlands to remove Se from agricultural drainage waters prior to impoundment in evaporation basins. A mathematical model was developed to describe Se transformations and transport in Tulare Lake Drainage Districtʹs pilot flow-through wetland cells in California having dimensions of 15 m width and 76 m length. In the model, the wetland cell is subdivided into five subcells and each subcell consists of 10 internal compartments and Se can be transferred between the compartments by physical and chemical processes. Physical processes include water movement, plant litter drop, and physical material breakdown. Chemical processes include Se reduction from selenate (SeVI) to selenite (SeIV), and further reduction to elemental selenium (Se0) and organic Se (Se-II). In the chemical processes, the Se transformation reaction was assumed to obey first-order kinetics and the Arrhenius equation for temperature dependency. A total of 33 ordinary differential equations were written to describe all the processes within and between the internal compartments. A Fortran program with a numeric subroutine using the fourth order Runge–Kutta method followed by the four-step Adams–Bashforth– Moulton predictor-corrector method was written to solve the equations simultaneously. The daily rate model was successfully calibrated with data from a test plot planted to Smooth cordgrass and validated with data from test plots planted to Rabbitsfoot grass, Saltgrass, and Cattail. This model provided seasonal variations in Se mass distribution and Se speciation in different compartments by considering seasonal changes in water temperature, Se volatilization rate, and evapotranspiration. Sensitivity analyses on physical parameters such as water inflow rate, water depth, or longitudinal length of a wetland showed that decreasing the inflow rate or increasing the water depth or increasing the longitudinal length of a wetland cell can increase the accumulation of Se in compartments of a wetland cell and reduce the total mass of Se in the outflow water. Such information will be useful for developing engineering design criteria for constructed flow-through wetlands.