Climate Change Will Cripple Coastal Septic Systems

Backyard units need to adapt to changing environmental conditions.

By Brett Walton, Circle of Blue

Adapting sanitation systems to a fast-changing ecological reality is a challenge not only for big municipal institutions. According to new research, many septic systems — which are simple, backyard devices for addressing the ceaseless problem of toilet waste — are unfit for future climate conditions. Because of a higher water table linked to higher sea levels, conventional septic systems near the coast will not be as effective at removing harmful bacteria and nutrients.

“We really need to start thinking about how to install these systems now before they become a huge issue for pathogens and nutrients in water,” Jennifer Cooper, a post-doctoral researcher at the University of Florida, told Circle of Blue.

Cooper would know. She is the lead author on the first academic study to assess how coastal septic systems will fare in a warming world. The peer-reviewed study was published online earlier this month in the journal PLoS ONE. The diminished capacity of septic systems to filter waste, she says, is another disruptive consequence of a changing climate that regulators and homeowners ought to confront.

Soil Essential for Waste Treatment

Water utilities, especially those located near the tide line, already recognize the stress caused by climate change. Coastal wastewater utilities have known, in some cases for decades, that rising oceans will upend the precision engineering that moves sewage from treatment plant to ocean outfall. Utilities on the Atlantic, Pacific, and Gulf coasts are spending tens of millions of dollars to prevent saltwater from corroding pipes and to avoid disruptive backups in a system that relies on gravity to eject waste.

Septic systems, on the other hand, are a low-tech sanitation solution that has largely avoided climate scrutiny. Septic systems were designed to dispose of toilet waste in areas without central sewers. A basic system is a three-step process: a tank to trap the solids, perforated pipes to remove the liquid, and an expanse of soil, called the drain field.

The drain field is important. Microbes in the soil, like a sanitizing coffee filter, consume harmful bacteria as the liquid waste percolates downward. Proper treatment depends on having a wide unsaturated zone that allows the microbes time to work before the waste reaches the water table. It is the change in water table due to rising seas that Cooper and her colleagues wanted to test.

To assess the performance of septic systems in an altered climate, Cooper moved the field into the lab. The research team cut samples of Bridgehampton silt loam soil, a soil type common in Rhode Island where Cooper completed the study as part of her doctoral work. The cylindrical cores — five feet long and six inches in diameter with septic waste dripping through them — were exposed to current conditions for 18 months to record baseline data.

Then the research team changed the climate. They modeled potential conditions in New England toward the end of the century: a one-foot increase in the water table and a five degree C increase in air temperature. Two septic system types were used: a conventional system that is installed deeper underground and a shallow system that uses a filter so that the liquid waste drains into the soil closer to the surface.

Under these late-21st century climate conditions, there is less oxygen in the soil and more water-filled pore space as rising seas push the underground water higher. With a shallower water table, the septic systems in the study were not as effective in removing fecal bacteria, which contributes to stomach illnesses, or phosphorus, a plant nutrient that fuels algae blooms. In effect, wetter conditions lubricated the soil, allowing bacteria to slip by. Though the results for nitrogen removal were less conclusive, the general finding is that the systems did not perform as well in under future conditions.

Interest from Regulators

Other researchers who study septic systems were pleased to see attention directed at the connection between climate change and water pollution. Laurel Schaider, a research scientist at Silent Spring Institute, a Massachusetts-based science organization, told Circle of Blue that the study is “a strong paper” that points out potential septic system failures.

“I would say that in areas that become inundated by large storms like Hurricane Sandy, the effects on septic systems are obvious because the systems may become flooded and release raw wastewater directly into the flood water,” Schaider said. “This new study also points to more subtle effects on contaminant removal due to rising water table levels and higher temperatures in many septic systems in coastal regions, even those that aren’t in areas prone to flooding.”

The results of the study are a warning signal, Cooper said. Septic systems are contributing to an ecological and public health mess in coastal communities from Cape Cod to Long Island to Florida. The U.S. Geological Survey predicts that water tables 10 kilometers inland on Cape Cod would rise by two feet in response to a six-foot rise in sea levels. Cooper found that performance diminished with only a one-foot increase in the water table. In addition, climate change is expected to increase the frequency of severe storms. If those storms saturate the ground, they could interfere with septic system performance for days or weeks.

Regulators are interested in Cooper’s work. She fielded questions from state authorities and presented the findings to officials at the U.S. Environmental Protection Agency’s New England regional office. The EPA regional office wants to improve septic systems on Cape Cod, where more than eight in 10 homes on the low-lying peninsula uses a septic systems and groundwater is polluted with nitrates and other contaminants. The office is involved with a separate assessment that is looking at more effective methods of removing nitrogen.

Above all, Cooper’s study is a reminder that septic systems rely on the soil for treatment. A better grasp of the underground interactions, which vary by region, soil type, and climate is essential for public and environmental health. More treatment of septic waste — to filter bacteria and nutrients — before it reaches the drain field might be necessary, Cooper said.

“The soil is a good remover of contaminants,” Cooper noted. “But we shouldn’t simply be putting wastewater into the ground and hoping that the conditions are right.”