Comparison of rhodomine-WT and sodium chloride tracer transport in a 4th order arctic river

Conservative tracers are useful for tracking a parcel of water through a river reach and understanding tracer transport phenomena (i.e. advection, dispersion, and transient storage). Rhodomine- WT (RWT) and sodium chloride (NaCl) are two popular stream tracers. NaCl is considered to be conservative and relatively inexpensive, yet it cannot be detected at very low concentrations. On the other hand, RWT can be detected at very low concentrations (<0.1 ppb), but it is known to photo-degrade and sorb to organic materials. Previous work has compared these tracers with small-scale laboratory analyses and field experiments on small headwater streams. The limitations and advantages to each of these tracers, as applied to large river slug injections, are not clearly understood. This work seeks to answer the following questions: 1) Does RWT improve the tracer window of detection (time of tracer arrival to time of tracer non-detection), compared to NaCl? 2) Are there differences in the late-time tailing behavior of each tracer? More specifically, can we compare RWT and NaCl breakthrough curve tail shapes to understand processes contributing to late time solute transport (transient storage or sorption-desorption)? During the summer of 2012, combined slug additions of RWT and NaCl were injected into a 1.5-kilometer reach on the Kuparuk River, a 4th order tundra river underlain by continuous permafrost located on Alaska's North Slope. Fluorescence and electrical conductivity were continuously logged at the upstream and downstream ends of the reach. Preliminary results show that the window of detection is expanded when using RWT under both high and low flow conditions by 0.2 times the advective transport timescale. Tail shapes are more similar under higher discharge conditions and dissimilar under lower discharge conditions. For example, using an exponential regression model (c(t) = eat) to quantify tail shapes, at Q = 500 l/s the exponential coefficient ratio, aRWT:aNaCl, is 0.80, while at Q = 1400 l/s aRWT:aNaCl is 0.98 (Figure 1). We expect to expand the scope of our results by analyzing a larger set of experiments and exploring different modeling techniques of BTC tails. These results have implications for experimental design of conservative tracer additions in large rivers. Differences in RWT behavior at various flow conditions indicate limitations for its use as a conservative tracer at lower discharges on a given reach. At higher discharges, RWT provides the benefit of a broader data set without considerable effects from sorption-desorption processes. Figure 1: NaCl and RWT BTC tails (tpk - t99) under two contrasting discharge conditions. A more defined difference in tail shape is present at 500 l/s compared to 1400 l/s. Also, the window of detection is longer for RWT compared to NaCl under both flow conditions.