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Science in the First Person: Michael Gonsior on tracing the impact of septic systems on local waterways

April 5, 2018

Environmental chemist Michael Gonsior has been dipping into streams in southern Maryland to figure out if septic systems in residential areas of Calvert County are contributing to nitrogen pollution in local streams—and ultimately the Chesapeake Bay. He’s been using a surprising clue to track down the pollution: sucralose, an artificial sweetener used in many food products.

What are you working on?

I am the principal investigator of a Maryland Sea Grant funded research project to develop chemical tracers to identify the presence of septic wastewater in streams and to link these tracers with nitrogen loading in southern Maryland. We wanted to know how important are septic systems in supplying nutrients and what are they doing to the local streams.

An estimated 20% of U.S. homes use septic systems for wastewater treatment. The old systems don’t remove much of the nitrogen, so most of it goes into the groundwater and eventually into streams, along with chemicals from personal care products, pharmaceuticals, and cleaning detergents. All these kinds of things end up in the groundwater and ultimately in the streams.

One aspect of the project was a monthly sampling of nine streams in Calvert County, Maryland—six potentially impacted by septic systems—and three forested basins to act as reference sites.

Some of the homes have traditional systems, meaning the wastewater from a septic tank goes into a leach field where it percolates into the ground or soil. Old systems only remove approximately 10 to 20% of nitrogen. New systems are designed as mini wastewater treatment systems and remove on average 50 to 75%. However, we still need to understand what role septic systems in general play in supplying nutrients to our streams.

Michael Gonsior testing one of the streams. Photo by Mindy Forsyth

Knowing what the sources of nitrogen are in streams is important to mitigate and to find solutions to potentially reduce them or at least find ways to reduce nutrient loading in waterways in the most cost-effective way.

Basically, we wanted to sample a stream and find out if this stream is directly impacted by septic wastewater – yes or no?

How did you find out if the streams were impacted by septic systems?

We use a multi-tracer approach to actually know that this stream is impacted. A tracer is a chemical compound which is not in the forest, and it is not naturally in the stream. It comes from us and septic systems. We can quantify these specific organics, and then we can estimate how much of the water in the stream is actually coming from septic waste water.

For this purpose, we use a very stable wastewater tracer, an artificial sweetener called sucralose—Splenda is the trade name—but it is also in soda and a lot of artificially sweetened food products. Sucralose is very stable, it practically doesn’t degrade in the environment. That means if you know what the concentration in the septic tank is and you know what the concentration in the stream is, you can calculate how much water comes from septics. We can analytically measure sucralose very precisely in the lab.

What did the tracer tell you?

Our preliminary data show very high concentrations of sucralose in some of our streams. Now we have direct evidence that some of the streams are definitely directly connected to the wastewater coming from septics.

State-of-the-art analytical methods also confirmed the presence of other wastewater tracers, such as surfactants that can be typically found in detergents, shampoos, and soaps.

This diagram shows how wastewater from a home can leach into nearby waterways.

This data correlates quite well with the nitrogen loading, so we have indirect correlation that the nitrogen we see is likely to be coming from the septic systems. We’re talking about 6-8 milligrams per liter of total nitrogen in the worst impacted streams, which is quite a lot.

These highly impacted streams are not functioning as normal streams. There are likely no fish present in some streams due to very high ammonia concentrations, which is toxic for fish. These streams are obviously impaired and should not just be understood as conduits for supplying additional nitrogen to the Bay.

How does this impact the Chesapeake Bay?

In Maryland, only few studies have evaluated the connectivity between septic systems and the stream network. These are small streams but we have a continuous supply of nitrogen. We are sampling every month, and we don’t see a lot of variability. We know now how much of the water in the stream originates from the septics. Now we have a lot of analytical work ahead of us to identify how much of the nitrogen is actually coming from septics, which is a bit more challenging.


Watch Michael Gonsior’s Science for Citizens talk on his septic system research here.

Maryland Sea Grant Fellow and graduate student Katie Martin is analyzing the data collected in the field. Read more about her experience.