What Lies Beneath?

Rainwater containing pollutants, waste and motor oil are flowing into ponds across the country. As a result, thick algae grows at the bottom of these ponds creating environmental issues. In this segment, we visit some Researchers in North Carolina who are searching for a solution to this common problem.


Rainwater containing pollutants, waste and motor oil are flowing into ponds across the country.

As a result, thick algae grows at the bottom of these ponds, creating environmental issues.

In this segment, we visit some researchers in North Carolina who are searching for a solution to this common problem.

Have you ever wondered what lies beneath the waters of those nicely landscaped retention ponds that are a pretty common sight in the urban landscape?

It's a pretty silty and sometimes slimy world underwater, and that's a problem.

New research shows retention ponds may look nice above the water, but they aren't working the way they're supposed to underwater.

The water, when it rains, flows into these wet ponds that are one of many of a class of storm water control measures that people use, and these wet ponds are a part of the landscape.

Our story begins with a question.

What can be done to reduce the amount of rainwater sitting on roads and on developed land?

That storm water carries pollutants such as fertilizers, yard and pet waste and even motor oil.

And as storm water carried all of that pollution into streams and rivers, the water supply along with people and the environment could be seriously harmed.

Because the presumption is they capture everything during the storm and fewer things leave, and if you use that in a general sense, that can mean water, which is accurate.

It does.

It captures the water.

It takes the peak off of the flow so that you don't have water rushing out into streams during storms.

But if you think about everything else like sediments and nutrients in particular, it's not as simple as they get captured in the pond and they just stay there.

State law requires developers to reduce the impact of storm water runoff.

The ponds are specifically designed to not only hold the water and let solid pollutants settle out but also to remove 30 percent of the nitrogen and phosphorous in the runoff.

That happens through a process called denitrification.

Where an available form of nitrogen, which is a nutrient that can stimulate algal blooms, comes in and through a microbially mediated process is removed.

So the presumption was that's what was going on.

Essentially, the microbes and plants in the water are supposed to take up the nitrogen.

Some is consumed.

Some is released into the air as N2 gas.

Problem solved.

A lot of those nutrients in the water are gone.

But that's not always what happens.

Some of our early work found out that not only was that not going on but the opposite of that -- nitrogen fixation was occurring.

So more nitrogen, new nitrogen from the atmosphere, was being added.

Yeah, just maybe a smidge lower if you can, and then we can always release some later in the lab if we have to.

It turns out all the action in a wet retention pond happens at the bottom, so researchers collected core samples.

You see, like, the fluff right here?


There's a bunch of just algal detritus.


Yeah, gunk on there.

It gets sandy underneath there.

You can kind of see...


...maybe some plant roots as well.

So what we do is we have our line connected to a core, so this one right here, where the water is pumping in the top, and it's being pulled out close to the sediment water interface.

So right down here is where we want to pull our water from, so then we can get a signal of what the sediments are doing, whether or not they're adding nitrogen or removing nitrogen.

What we would want to happen is denitrification where the sediments are converting nitrate, which is a nutrient, into N2 gas, which is a gas that organisms can't use to grow.

What we're seeing instead, sometimes, is net nitrogen fixation, which means that N2 gas is being taken from the water by the bacteria and turned into nutrients that they can use to grow.

Which explains why some ponds have a thick layer of algae on the surface and on the bottom.

Usually during the summer months, we're seeing actually net nitrogen fixation in the sediments, so instead of removing the nitrate, they're actually adding nitrogen into the system through nitrogen fixation.

It's bad because it's a net influx of nitrogen into the system, so instead of being net sinks, so an area where nitrogen is removed from the system where things can't use it, it's actually a net influx of nitrogen.

There's more nitrogen coming into the system than before.

And those explosions of algae can threaten streams and rivers and lakes farther downstream from the pond.

In a world where we have a lot of increasing urban development and a lot of impervious services and storm water increasing every time that you're building another Walmart or another parking lot, it's really important to think about where that water is going and how that water is affecting the wildlife community, the kind of microbes that you don't see and the nutrient flow that you also don't see, so yeah.

It's a key component for sure.

The solution, researchers say, is to routinely excavate the pond to prevent the buildup of organic material.

Installing aerators that mix the water in the pond, including fountains, can also help promote algae removal.

As they receive more and more water, they're receiving more and more sediment and other organic material, and it builds up, and so you have the physical filling of the pond, but you also have a big change in what the sediment composition is.

And if it gets too organic-rich, that doesn't favor the removal processes that the microorganisms do.

That favors the recycling of nutrients into the water column and is more likely to make there be nutrients passing through.

Just to the sediment surface is where all the action is, and so maintaining it, where you scoop it out and start anew, does an awful lot to change the conditions and potentially create conditions that are more favorable to removal.