Visualizing bromide and iodide water tracer in soil profiles by spray methods

In this study we developed and tested a spray method to visualize bromide water tracer in soil profiles. The method is based on the transformation reaction of a white precipitate into a colored one (Prussian blue) in the presence of Br-. After application of water containing bromide (0.2-0.4% wt.), a soil profile is dug out from the irrigated area and sprayed with a Br- indication suspension containing ferric ion and silver ferrocyanide precipitate. About two hours later, the pattern of irrigation water movement in the soil profile appears due to the formation of Prussian blue complex. We describe the method and demonstrate its use in a field experiment to visualize water flow paths. Since this method might be subject to possible interference from Cl-, a newly designed method with iodide ion as a water tracer and its indication solution containing soluble starch and ferric ion is also presented and recommended for use in soils with high chloride background.