On-line fluorometry of multiple reactive and conservative tracers in streams

Performing tracer tests using artificial tracer compounds is a common practice to characterize natural streams regarding their (reactive) transport properties. Recently, the fluorescent compound resazurin was introduced as a reactive stream tracer to quantify hyporheic exchange and metabolic activity of streams. This tracer, together with its reaction product resorufin and a conservative tracer (in our case fluorescein), provides additional information about transport properties of the stream and its hyporheic zone and can therefore overcome restrictions that are commonly affiliated with the use of conservative tracers alone. However, all previously published studies using this tracer system were based on manual sampling of the water. This usually limits the number of measurements and thus the achievable temporal resolution, and potentially endangers data quality due to inadequate handling of samples. In this paper, a modified version of the GGUN-FL30 on-line fluorometer is presented in which the optics have been modified to allow measuring the concentrations of all three tracers simultaneously at intervals of 10 s. Experiments under controlled and natural conditions showed that the performance of the on-line fluorometer regarding tracer separation efficiency and practical detection limits is comparable to a high-performance laboratory spectrofluorometer. Furthermore, suggestions are given on how to correct tracer signal fluctuations caused by temporal changes in temperature and pH that might occur during a field tracer test.