THREE-DIMENSIONAL SIMULATION OF THE TEMPERATURE AND RESDIENCE TIME DISTRIBUTION DYNAMICS OF A CONSTRUCTED AGRICULTURAL TREATMENT WETLAND

This study demonstrates the development, calibration, and use of the 3-dimensional Environmental Fluid Dynamics Code (EFDC) hydrodynamic and mass transport model to study the environmental controls on the mixing dynamics of a 1.3 hectare constructed agricultural treatment wetland located in Story County, Iowa. Incorporating measured time-varying flow boundary conditions and atmospheric forcing, the EFDC model was calibrated against a set of measured wetland basin state variables including internal temperatures, basin hydraulic characteristics, and dye concentration observed at the outlet over the course of 6 independently performed field-scale hydraulic tracer studies conducted in the absence of submersed aquatic vegetation. Model calibrations indicate that EFDC reasonably reproduces observed basin internal hydraulics, temperatures, and mass transport dynamics, with mean absolute relative errors ranging from 0.02 to 16.3%. Model errors were highest for basin volumes and water depths, and lowest for temperatures, dye concentrations, and outflow discharge. The comparatively close correspondence between the observed and modeled volume-based residence time distribution (RTD) characteristics for each tracer study, as indicated by lower relative errors, further affirm the efficacy of EFDC in simulating the RTD dynamics of this system. Sensitivity analyses of the primary environmental factors of external wind shear, time-varying inflow rates, and atmospheric thermal forcing suggest that wind shear at the water surface exerts the greatest control on the development of the modeled (and by extension, observed) RTDs for this system for the environmental conditions encountered. The influence of wind was primarily realized in measures of short-circuiting. Internal temperature dynamics were also shown to maintain considerable influence on RTD characteristics, particularly bulk dispersion, during warmer operational periods. Transient flow conditions, however, were shown to have only nominal influence on most RTD characteristics for all tracer studies, save for mean and median residence times.