Watershed Tracer Studies

Water quality is one of the most significant considerations in working forests, and many of our current best management practices have been designed to protect our water resources. Diana Karwan, Assistant Professor in forest hydrology and watershed management, works with watershed tracer studies to better understand how sediments move through and interact with streams. In turn, her work can then help improve and design best management practices that are scientifically sound and impactful in forest watersheds.

There are two different styles of watershed tracer studies, and Karwan works with both varieties. One style of study introduces something new to a stream or watershed to watch how it moves. This introduced signal can then be followed through the stream to study movement, time, and interactions with the streambed. In her studies, Karwan has introduced suspended materials like titanium dioxide. She says, “It’s a clay sized particle so it can travel either floating, suspended in the water or it can react with the stream bed, settle to the bottom, or stick to other portions of the stream channel.”

The titanium dioxide, when used as a tracer, can turn an entire stream white with relatively small amounts. Karwan explains, “The fact that you can see it provides a check on when your plume has passed by when you’re in the field, but the data actually comes from the chemical analysis of the tracer.” The amounts of tracer released into the stream are carefully added by finely calibrated pumps and then measured downstream at various points. This allows Karwan to accurately measure the amounts being introduced and make better assessments of how the tracer behaves through data gathered at the downstream stations. “We add the plume, sample downstream, and sample over time,” Karwan explains. “That allows us to study how the sediment moves differently than the water that carries it.” 

The other type of watershed tracer study, known as sediment fingerprinting, does not involve any additions to the stream. Instead, the unique chemistry along a stream is used to identify chemical signals that can then be measured downstream. For example, the upslope sediment chemistry might be unique to litter from the forest floor while downstream locations could be experiencing bank erosion that is providing a different set of chemical inputs into the stream. These unique chemical signals can then be measured through the watershed and picked apart to understand where materials in the stream are coming from and how they move through the environment. 

This process of fingerprinting chemical signals from sediment or other sources takes a long time to set up, sample, and analyze. In a single study on one watershed there might be dozens upon dozens of different chemical tracers being tested. From there, the data analysis begins.

Sediment fingerprinting offers an appealing way to study watersheds, but the methodology is fairly new. These types of studies have been used for a decade but interest in them has increased in the past few years. Karwan notes, “What’s happening now in the scientific literature is that people are applying this in different ways and there are lots of questions, of course. This could be a great tool but we need to make sure the science behind it is on solid footing so that we’re applying it in a proper way.”

Karwan is working with introduced tracers in working forests to then apply the results to management questions. She has conducted watershed tracer studies in Idaho and Connecticut but is also working to develop basic monitoring stations on forested land in northern Minnesota. Once the baseline information is gathered, experiments can be applied and key research questions can be addressed.

The issue of time is another area Karwan would like to explore. “Some of the foundational work was done in the geology community, and they have a very different sense of time,” Karwan notes, compared to a water quality specialist who is interested in immediate effects. “It’s one thing to say during this storm event this is what we see coming out of the stream. That notion of time is interesting, though, and how long did it take it to get there? It is an important issue that’s not as easily understood.”

In the end, Karwan is interested in helping refine watershed tracer methodology and applying findings to forest management. She says, “I would like to see a way that we can make sure this toolkit, where there is a lot of interest, is being applied in a manner that is scientifically sound. We need that on the chemistry side, on the stream transport side, and on the statistical analysis side. I would hope we could gain some better understanding of when we get pulses of materials coming out of streams, what brought them there? And on what timescale? So if you are applying a best management practice within your watershed, you have some notion of when you should see a change take place downstream.”