Response to the slug injection of a tracer : a large-scale experiment in a natural river

A unique, large-scale tracer test performed along a 90-km reach of a natural river is presented. This method was crucial for evaluating the impact of a retention reservoir on protected areas of the river downstream, and to assess the threats due to potentially catastrophic releases of toxic substances into that river. The response to the slug injection of a soluble tracer is assumed to imitate the characteristics of a soluble pollutant, so an understanding of how tracers mix and disperse in a stream is essential to understanding the processes of pollution transport. The procedure applied during this experiment consisted of the instantaneous injection of a known quantity of Rhodamine WT into the stream and the determination of the temporal variation in concentration of the tracer at sites as it moved downstream. The results were analysed from the perspective of a transient storage model. Relevant model parameters were evaluated by fitting the computed breakthrough curves to the observed ones on a reach-by-reach basis.