Sorption and desorption characteristics of the dye tracer, Brilliant Blue FCF, in sandy and clay soils

Dye tracers are widely used for tracing and visualising hydrological flow paths in soils. Despite evidence of non-conservative behaviour (i.e. sorption) in recent years, Brilliant Blue FCF continues to be used extensively for dye tracing in soil. However, previous studies have not considered hysteresis in the sorption mechanism. The objectives of this study were (1) to test the hypothesis that sorption of Brilliant Blue FCF on soils with widely contrasting textures could be described with a single sorption isotherm based solely on sorption by the clay fraction and (2) to determine whether desorption hysteresis is likely to confound the estimation of retardation factor in modelling dye transit through soil. We measured the sorption and desorption of Brilliant Blue on four soils of contrasting texture, ranging from a light sand to a heavy clay, and resolved a single Freundlich sorption isotherm for the clay fraction in all soils. An attempt to include silt and humus fractions as additional adsorbents with independent Freundlich parameters was unsuccessful. The sorption of Brilliant Blue FCF on the four soils studied was not fully reversible; there was apparent sorption–desorption hysteresis in all cases. Therefore, even though the sorption strength of Brilliant Blue in soils might be less than that of other dye tracers, it is possible that the visualised flow pathways may not accurately reflect those of water in soil. From this we suggest that Brilliant Blue may not be well suited for all forms of dye tracing in clay soils and it is therefore crucial that the specific environmental conditions, soil type, dye concentration and objectives of the study are considered each time when choosing the appropriate dye tracer.