Rising sea levels mean rising groundwater—and that spells trouble for
coastal septic systems
Denver, Colo., USA: Sea level rise and big storms are hammering coastal
communities, causing increased flooding and land loss, saltwater intrusion,
wetland loss/change, and impacts to local infrastructure.
Communities along the coast often have their individual, onsite wastewater
treatment systems, also called septic systems. In North Carolina alone,
there are about 1 million homes with septic systems that are either on a
coastline or are located in watersheds that drain into the ocean.
To operate effectively, coastal septic systems rely on unsaturated soils to
filter wastewater and direct flow away from homes. But in some communities,
the shallow groundwater table is rising, leaving homeowners in a precarious
position.
“Typically, when you permit the septic systems in North Carolina, they want
to have about one and a half feet of unsaturated soils below the system,”
explains
Michael O’Driscoll, associate professor at East Carolina University. O’Driscoll is a coauthor
of a new study being
presented
at the Geological Society of America’s annual meeting tomorrow.
However, as groundwater levels rise in some of these areas, septic systems
are no longer functioning as they were designed.
“You can have a mixture of groundwater and untreated waste getting up to
the surface,” says O’Driscoll. While unpleasant, it can also be a health
hazard, as the normal filtration of bacteria and viruses doesn’t happen.
Adding to the problem, this water can back up into homes, causing a real
issue for residents. Some residents have waited days to weeks for the water
to recede before being able to use their septic system.
As part of the larger
North Carolina Sea Grant research effort, O’Driscoll and colleagues focused on understanding rising groundwater
levels in coastal areas. “We wanted to get an idea of where the more risky
areas are, to help understand and try to come up with some solutions for
how to adapt to this change,” he says.
The team focused on the town of Nags Head, a popular tourist destination in
Dare County, North Carolina. “When you look at sea level rise along the
Atlantic coast, that part of Northeastern North Carolina and Southern
Virginia around the Chesapeake Bay, have some of the highest rates of sea
level rise,” says O’Driscoll. “Some of the town officials noticed that
they're having more challenges with groundwater-related flooding and
stormwater issues.”
The region has a long record of groundwater level data from eight NC
Department of Environmental Quality aquifer wells, reaching back to 1983.
Along with these state-operated wells, the researchers installed
instruments on five additional wells to look at how groundwater
fluctuations affected septic systems.
They found that groundwater levels are rising with the sea level rise. This
change has shrunk the unsaturated soil thickness necessary for functional
septic systems. By looking at land surface elevations of septic systems and
groundwater levels, the team found that in the Nags Head area, homes that
were less than about 2.6 meters above sea level were more likely to
experience insufficient unsaturated soils, and would likely have trouble
with on-site water treatment systems.
Considering state regulations require 45 cm of unsaturated soils for septic
systems to work as designed, having groundwater levels withing a meter of
the land surface means problems, says O’Driscoll. “The systems that are in
the lower elevation areas, they're having problems right now,” he says.
During their study, one home they were monitoring had groundwater in the
drain field about 70% of the time. “That tells you there’s no treatment…
and it can just get worse over time.”
To combat dysfunctional onsite wastewater systems, O’Driscoll says some
builders are creating an artificial buffer. They put septic systems in
mounded sand, effectively raising the drain field and adding unsaturated
soils. In some communities, like Nags Head, officials have created their
own septic health initiatives to identify problem areas and deal with
failing systems that need to be replaced.
O’Driscoll notes that big municipal wastewater treatment facilities are
cost prohibitive for some communities. He says that if there is an area
where there's a hotspot of failing systems, an advanced treatment system
called a package plant might be an option for a community. They’re more
expensive than a septic system, but much less than building a municipal
wastewater treatment center.
The team’s research shows that updated septic system regulations are a key
for coastal communities. Current septic system regulations only consider a
static and stable groundwater level, not the potentially ever-rising
amounts the researchers observed. O’Driscoll says that new regulations have
to consider a longer-term approach more related to the life-span of a home.
CONTACT: Michael O’Driscoll, East Carolina University, Department of
Coastal Studies, odriscollm@ecu.edu
Paper No. 248-2:
Climate Change and its influence on groundwater levels in coastal North
Carolina: Implications for onsite wastewater treatment
https://gsa.confex.com/gsa/2022AM/meetingapp.cgi/Paper/382531
Wednesday,12 Oct., 2022, 1:50 p.m. MDT
Session 248: D15. Coastal Hydrology in an Age of Rising Seas and Other
Advances in Hydrogeology
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