FLOW SEPARATION IN UNDISTURBED SOILS USING MULTIPLE ANIONIC TRACERS .1. ANALYTICAL METHODS AND UNSTEADY RAINFALL AND RETURN-FLOW EXPERIMENTS

Tracers provide one of the few ways of obtaining realistic information on the hows of water and solutes in undisturbed structured soils. Thr ee fluorinated organic acids [pentafluorobenzoic acid, 2,6-diflurobenz oic acid and o-(trifluoromethyl) benzoic acid] and bromide were tested as anionic tracers in situations where the separation of different fl ow components is of interest. The fluorobenzoates were relatively cons ervative (approximately 90% for loam soil) although, in some instances , co-elution or complexation may lead to apparently non-conservative b ehaviour. Tracer mixtures applied to soils, were separated and quantif ied by high-performance liquid chromatography. The relative mobilities of the tracers were studied during unidirectional steady flow in larg e undisturbed soil columns brought back to the laboratory. The breakth rough curves showed highly dispersive behaviour, with very early break through of solute and a long tail. Contributions to the column outflow of different simultaneous applications of rainfall and return flow, l abelled with different tracers, were separated using multicomponent mi xing equations. The results show the importance of preferential flow a nd relatively immobile storage in the transport process.