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Gauging the flow of Southeastern waterways

A few years ago, the  reached out to IRIS Director Brian Bledsoe with a concern. Waterways across its Southeastern corporate footprint were running lower than in the past—a phenomenon that could have significant business implications—and its leaders wanted some empirical data.

Southern Co. uses natural waterways to cool its power plants, especially those with nuclear reactors. It does this in two ways: By piping in stream water to cool reactors, and by releasing heated water back into the waterway. In the first process, low-flowing streams may actually drop below the level of a plant’s intake pipes. In the second, the heat itself acts as a contaminant, as aquatic ecosystems can be greatly affected by shifts in water temperature.

“You’ve heard the saying, ‘Dilution is the solution to pollution,’ right?” asked Bledsoe, UGA Athletic Association Professor in Resilient Infrastructure in the . “If you have more water in the river and you put in a certain load of pollutant, the concentration depends on how much water is there.”

Using data provided by 349 U.S. Geological Survey (USGS) stream gauges across the study area, Bledsoe and former Ph.D. student Timothy Stephens  going back as far as 100 years. They identified the historical “Q7,” or the lowest-flowing seven-day period each year, at time intervals of 25, 50, 75, and 100 years, then analyzed the comparative data.

Assessing wastewater and sewage challenges

Many cities along the I-85 corridor rely on small rivers for water supply and waste assimilation. However, as populations continue to grow, infrastructure is beginning to strain.

The U.S. Southeast is a global hotspot for freshwater biodiversity, and rivers along the corridor have been likened to “underwater rainforests.” However, rivers face many of the same problems that threaten aquatic animals and human well-being, including low water supply during droughts, seasonally low flows for wastewater dilution, increasing drought and precipitation extremes, and downstream “eutrophication,” a process by which an area of water becomes overly saturated with nutrients, leading to the growth of simple plant life such as algal blooms.

While more populated cities and counties have sewage systems to manage wastewater, many homes along the I-85 corridor rely on septic systems. Aging and failing septic systems are often linked to declining water quality in rivers and lakes, but septic management is largely the responsibility of homeowners. Most communities do not maintain detailed information about septic infrastructure.

With the anticipation of more population growth, Krista Capps, an associate professor in the  and the , is just one of the many experts searching for ways to address complex issues associated with septic infrastructure.

“Many communities don’t know where their septic tanks are located, how many systems are in use, or what condition they’re in,” Capps said.

Without proper oversight, even relatively new septic tanks can malfunction and release untreated wastewater into the surrounding environment, possibly affecting downstream communities. Many streams along the I-85 corridor are polluted by gut bacteria such as E. coli, which is often attributed to untreated sewage from septic systems.

After all, Capps pointed out, we all live in watersheds, and water management decisions affect everyone in the region.

“In other words, we are drinking water that has been recycled through other people,” she said. “One of my research goals is to support communities in making informed water management decisions and reduce negative impacts on the quality of water moving downstream.”

To support more effective management of septic infrastructure, Capps and colleagues are working with local, regional, and state water managers and public health officials to assess how septic system function changes with extreme weather events, including intense precipitation and droughts. They are also working to develop new resources for local governments to support evidence-based septic management decisions.

Mitigating urban climate effects for ‘thermal and flood justice’

Ask the average American about the deadliest weather events and you’ll probably get one of two answers: tornadoes or hurricanes.

“They’re the most telegenic atmospheric forces that draw my colleague Jim Cantore into those dramatic news segments,” joked Marshall Shepherd, Georgia Athletic Association Distinguished Professor in the , in reference to the well-known . “In reality, [the most dangerous events are] actually heat and flooding.”

Between 2018 and 2020, more than 3,000 people in the U.S. died from heat-related causes, according to the Centers for Disease Control. Flooding, meanwhile, has cost U.S. taxpayers more than $850 billion since 2000, more than two-thirds the cost of all natural disasters.

Research from Shepherd and colleagues has demonstrated that heat, rainfall, and even lightning are more pronounced in urban centers and what he describes as “climate archipelagos,” or urban chains like the one from Atlanta to Charlotte along the I-85 corridor.

It presents unique challenges in areas often marked by already vulnerable populations.

Urban heat islands (UHIs) are a significant part of the discussion. They form as hard surfaces like asphalt, pavement, and roofs absorb solar radiation and re-radiate it in the form of infrared heat. The lack of urban vegetation also reduces cooling, while buildings and surfaces exchange heat, and car engines and other machinery also contribute their own waste heat.

Using data from 1984 to 2007, Shepherd’s lab found a mean UHI magnitude of 1.31 degrees Celsius when comparing Atlanta to the relatively smaller Athens area.

The figure here illustrates the concept with a pair of satellite images taken of Atlanta in 2000, demonstrating temperature variability of up to 12 degrees Celsius in developed urban centers.

But it isn’t just the heat.

Rainfall also is a major challenge, with increased rainfall totals around Atlanta during the warm season. Lightning prevalence is another; during the period from 2018 to 2021, Shepherd and his former student Jeffrey Burke found increases in total flash rate density (14.3%), flash days (8.3%), and average flashes per day (5.5%) between urban and rural regions.

“It’s driven by a combination of a few factors,” Shepherd said. “Cities are warmer and tend to have more destabilized air. Buildings are taller and surfaces rougher, so air is more convergent at low levels. There tend to be more aerosols, or pollution, in the air.

“We think a combination of these is driving anomalies in rainfall and lightning that we’ve seen.”

Taken in aggregate, each of these factors can have an even larger impact on urban archipelagos.

“If you think about a mountain, it can impact rain or snowfall,” Shepherd explained. “Now think of a range of mountains like the Rockies and the extended spatial impacts of a system of mountains on the rainfall patterns. That’s what we think exists in these urban centers.”

Socially and economically vulnerable populations living along the I-85 corridor feel that burden more acutely, Shepherd said, in the form of things like higher electricity bills for air conditioning. But there are mitigation techniques: increased urban vegetation, more permeable surfaces that allow water to filter to the soil underneath, and an entirely new approach to stormwater management.

“Many of our cities are engineered under the assumption of what my colleague Brian Bledsoe in IRIS calls ‘stationarity’—that storms in 1970 will look like the storms of 2024,” Shepherd said. “That’s not the case—they have grown in intensity. We propose radically reimagining how we plan and design our cities to engineer them for what we call ‘thermal and flood justice.’”

Protecting our lakes and streams

Agriculture is Georgia’s No. 1 industry. The state is a primary producer of goods like cotton, peanuts, onions, pecans, and corn. According to the , farmland comprises some 26.7%—nearly 10 million acres—of Georgia’s overall land.

With all of this comes a challenge, however: Livestock produce manure, and crops require fertilizer and pesticides. Both pose threats to nearby water sources.

It’s something  Professor Rhett Jackson works to better understand in hopes of offering solutions to Georgia farmers and forest owners.

Jackson (lead author of the cited at the top of this story) examines non-point source pollution, or pollution that is being carried not through an engineered conveyance system—like a pipe—but through natural processes in the ground.

“Agriculture is really all about non-point source pollution,” said Jackson, John Porter Stevens Distinguished Professor of Water Resources and director of the River Basin Center. “Ag streams tend to be highly polluted, but none of it is coming from the ways we usually think about.”

Instead, agricultural pollution typically derives from excess fertilization, manure, or pesticide application, or livestock access to streams, from which solutes travel through groundwater into streams, lakes, and estuaries. The result may be algal blooms, toxic cyanobacteria growth, fish kills, and poisoning of wildlife.

But Jackson and colleagues study a natural solution that can mitigate this pollution.

Riparian buffers are zones of undisturbed soils and vegetation along streams, usually featuring shallow water tables and copious organic matter. Within these zones, a process called denitrification can cycle nitrates from the ground back into the atmosphere.

Heterotrophic bacteria—bacteria that absorb nutrition through other sources of organic carbon like plant or animal matter—turn the nitrogen into gas and expel it back into the atmosphere, eliminating many of the harmful nitrates that pollute the stream.

Jackson’s team aims to quantify just how effective these riparian zones are. To study them, they overfertilized a pine forest near a stream and identified excess nitrates that contaminated the forest’s groundwater.

“We saw the contamination in the groundwater, but we never saw nitrate in the streams,” he said. “Enough time had elapsed that it should have been there.”

The denitrification rates?

“Over a 90% reduction,” Jackson said.

To Jackson, the takeaway is clear: We should encourage farmers and those who are fertilizing these lands across the state to avoid removing crucial vegetation that provides a buffer between pollutants and nearby streams.

“This,” he said, “is a very cost-effective strategy for reducing non-point source pollution.”