Characterizing unsaturated solute transport by simultaneous use of dye and bromide P. O ¨ hrstro ¨m a, * , Y. Hamed b , M. Persson a , R. Berndtsson a a Department of Water Resources Engineering, Lund University, Box 118, S-221 00 Lund, Sweden b Faculty of Engineering, Suez Canal University, Ismaelia, Egypt Received 15 October 2003; revised 20 October 2003; accepted 31 October 2003 Abstract Variability in solute transport properties in the unsaturated zone is of growing concern due to environmental hazards. By combining dye and salt tracers, a detailed description of the solute transport can be made in both time and space. In this paper, qualitative and quantitative effects of preferential flow were investigated in a sandy loam. The experimental site was equipped with 64 TDR probes. The plot was irrigated with pure water for 12 days to achieve steady state conditions. Then the plot was irrigated with a solution containing a dye tracer and KBr over three days. During the experiment, bromide concentration and soil moisture were measured hourly by the TDR probes. On the day following the last day of irrigation, a trench was dug and 14 vertical slices were excavated and photographed. A Sigma Probe (SP) was used to investigate the bromide concentration in an individual finger. The SP is a new dielectric measuring technique that gives readings of soil solution electrical conductivity. The dye photographs provided visualization of finger flow. Horizontal flow was evident in the E horizon at 0.3 – 0.4 m depth. Also the bromide concentration was elevated at these depths. The occurrences of high bromide as measured by the SP concentration corresponded fairly well to the dye patterns. The visible lower limit of dye stained soil corresponded to a bromide concentration of 0.10. A comparison between the bromide and dye penetration showed that the retardation factor of the dye as compared to bromide was 1.5. This is within the range reported by other authors and shows that the SP can be a useful tool to investigate the retardation of dyes as compared to bromide. q 2003 Elsevier B.V. All rights reserved. Keywords: Bromide; Dye; Preferential flow; Time domain reflectometry 1. Introduction Solute transport properties have received increas- ing attention in recent years amid growing environ- mental concerns. The presence of preferential pathways may cause significant losses of water and nutrients to the plants (Bouma and Dekker, 1978; Kosmas et al., 1991). Furthermore, harmful sub- stances spread on the soil surface might be bypassed directly to the ground water (Bouma and Dekker, 1978). Uneven moisture distribution in the form of fingers has been detected in wettable as well as non- wettable field soils (e.g. Ritsema et al., 1996). Dye, as a tracer, is a useful tool for revealing spatial flow patterns, and has been used by soil scientists for years. 0022-1694/$ - see front matter q 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2003.10.014 Journal of Hydrology 289 (2004) 23–35 www.elsevier.com/locate/jhydrol * Corresponding author. Fax: þ 46-46-2224435. E-mail address: pernilla.ohrstrom@tvrl.lth.se (P. O ¨ hrstro ¨m).