Climate change is likely to render household septic tanks in coastal communities less effective over the next few decades, which could lead to the contamination of household drinking water, according to a recent study published at PLoS ONE by University of Rhode Island scientists.
Nearly one third of all homeowners in Rhode Island rely on septic tanks to handle all of the wastewater exiting their homes, as do many other coastal homeowners throughout New England,.
Septic systems are used primarily in rural areas where installing a sewer system that directly connects the home to municipal water treatment centers is not cost effective. Most homeowners who rely on a septic tank, also rely on a nearby well for their drinking water, food preparation, baths, laundry, and other household water uses.
Septic tanks are designed to prevent contaminated water from seeping into the adjacent wells. But even with just a foot of additional sea level rise, the new study suggests that many septic tanks may no longer be able to function in this protective capacity.
“You don’t really ever think about your septic system unless it’s not working…so climate change, septic system, who would think about making that connection?” says Jose Amador, a soil scientist and co-author of the new study.
A Reduced Filtering Function
In a simple septic tank, wastewater is sent from a house to the tank. There, the solids sink to the bottom of the tank and liquids remain just below a surface “scum” layer composed of fats and oils. As more water enters the tank it causes the water level to rise, and pushes out the water that has already been separated through a “T” shaped pipe that is partially submerged, so none of the scum or solids get caught in the moving water.
Then the water passes through a leach field, a layer of cleansing soil, and a series of trenches that disperses the water into the soil. Under normal conditions of low moisture and high oxygen, this layer of soil acts like a powerful filter, removing harmful pathogens and chemicals from the contaminated water.
“The idea in a leach field is that the more soil you have, the more soil there is for the water to go through, the better the chances for the water getting cleaned…” Amador explains.
But as climate change causes sea levels to rise to a foot or more, so will the water table in coastal areas, meaning less soil to clean the water. The temperature will also rise, which will impact the microbes in the soil that work to clean the water as it passes through the leach field.
The experiments carried out by Amador and his colleagues indicated that under such conditions, the filtering function of septic tanks is likely to fail.
Risk to Well Water
At some point in the next few decades, the loss of this filtering function will be a major problem for coastal area homeowners, as water contaminated with bacteria, phosphorus, or nitrogen seeps into wells and the surrounding soil.
“More pollutants getting through means that the well water could be at risk, and maybe it’s not your well water, but it could be your neighbors because ground water travels,” Amador says.
Homeowners and municipalities will face significant financial burdens in replacing current septic systems in favor of more advanced ones, or in connecting households to a municipal waste water system.
“We need to change the way we think about septic systems. Now we kind of put the contaminants into the soil and hope the conditions remain the same,” says Jennifer Cooper, a co-author of the study. But this will not be an option in the future. Instead, Cooper says she would like to see more pretreatment of wastewater before it gets added back into the soil and the leach field.
But pretreatment of wastewater could potentially be very expensive, and would require technology that normal septic tanks do not have. The process of pretreatment would involve the removal of certain pathogens or contaminants through the use of pumps or recirculation systems before sending the water to the septic tank.
The two scientists acknowledge that they are among the first to identify the threat to septic systems from climate change, and that more research is needed. But one thing’s for sure, “We’re not going to get rid of septic systems,” Amador says.
—Nina Godlewski (@NinaGodlewski) is a fifth year Communication Studies student at Northeastern University. She has previously written for The Boston Globe, Business Insider, and Boston.com.