Septic Tanks Aren’t Restraining Faecal Bacteria From Contaminating Waterways

– A new study from Michigan State University refutes the long held perception that septic tanks can hold back faecal bacteria, as researchers trace back the bacteria in the water bodies to leaky septic tanks.

This research work is the largest watershed study of its kind to date, involving taking samples from 64 river systems representing the watersheds in the Lower Peninsula of Michigan, USA.

Professor Joan Rose from Department of Fisheries and Wildlife, Michigan State University, who led the research team, says “All along, we have presumed that on-site wastewater disposal systems, such as septic tanks, were working. But in this study, sample after sample, bacterial concentrations were highest where there were higher numbers of septic systems in the watershed area.”

The general understanding until now has been that, soil acts as a natural treatment system and filters out human sewage. With this notion, discharge-to-soil methods like, a simple hole dug in the ground under an outhouse for example, is quite common and have been in use for many years around the world. However, the new discoveries by Dr. Rose and her team of water detectives points to the fact that, these age old systems do not keep the E. coli and other pathogens from contaminating the nearby water bodies.

(Image credit Daniel Friedman)
(Image credit Daniel Friedman)

For years we have been seeing the effects of faecal pollution, but we haven’t known where it is coming from…. Pollution sources scattered in an area – called non-point – have historically been a significant challenge in managing water quality.

Dr. Rose.

To do this study, researchers used a novel method called the “source-tracking marker”, to sample river systems in Michigan for E. coli and the human faecal bacteria B-theta. What is noteworthy about this tracking system is that, it allows scientists to track down the origin of non-point pollution sources very accurately.

How it functions is quite simple. Water sample from a source is taken and the bacteria in them are picked and their DNAs extracted to know their identity and source. Different kinds of bacteria indicate specific types of hosts. Some markers indicate pig waste, some cow waste and some others chicken waste. Likewise, faecal bacteria B-theta is a marker linked to human sewage.

During their investigation in the watershed regions of Michigan, researchers started observing certain zones where the human marker was found in relatively higher concentrations, indicating the presence of more human waste. Later, while looking at the landscape, they started noticing the association of the presence of human faecal markers with the concentration of septic tanks in the watersheds. As the numbers of septic tanks in the region rose, so did the numbers of B-theta human marker in the waterways. This is a disturbing revelation because across the world, millions use septic tanks to dispose off their wastes and any exposure to human wastewater can cause diarrhea, chronic fatigue and meningitis.

(Image source
(Image source

Apart from the leaking septic tanks, human wastes finds their way into water bodies also through sewage overflows caused by occasional big storms. Even though it’s in large volumes, they are single, onetime event and it is mandatory in US to fix combined sewer overflows caused by such events. Unfortunately, there are not sufficient regulatory rules and measures to take care of leaking septic tanks.

Statistics points out that, about 30 percent of the house hold in US have septic tanks and 77 million people living in small communities scattered across the country side make use of them to store their wastes. In the larger communities living in the urban and semi urban regions, the usage of septic tanks is fairly less and amounts to about 7 percent.

The study is an eye-opener, as it brings out the falsity in the long held notions about septic tanks.

This study has important implications on the understanding of relationships between land use, water quality and human health as we go forward…… The information’s are vital for determining the impacts of septic systems on watersheds, and improving management decisions for locating, constructing and maintaining on-site wastewater treatment systems.

Dr. Rose.

Dr. Rose feels that the long-trusted methods of waste disposal systems may no longer be reliable and any further use may come at a hefty price. Pollution arising from septic system discharges is likely more important than previously realized, affecting many communities. Till now, no one knew that this problem really existed at this large scale. If one considers large watersheds spanning more than one county, or more than one state or more than one country, communities from all these concerned geographic regions should be brought to the discussion table to fix the problem.


Linking faecal bacteria in rivers to landscape, geochemical, and hydrologic factors and sources at the basin scale”, Marc P. Verhougstraete et al., PNAS vol. 112 no. 33, August 3, 2015.

Feature Image (Image source This image has been cropped.

Ravindra Krishnamurthy

Ravindra Krishnamurthy is a freelance science writer covering science, tech, the environment, health, food, and culture.

One Comment

  1. I live in a rural area with a septic, and have no illusions that there are tree roots in it. A root hair can be as in the league of micrometers, and any broach this miniscule root will find its way in and then grow feeder roots (exactly what we found in ours). In Australia, where I live, the dominant tree in many habitats are eucalyptus ( with 800+ species) and they have a well established reputation of being hardy and resourceful- yes, they are in our septic. I would like to consider that human sewerage is (within context) no different from other organic matter- it has to be incorporated into the ecosystem appropriately for its nature. I have rainforest and creeks near me. I also am very conscious of the watershed of my property, and I know what plants are using water and nutrient at what level throughout the year. In other words, growing is based on two premises: 1/ One cannot create or destroy energy/ matter, it can only be changed and moved around. 2/Everything you can see, feel, smell or touch is nutrient for something. It’s about aligning nutrient with maximal and most appropriate use, and about holding nutrient on block for as long as possible- minimise leaching and volatisation. I don’t mean to be simplistic about pathogen loads of humanure. But building high carbon, quality, biodiverse soils with many layers of above and below ground complexity and diversity is the best way to filter out that pathogen containing water, incorporating many of these disease causing facultative anaerobes or anaerobes into a healthy soil web where they will be taught good manners, outcompeted or digested by more fit members of a high organic, well structured, aerobic soil.

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