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Paige Becker’s Research Showcase: Comparing tracer techniques to explore flow paths and transit time

I’m a third year Ph.D. candidate at Indiana University researching the hyporheic zone and river corridor in the central cascades of Oregon. I use a combination of modeling and field work to improve our understanding of fluxes and transit times of headwater mountain streams. Currently, I am living in Oregon at the H.J. Andrews Experimental Forest (HJA), which is part of the Long Term Ecological Research Network, where I have been conducting research since summer 2018. Most of my research is in a small catchment named Watershed 01 (WS01), which has been heavily studied by hydrologists since the 1990’s.

The goal of my fieldwork this summer is to compare the use of tracer experiments to understand how they represent various flowpaths and timescales within the river corridor. A tracer experiment uses a solute to ‘trace’ the water particles and monitor concentration of the tracer over time to develop a breakthrough curve (BTC). From the BTC, I can get advection and dispersion, transit time distributions, discharge, and mass gained and lost, etc. (Stream Solute Workshop). Previous use of tracer experiments has provided a stream-centric understanding of river corridor exchange, highlighting advective timescales and individual feature scales, limiting the ability to measure multiscale flow paths within the river corridor (Ward et al, 2013; Payn et al, 2009; Bergstrom et al., 2016). Multiscale flow paths can take days to weeks to experience complete turnover, but are important for processing of nutrients and various reactions within the stream.

The experiment I am working on this summer is to implement a channel water balance with a combination of salt and uranine tracers to measure these multiscale flow paths. The segments I’m studying are between two bedrock outcrops. I can calculate the total flows into and out of the segments with dilution gaging. Any additional discharge between the upper and lower ends are quantified as hillslope inputs. Salt tracers are used to quantify the advective flow paths and shorter hyporheic transit times. A multi-hour constant rate injection of uranine is used to trace longer flowpaths and extend the window of detection that is limited by salt tracers. From this, I hope to gain a better understanding of multiscale flowpaths and long-term subsurface flow paths and exchanges normally missed by salt tracers. Photos from this experiment can be seen below.

Figure 1: Raw data from the uranine sensors following a 6 hour injection. Note, this has not been cleaned in any way, nor converted to concentration of uranine. The aim is to highlight the patterns being observed.

The figure above shows the raw data from the uranine sensors following the injection. I injected uranine for 6 hours on the afternoon of the 29th about 360 m upstream from the gage house at WS01. The legend indicates the distance upstream from the gage that the uranine sensors are located. During base flow conditions, WS01 experiences diel fluctuations of discharge (discharge at the gage house can be found here) and the diel fluctuations of uranine that the sensors are picking up coincides with the discharge. As seen in the figure above, the stream has still not achieved background levels days following the injection. Salt tracers however, with their limited window of detection, have shown that background levels are reached within hours to a day at most, indicating that salt tracers are great for advection-dominated fluxes, but miss multi-day flow paths that exist within the watershed. I plan to continue the monitoring and ideally repeat the experiment once more but am looking forward to the in-depth analysis of the data I have collected thus far!

As with any field work, I have experienced challenges that have required me to make adjustments to my planned experiment. One of the major obstacles has been changes in the landscape that has shifted the stream within the watershed or caused fallen trees to block my access. In early 2019, the area experienced a snowstorm causing many trees to fall under the weight of the heavy snow, opening up the canopy, shifting sediment, and creating a mess of fallen trees to navigate (Figure 2). Then, in September of 2020, the Holiday Farm Fire came through, burning many of those fallen trees, further opening the canopy, increased sediment movement, and increased hazards along the way (Figure 3). Despite the obstacles (literally clambering over trees), I really enjoy field work and a break from the computer.

Figure 2: Photo of me in WS01 against a backdrop of fallen trees from the 2019 winter storm. Photo credit: Lina DiGregorio.

Figure 3: Burn scar in WS01 at HJA following the Holiday Farm Fire. Photo credit: Mark Schulz

Below is a collection of photos I have taken from my fieldwork including from the experiment.

Trashcan of uranine/ the injectate

The set up

The dye being added to the stream at the start of the injection

The stream just below the injection point after about two hours

Me taking instream readings after the injection of uranine

Me adding the uranine to the water

Me getting an instream reading of uranine concentration during the injection and the stream is visibly green

looking upstream at a large log jam that has caused the stream to split into two channels (one center left in the image and other bottom right)

The rough skinned newt, mascot of the HJA. They are everywhere and very cute.

By Paige Becker, PhD Candidate, Indiana University Bloomington, USA

Citations:

The University of Mississippi. 1990. Concepts and methods for assessing solute dynamics in stream ecosystems: Stream Solute Workshop, February 1-5, 1989. Journal of North American Bnthological Society. 9(2): 95-119.

Ward, AS, RA Payn, MN Gooseff, BL McGlynn, KE Bencala, CA Kelleher, SM Wondzell, T Wagener. 2013. Variation in surface water – groundwater interactions along a headwater mountain stream: Comparisons between transient storage and water balance analyses. Water Resources Research, 49(6), 3359-3374. doi: 10.1002/wrcr.20148

Payn, R. A., Gooseff, M. N., McGlynn, B. L., Bencala, K. E. & Wondzell, S. M. Channel water balance and exchange with subsurface flow along a mountain headwater stream in Montana, United States. Water Resour. Res. 45, (2009).

Bergstrom, A., Jensco, K. & McGlynn, B. Spatiotemporal processes that contribute to hydrologic exchange between hillslopes, valley bottoms, and streams. Water Resour. Res. 52, 4628–4645 (2016).

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