Bromide transport at a tile-drained field site: experiment, and one- and two-dimensional equilibrium and non-equilibrium numerical modeling

Systematically tile drained field sites have been recognized as one major source for surface water contamination with agrochemicals. To study the effects of tile drainage and physical non-equilibrium on solute transport in structured soil, bromide (Br−) transport experiments were carried out on three plots (A, B, C) with different tile drain depths (128, 101, 96 cm) and spacings (16, 18 and 12 m) at the Infeld experimental field site (North–West Germany). Tile drain outflow along with Br− concentrations were monitored over a half-year period. For all three plots, fast Br− breakthrough was observed with Br− concentrations fluctuating around levels below 6 mg/L during the experiment without a distinct concentration maximum. Experimental observations of plot B were analyzed using one-dimensional (1D) and two-dimensional (2D) single-porosity model (SPM) and mobile-immobile model (MIM) approaches. All SPM and MIM parameters were obtained from independent measurements, except for the calibrated MIM water and solute transfer coefficients. Water flow and Br− transport were then predicted for the A and C plots using the model parameters as obtained for plot B. Measured plateau-like Br− concentrations could only be consistently calibrated (plot B) and predicted (A and C) using the 2D-MIM approach, while the 1D-MIM, 2D-SPM, and 1D-SPM approaches (in this order) increasingly deviated from the experimental data. The MIM simulations suggested that solute transfer into the immobile region represented more than 60% of the surface applied Br−. Non-equilibrium transport with advective and diffusive mass transfer increased early mass loss and entailed extended, slower leaching as compared to equilibrium transport. Furthermore, model simulations suggested that the two-dimensional flow field as induced by tile drains enhanced Br− dispersion and accelerated Br− appearance in the drain. This study showed that both the variably-saturated 2D flow field and physical non-equilibrium transport should be explicitly accounted for in physically based model simulations of solute transport in tile-drained structured field soils.