When a massive chemical spill contaminated West Virginia’s Elk River in January, up to 300,000 residents were without access to potable water. Officials began lifting the ban on using tap water only a few days later, citing lowered concentrations of 4-methylcyclohexane methanol (MCHM), the licorice-smelling chemical used in the separation and cleaning of coal products.

But Tracie Jackson, a recent graduate of the Water Resources Engineering Ph.D. program at Oregon State, knows the problem might not be so short-term. With funding from the National Science Foundation, Jackson applied a fluid mechanics perspective to understand how long contaminants stay in a river system and where they end up — a concept called transient storage. Under the guidance of Sourabh Apte, associate professor of mechanical engineering in the College of Engineering and Roy Haggerty, professor of environmental geology in the College of Earth, Ocean, and Atmospheric Sciences, she developed tools to measure the residence time of contaminants within pools and other slow-moving areas, where solutes get trapped in the recirculating flow. Her interdisciplinary research is shedding new light on the complex way these contaminants move through a river system and impact water quality.

“If you understand how long a contaminant resides in one of these slower-moving areas and generates something called a residence time, you can understand how well the stream is working as a natural filter,” Jackson said. “Increased residence time increases the potential for pollutants to undergo different reactions in the stream, which can remove these pollutants and improve water quality. Besides helping a river flush out harmful substances, transient storage zones provide critical habitat for rearing fish and other aquatic life.”

Streamlining her approach

While she eventually co-authored seven publications on transient storage, Jackson admits that bringing together engineering and geosciences was initially challenging.

“When I started at OSU, I did not have an engineering background,” she said. “I have a B.S. in geology with a minor in math, and an M.S. in hydrogeology. So I had to start in the lowest fluid mechanics course for a graduate student and work my way up to the more challenging computational fluid dynamics and turbulence courses.”

Undeterred, Jackson immersed herself in the literature, reading everything from fluid mechanics, to solute transport theory, to residence time theory, to geomorphology, to sedimentology. Her days were often 16 to 18 hours long.

Roy Haggerty, her adviser from geosciences, said Jackson was determined to figure out the best way to study how contaminants move through a stream. “When I first asked her to research residence times distributions in water, Tracie read hundreds of journal articles,” he said. “She wouldn’t stop digging until she got to the bottom of it.”

It was during her literature review that Jackson realized current models insufficiently characterize solute transport in streams with transient storage zones. “Right now, we have very simplistic models with arbitrary parameters that can have a wide range of values. This creates a problem because these parameters often do not translate either to the same stream under different flow conditions or among different stream types. We need a new solute transport theory to understand how things move from A to B.”

A watershed moment

Jackson had a watershed moment — literally — when she went to a stream, poured dye into the water, and watched it move downriver. She realized that all localized stream features have something in common: a process driving solute exchange between the main channel flow and nearby storage zones that can be explained using fluid mechanics principles.

“Using a visual dye, I could observe how water in the main channel was exchanging with different localized stream features, such as submerged vegetation, meander bends, pools, and other transient storage zones. I was able to identify the types of flows that were driving solute exchange.”

Inspired by her observations, Jackson developed her own fluid-mechanics-based classification scheme that describes the most prevalent types of transient storage zones in streams, with the goal of understanding and quantifying their effect on solute mean residence time.

“The classification journal paper was one of my greatest achievements,” she said. “It took about two years to write because of the comprehensive literature review; however, the review really helped me to understand how different scientific and engineering disciplines view and study streams,” Jackson said.

Calculating flow dynamics

Jackson went on to write six additional publications based on her fieldwork, laboratory flume experiments, and numerical modeling.

“Tracie basically did everything,” said Sourabh Apte, Jackson’s adviser from engineering. “She did modeling, went into the stream, characterized the stream bed geometry, and worked in a lab. She is going to be a great researcher because she has a background in fundamental fluids as well as the geosciences. And she published really good work.”

Jackson also developed five equations to estimate solute mean residence times for different kinds of riverbed substrates, such as bedrock, gravel-bed, or clay. The equations are easy-to-use and less time-consuming than injecting dye into a stream — the standard method to calculate mean residence times.

“The great thing about these equations is hydrologists and water managers can go into the stream with nothing more than a flow meter to measure stream velocity and a measuring tape to calculate the width, length, and depth of the storage zone. It requires very little cost, time, or instrumentation and provides a good estimate of mean residence time,” she said.

Applying it to the real world

Jackson’s research has several important applications. In the case of a contaminant spill such as Elk River, her equations will help scientists more accurately quantify the downstream migration of pollutants and how they impact drinking water. Restoration engineers wanting to add transient storage zones to improve habitat and water quality could use the equations to optimize zone placement and size.

More broadly, her work sheds light on how river contaminants — such as nitrates from farms and lawns — eventually end up in the ocean and impact coastal wildlife.

Jackson is continuing her research as a hydrologist with the U.S. Geological Survey. There, she will be modeling groundwater flow within the Nevada National Security Site to assess the potential health and safety hazards of nuclear waste byproducts resulting from decades of nuclear testing.

Jackson acknowledges the support of her advisers and a few key contributors during her program, including David Hill in the School of Civil and Construction Engineering and mechanical engineering master’s student Kevin Drost. The disciplinary cooperation perhaps gave Jackson a more fluid perspective on how rivers work.

“Now when I look at water, I don’t look at it from one particular discipline or perspective,” she said. “I’m automatically thinking about how the interplay between fluid mechanics, channel hydraulics, and geomorphology drives the movement of water. It has given me a unique perspective as a hydrologist.”

— Abby P. Metzger