Sea-Level Rise to Reduce Effectiveness of Home Septic Systems

By ecoRI News staff

KINGSTON, R.I. — A team of researchers from the University of Rhode Island is recommending that state and federal officials rethink the regulations for the installation and management of home septic systems, especially in coastal zones, in light of research they conducted that demonstrated that warming temperatures and rising sea levels will reduce the effectiveness of conventional septic systems.

The study by doctoral student Jennifer Cooper, professor Jose Amador and research associate George Loomis found that a 1-foot increase in the height of the water table due to sea-level rise and a 5-degree-centigrade increase in air temperature would reduce a septic system’s ability to filter out phosphorous and nitrogen before it reaches groundwater and nominally affect bacteria and carbon removal.

“In conventional septic systems, we rely on the soil to remove bacteria, phosphorous, carbon and nitrogen before treated effluent reaches the groundwater,” said Amador, a professor of soil science. “But if the drainfield is not performing as it should because of climate change, then we may be degrading our groundwater quality.”

The researchers say the issue is of national concern. About a quarter of U.S. households use septic systems to treat wastewater, and in some areas where municipal sewers are unavailable, septic systems are the only option. In Rhode Island, one-third of households have on-site septic systems.

In an experiment that lasted more than two years, the researchers used a series of mesocosms — an outdoor experiment that examines the natural environment under controlled conditions — to simulate the conditions in several types of conventional and alternative septic systems and drainfields.

For the first 18 months, the experiment tested the effectiveness of the systems under current climate conditions. As expected, almost all bacteria, phosphorous and carbon were filtered from the wastewater before it reached the groundwater, and between 5 percent and 12 percent of the nitrogen was removed.

But in the next 10 months of the experiment, in which warmer conditions and a rise of the water table were simulated, contaminant removal declined across the board.

“When sea-level rises, it makes the water table rise, and that reduces the distance between the groundwater and the drainfield,” Amador said. “It means there is less of an opportunity for the soil to treat the wastewater before it reaches the groundwater.”

Loomis said the researchers had been thinking for some time about how climate change is going to impact infrastructure.

“If you don’t have good wastewater treatment, that’s going to have a significant impact on public health and environmental health,” he said. “It not only affects the groundwater, but also coastal waters, inland fresh waters, and drinking water.”

The researchers said public officials responsible for managing existing home-based wastewater treatment systems should immediately begin thinking about how to upgrade existing systems in at-risk areas and modify requirements for the design and installation of new systems.

“The sensible thing to do in coastal zones would be to use advanced nitrogen removal treatment systems, which rely less on the soil for treatment, because there will be less soil available to do the work,” Loomis said. “Shallow pressurized drainfields should be used for final dispersal of effluent to build in as much separation distance as possible between the drainfield and the water table for phosphorus and bacteria removal.”

Septic systems in inland areas not affected by sea-level rise will be less impacted, according to the URI researchers, although the expected increase in the frequency and severity of rainstorms will occasionally raise the water table and temporarily decrease the effectiveness of septic systems.

In areas where climate change will bring increasing drought conditions, it’s predicted that water conservation efforts will result in a more concentrated wastewater effluent that will be more difficult for soil to treat.

“Our experiment didn’t address that issue, but it’s part of the big picture of how climate change will affect wastewater treatment,” Amador said.