Research by scientists at the University of Rhode Island suggests that warmer temperatures and rising sea levels may reduce the effectiveness of conventional home septic systems.

A study carried out by doctoral student Jennifer Cooper, Professor Jose Amador, and Research Associate George Loomis found that a one-foot increase in the height of the water table due to sea level rise and a 5-degree Celsius increase in air temperature would reduce a septic system’s ability to filter out phosphorous and nitrogen before it reaches groundwater, while nominally affecting 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,” says Amador. “But if the drainfield is not performing as it should because of climate change, then we may be degrading our groundwater quality.”

About one-quarter of U.S. households use septic systems to treat their wastewater. In some areas where municipal sewers are unavailable, septic systems are the only option. In Rhode Island, around one-third of households have on-site septic systems.

Researchers advocate for nitrogen-removal treatment systems in coastal zones that have high water tables. Image credit: Patsy Lynch/FEMA.In a two-year study, the scientists used a series of mesocosms 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 5-12% of the nitrogen was removed.

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

“When sea level rises, it makes the water table rise, and that reduces the distance between the groundwater and the drainfield,” Amador says. “It means there is less of an opportunity for the soil to treat the wastewater before it reaches the groundwater.” Substandard wastewater treatment, says Loomis, can impact public and environmental health, as it affects not only the groundwater, but also coastal waters, inland fresh waters, and drinking water.

The researchers say that one approach 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. To account for the rising water table, 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.