Heavy rains are expected in NC’s future. Here’s why Charlotte waterways could suffer.
Incidents of heavy rainfall, which North Carolina climate experts say have increased in recent years and will continue to in the future, might also cause more large sewage spills in Mecklenburg County.
Since 2017, 250 million gallons of sewage have leaked from pipes in North Carolina, a Charlotte Observer analysis found. Of that total, about 27.8 million gallons — enough to fill 42 Olympic-size swimming pools — spilled in Mecklenburg. Only Harnett County, south of Raleigh, had a higher total.
Last month, nearly 850,000 gallons of raw sewage spilled from a Charlotte Water line into the Catawba River near Old Dowd Road. The incident added to the abnormally long list of large spills that have occurred in the county over the past four years.
The largest sewage spills by Charlotte Water in the past decade all occurred between 2017 and the end of 2020 — a series of spills of more than 1 million gallons each not seen since 2003, according to annual reports by Charlotte Water.
While the July spill was attributed to a construction error, all four of the other large spills in the past four years were related to flooding and trees falling due to excessive rain, the reports show.
Before 2017, the last spills of over 1 million gallons in the county were in 2003, an abnormally rainy year according to data from the National Centers for Environmental Information, a federal agency with an Asheville office. Four of the five major spills that year were related to heavy rain events, Charlotte Water records show.
“I’m not at all surprised,” Brian Magi, an associate professor of atmospheric sciences at UNC Charlotte, said of the recent rain-related spills. “The pressure is on with all of our built systems.”
The number of extreme precipitation events in NC has increased over the last decade, Magi said. As climate change warms the atmosphere, scientists say, the amount of water vapor in the atmosphere increases. That means more heavy rainfall events.
While the number of storm and flooding events in North Carolina over the past 20 years has fluctuated significantly, NCEI says, 2020 saw more floods and flash floods in Mecklenburg County than in the previous three years combined.
A report by the N.C. Institute for Climate Studies, a University of North Carolina system research institute, updated in 2020, says extreme precipitation frequency and intensity in North Carolina is very likely to increase thanks to increases in atmospheric water vapor content. Heavy rainfall accompanying hurricanes that pass near or over the state is also very likely to increase, the report said, increasing the potential for freshwater flooding in the state.
“Storms are unpredictable, but we can predict that there will be more storms,” said Jennifer Frost, public affairs manager of Charlotte Water.
Still, heavy rain doesn’t always prompt big sewage spills in Mecklenburg County, Charlotte Water data shows. The year 2010 had as many heavy rains as in 2003 and 2020, according to NCEI, but recorded no spills of more than 1 million gallons and one of the lowest total volumes of spills of the past 20 years.
Pipe failures, line-cracking tree roots, and clogs from debris and grease cause most overflows, Charlotte Water records show.
“A lot of it is old, aging infrastructure,” said Brandon Jones, the Catawba Riverkeeper. “If you look at the number of spills they have per mile, Charlotte Water stacks up pretty well. The problem is, they are moving so much waste, when they have a spill, it can be extremely large, and a lot of their pipelines are on drinking water reservoirs.”
But extreme precipitation can cause the soil around the bases of trees to wash away, exposing their roots and making them more vulnerable to being uprooted, UNCC’s Magi said.
In October 2017, 4.8 million gallons of raw sewage spilled into Mallard Creek due to a tree falling on a pipe after a storm, Charlotte Water records show. Six months later, a tree falling in heavy rain caused soil erosion under a pipe, resulting in a break that dumped 15 million gallons of sewage into Long Creek. In September 2018, overnight storms eroded creek banks, destabilizing a tree that fell and broke a pipe, spilling 2.6 million gallons of wastewater into Taggart Creek. In February 2020, over 2 million gallons spilled into Stewart Creek after heavy rain eroded a creek bank, causing a wastewater pipe to break.
Health, environmental risks
Charlotte Water mostly relies on gravitational flow to move wastewater through pipes, following the natural topography of the land. That’s why sewage lines often run near creeks and rivers, Frost said.
When untreated sewage spills into bodies of water, bacteria, viruses, parasites, molds and fungi are also released. Exposure to these microorganisms, such as while swimming, may lead to stomach cramps and diarrhea or even life-threatening ailments such as cholera and hepatitis, according to the Environmental Protection Agency.
Sewage is also full of the nutrients nitrogen and phosphorus, Riverkeeper Jones said. Too many nutrients in bodies of water can cause algal blooms and eutrophication, which is characterized by excessive plant and algal growth that can rob the water of oxygen and lead to fish kills.
Preventing future spills
Charlotte Water has more sewer lines than any other system in the state, 4,300 miles of them, almost double Raleigh’s 2,500 miles of lines. The utility says 99.99% of all wastewater that passed through its system in the 2020 fiscal year was safely collected, treated, and discharged.
With more than 450 spills between 2017 and mid-2021, Mecklenburg County racked up 60% more overflows than Wake County, which has a similar population size. Mecklenburg’s total volume spilled, almost entirely by Charlotte Water, was 27.8 million gallons to Wake’s 23.7 million. Wake’s figures include spills by Raleigh and other wastewater systems. Both counties had four overflows of 1 million gallons or more.
Even with last month’s overflow into the Catawba River, the number of spills and the total wastewater released in Mecklenburg County is down so far this year compared to the same period last year. Spills are also down statewide compared to 2020, data show.
To prepare for more extreme weather ahead, Charlotte Water does vegetation maintenance near wastewater pipes, such as by cutting unstable trees, and addresses creek bank erosion before it causes a collapse. A rehab program replaces or recoats the interiors of old pipes.
“Quite a bit of our system has been there since the ‘70s and ‘80s and is expected to reach the end of life soon,” Frost said.
The utility also uses closed-circuit TV cameras and sound wave technology to search for pipe blockages. The number of sewage spills caused by debris has increased by about 10% since 2017, making it the most common culprit, according to Charlotte Water’s annual wastewater performance reports.
“Debris is code for disposable wipes,” Frost said. “They have gotten more and more plentiful. They will decompose over time, but there’s not enough time between flushing and how long it takes to get to the treatment plant for it to degrade. Toilet paper takes a minute to degrade. Wipes take days or weeks.”
Worst of all, she said, are “fatbergs” that form when grease poured down sinks combines with debris like wet wipes and cotton buds to create a solid, stinky block that is virtually impossible to break down. They can cause huge sewage blockages that lead to spills.
Residents can help reduce spills by flushing only toilet paper down the toilet and pouring cooking grease into the trash can rather than down the sink, the utility says.
“The public is part of the solution to this,” Frost said.