PARSING THE SOURCES OF GROSS GAINS IN STREAM FLOW BASED ON MASS RECOVERY OF CONCURRENT INSTANTANEOUS AND CONSTANT-RATE TRACER RELEASES OVER MULTI-SCALED REACHES

Abstract:

Reach-scale solute transport studies are often used to characterize transient storage of solutes or groundwater-stream exchanges, yet the results are limited by window of detection issues (the limit of temporal and spatial scales over which one can infer processes from observed breakthrough curves). To determine the influence of exchanges into and out of reaches of different spatial scales, we conducted two simultaneous tracer injection types in a headwater stream in the Tenderfoot Creek Experimental Forest in central Montana, USA - one constant rate Rhodamine WT (RWT) injection (8 days) over 2.8 km of stream, and synoptic chloride releases in 28 100-m and 14 200-m subreaches when the RWT concentrations were at quasi-steady state. From the synoptic releases, we examined the spatial distribution of stream flow exchanges. Stream discharge increased from 0.8 L s^-1 (at the head) to 27 L s^-1 (at the base) along the 2.8-km study reach. Although the stream gains water at the scale of the 2.8-km study reach, stream water balances in the 100-m and 200-m subreaches demonstrate gross gains and losses. When comparing the summed gains and losses of the two 100-m subreach exchanges to that of the encompassing 200-m reach, the combined 100 m reaches always exhibit greater gain and loss. We interpret this as an indication that the 200-m reach includes return flows that appear to be losses from the 100-m subreaches. When we evaluate the change in RWT load at 100-m and 200-m subreach scales, we find typically less mass loss of RWT than is computed from the chloride slug releases, indicating the gain of RWT from long hyporheic flowpaths that originate upstream of the subreach of interest (even when RWT is likely to be a less conservative tracer than chloride). We can further parse gains of stream water at the 100-m and 200-m scales into estimates of hyporheic water (i.e., streamwater labelled with RWT returning to the channel) and lateral water (not labelled with RWT), and we find that a majority of water that enters the 100-m and 200-m subreaches is hyporhiec, rather than lateral inflow from sources unmarked by tracers.