Analysis of Rhodamine– WT and sodium chloride as stream tracers
Conservative tracers help to understand solute transport processes such as advection, dispersion, and transient storage. Two popular stream tracers are Rhodamine- WT (RWT) and sodium chloride (NaCl). NaCl is considered to be conservative, yet it cannot be detected at very low concentrations. RWT, on the other hand, can be detected at very low concentrations (<0.1 ppb), yet it is known to sorb to organic materials and photo-degrade. I am studying the limitations and advantages of both RWT and NaCl as applied to large river slug injections. Our research team seeks to analyze the window of detection for each tracer (time of tracer arrival to time of tracer non-detection), as well as the late-time tailing behavior of each.
For the past few weeks, I have been working to complete various combined slug additions of RWT and NaCl along a 1.5-kilometer reach on the Kuparuk River, an Arctic river located on Alaska’s North Slope. Both tracers are simply mixed together with river water in large buckets until the NaCl completely dissolves. Fluorescence and electrical conductivity (EC) are continuously sampled at the upstream and downstream ends of the reach prior, during, and after the injection, until fluorescence and EC return to ambient (pre-injection) levels. Various flow conditions have been characterized, and I plan to complete additional injections during the rest of my stay at Toolik Field Station. With a larger data set, the results will contribute to experimental design of conservative tracers in large rivers, and weigh the benefits of RWT versus NaCl.