Field experiment and numerical simulation of point source irrigation with multiple tracers.

Dyes like Brilliant Blue have similar adsorptive behaviour as some organic contaminants, e.g., pesticides. Bromide ions, on the other hand, move much like NO3-N (fertilizer) in soil. Consequently, by using these two tracers, it is possible to in a general way mimic how organic contaminants and fertilizers may move through soils. Three plots with sandy soil in semiarid Tunisia were irrigated during three successive hours using a single irrigation dripper and high-saline solution (10.50 dS m-1) containing dye and bromide. Fifteen hours after cease of infiltration, horizontal 5 cm trenches were dug in the soil and dye pattern, bromide concentration, and soil water content were recorded. Preferential flow occurred to some degree, however, it did not dominate the solute transport process. Therefore, drip irrigation can be recommended to improve plant culture for a better water and soil nutrient adsorption. Numerical simulation using HYDRUS-2D/3D was performed to replicate the field experiments. Observed soil water contents before and after infiltration were used to run an inverse parameter estimation procedure to identify soil hydraulic parameters. It was found that for both field experiments and numerical simulations the mobility of bromide is different from the mobility of dye. The dye was retarded approximately twice by volume as compared to bromide. The simulation results support the use of HYDRUS-2D/3D as a rapid and labor saving tool for investigating tracers' mobility in sandy soil under point source irrigation.