Sea-level rise and warming temps expected to impact onsite wastewater treatment
By MEREDITH HAAS/ecoRI News contributor
As the climate changes, will septic systems fail to protect sensitive coastal environments? Yes, according to George Loomis, soil scientist and director of the New England Onsite Wastewater Training Center at the University of Rhode Island.
“Changing temperature and precipitation patterns, compounded with periods of very wet conditions and sea-level rise, will contribute to septic system failure,” he said. Current septic system designs, especially those in the coastal zone, are vulnerable to saltwater intrusion from flooding as well as from a rising groundwater table.
Loomis and his colleagues at URI are looking at the current designs and parameters for septic systems, also referred to as onsite wastewater treatment systems, against various climate-change scenarios. Jose Amador, URI professor of natural resources and the team’s research leader, said this research maybe the first in the country to be looking at septic systems and climate change.
“It’s an issue that has been talked about all over the country, but we’re the first ones with an experiment,” he said.
Septic systems are an alternative treatment option to municipal sewage treatment plants, and generally consist of a septic tank and drainage field. Properly functioning systems don’t pose a real threat to drinking water sources, but when not properly designed or sited, they can pollute groundwater, and from there, can contaminate drinking water and coastal ecosystems.
About 30 percent of Rhode Islanders depend on septic systems to treat and disperse wastewater from kitchens, washing machines and bathrooms. According to the state Department of Environmental Management (DEM), there are about 5,700 onsite wastewater treatment systems in the coastal zone — the area 200 feet from any shoreline. Many more systems are further inland, but still near watersheds that drain to the coast.
While all septic systems will be impacted by climate change, Loomis said that those systems in the coastal zone are most vulnerable and most critical, because of their close proximity to sensitive habitats and water resources.
Long-term records from the Newport tide gauge show that Rhode Island has experienced about 8 inches of sea-level rise in the past century. This continuing trend has affected septic systems in two ways. As the sea level rises, denser saltwater pushes overlying freshwater upward, raising the groundwater table and decreasing the separation distance from the drainfield base to the groundwater.
The distance required by the state is 3 feet for inland areas and 4 feet for systems within some coastal zones. As sea level rises, so do tidal and storm-surge heights, increasing flooding events that make the soil saltier and wetter, which decreases the amount of dissolved oxygen in the soil and inhibits beneficial microorganisms from breaking down wastewater pollutants.
In addition to saltwater intrusion, high-intensity storms introduce heavy wind and wave forces that can destroy foundations, as happened during Sandy last October. This can compromise septic systems and potentially open a direct route for untreated wastewater to flow into nearby coastal ponds, according to Russell Chateauneuf of the DEM’s Office of Water Resources.
Onsite wastewater treatment systems are also challenged by warming surface-soil temperatures and increased precipitation that combine to decrease the levels of dissolved oxygen in the soil that are needed for effective wastewater treatment.
Good to be shallow
Prior to 1993, when advanced treatment technologies started to be used more commonly, homes either had a conventional septic system that used a drainfield to treat wastewater or a cesspool that provided no treatment at all. A 2007 state law enacted to enhance wastewater treatment requires that cesspools be completely phased out by 2015 in designated critical resource areas of the state, such as watersheds for coastal ponds and drinking water reservoirs.
Conventional systems consist of a septic tank, distribution box and drainfield, which disperses wastewater into the underlying soil where beneficial microorganisms can breakdown harmful contaminants. While conventional systems can be effective and cost-efficient, they require more space for adequate treatment and are placed lower in the ground than advanced treatment system drainfields, making them more susceptible to failures due to sea-level rise.
Advanced treatment systems incorporate additional wastewater processing steps that produce highly treated wastewater that is commonly dispersed in specialized drainfields in small, regulated doses controlled by a programmable timer so drainage areas aren’t overwhelmed by high peaks of outflow — laundry days and the morning shower rush.
“The fundamental difference between some advanced treatment systems and a conventional system is that they operate in a timed-dose fashion,” said David Kalen, the research team’s resident engineer. “For example, you could release eight gallons every half-hour during a 24-hour clock cycle with these advanced systems, whereas the conventional system is ‘socially dosed’ based on what happens in the home.”
Jennifer Cooper, a Ph.D. candidate working with Amador, is looking at the effects of climate change on conventional systems and two types of pressurized shallow narrow drainfield (PSND) technologies that both utilize advanced treated wastewater using sand filtration and the surface soil for treatment. These technologies are the most commonly used in Rhode Island. They enhance treatment by dispersing already-filtered wastewater in a time-dosed fashion closer to the surface, where grasses and a larger microbe population exist. This enables better recycling and breakdown of harmful nutrients found in wastewater. These shallow systems are also beneficial because they increase the separation distance to the groundwater, Loomis said.
“Many of the older conventional systems in the coastal zone have been around for 30 or 40 years with a drainfield design that pinched fractions of an inch to meet the required groundwater separation distance needed to get initial regulatory permit approval,” said Loomis, noting that these systems will be most problematic with sea-level rise.
The challenge, however, for the advanced shallow systems that will be tested is whether climate-altered conditions — sea-level rise, wetter and warmer soils — will interfere with the effectiveness of various wastewater treatment technologies. This raises the question about what technologies will be able to handle saltwater intrusion from periodic flooding events as sea-level rise combines with flooding episodes caused by more frequent and more intense storms.
“We’re stuck between a rock and a hard place,” Loomis said. “On one hand, you have the surface soils warming and becoming wetter from increased precipitation, and on the other, you have lower-lying soils being impacted by rising water tables and saltwater intrusion.”
Ivan Morales, a Ph.D. candidate working with URI professor of geosciences Tom Boving, will be modeling these systems to better predict how climate change will affect them.
Testing the theory
“We want to compare the newer technologies to the standard, conventional system,” said Cooper, who hopes to take lessons learned from this project to develop better wastewater treatment capabilities in developing countries.
The experiment tests nine soil core samples with three replicates of a conventional pipe-and-stone system, and the two types of PSND systems. All samples use the same soil and are dosed with wastewater from the same residential septic system that Cooper pumps once a week.
“We collect and analyze wastewater once a week,” she said. “We need to form a baseline from the data on current conditions.”
The water samples will be monitored over the following year to better understand how the various septic systems function under current climatic conditions. Once that dataset has been established, the water table will be raised a foot to mimic predicted sea-level rise, and surface temperatures will also be modified to expected levels using heat lamps.
“We can theorize what’s going to take place, but it’ll be nice to know definitively to make management projections that will enable decision-makers to mitigate those outcomes,” said Loomis, who recommended that, for the time being, local management agencies focus on mapping at-risk systems, evaluating technology and retrofitting failing systems to advanced technologies that rely less on the drainfield for treatment.
Silver and copper
The team’s next goal is to collaborate with Vinka Craver, URI professor of civil and environmental engineering, to find support for research on the application of silver and copper nanoparticles in drainfields to aid in pathogen reduction. Treatment with nanoparticles may be a cost-effective and sustainable way to augment the treatment that soil microbes normally provide and may help mitigate climate impacts to existing conventional systems in coastal zones.
“Silver nanoparticles have been used as water treatment in developing countries and can be found in socks, athletic clothing, and are used throughout the military as an odor and bacteria-killing agent,” Loomis said. “We want to see if, with conventional systems already in a coastal zone and operational, there is some way to apply silver nanoparticles to a drainfield, and convert that drainfield into a better treatment system that yields treated wastewater that is close to, or similar to, that of sand filtration. This might buy conventional systems another one to three decades of effective treatment under climate-change conditions.”