Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 153

Figure 84 Example of two different restitution curves for two neighboring springs during a
multitracer operation (from Mangan, 2000, unpublished).
Tracer tests are always delicate and costly operations, requiring a
rigorous operating and interpretation protocol. A few causes of failure
should be highlighted:
￿ quantity of tracer is too small;
￿ not enough points being monitored;
￿ tracer is injected into an impermeable compartment;
￿ monitoring period is too short;
￿ disappearance of monitoring equipment (theft, fl ooding);
￿ erosion of monitoring equipment or clogging by organic material;
￿ lack of circulation in the compartment (pronounced dry period). (This
last cause is very problematic, as the tracer will stay in storage until
the next rainfall. It can then modify the results of other tracer tests);
￿ contamination of samples by the tracing substance still present on
hands, injection equipment, car,….
The most commonly used tracers are fluorescent ones, as
spectrofluorimetric analysis is not costly and can detect very low
concentrations. Furthermore, field devices enable now to perform
continuous measurements.