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🔬 "Turning a Lake From Green to Clean"


Photo by manu schwendener / Unsplash

Table of Contents

Host: Travis Loop
Guests: Mike Mallin | Research Professor | Center for Marine Sciences at the University of North Carolina Wilmington  &
Fred Royal | Stormwater Services Manager | City of Wilmington
Category: 🔬 Research

Podcast’s Essential Bites:

[2:30] MM: “Greenfield Lake [is] not a natural lake. It's an impoundment and it was first filled in 1750 and then it was made a public park in 1925. […] What was interesting is [that in papers from] 1908 the lake was described as springfed in his unpolluted waters, and like a mile and a half south of the city of Wilmington. […] Since that time, the city of Wilmington has gone and wrapped itself all around Greenfield Lake. So […] the shorelines and the watersheds […] are one of the most highly developed in the city of Wilmington.”

[3:33] MM: “What's happened here is something that has happened probably to numerous lakes throughout North America. […] The development has led to increased nutrient pollution, increased fecal bacteria pollution and finally, a few years ago, this lake was placed on the state's 303 (d) lists for impaired waters. […] The situation in the lake right now is that every summer we have these massive blue green algal blooms, and occasionally those are accompanied by fish kills. […] We also have a problem with elevated fecal bacteria counts in the lake. […] This result is very common for all the built up area, [called] impervious surface area, in the watershed. So what happens there is that whatever it rains, all of the pollutants, fertilizers, animal waste […] end up washing down into the lake. And the lake is not well flushed, […] so the nutrients go in there and sit, get plenty of sunlight and thus we have these massive algal blooms.”

[6:24] FR: “[At] stormwater services for the city, we have two primary focal points. One is to reduce pollutants that flow into water bodies throughout the city and this primarily means nonpoint source flow of stormwater runoff, which means everywhere except sewer pipes, essentially. And it's a huge challenge with higher development, higher density and higher land use issues. So ways we can get to try to mitigate that and go in the other direction, are creating a green infrastructure, protecting more green infrastructure, creating more green infrastructure that removes nitrogen and phosphorusthat […] also reduces volume that runs off of paved surfaces, etc.”

[7:42] FR: “The other issue we deal with a lot is flooding […]. That's a huge issue here […] in eastern North Carolina [where] we have tropical storms and […] a lot of rainfall. So we have to balance [between] possible flooding issues [and] water quality issues. Flooding issues typically mean we are modeling and replacing antiquated drainage infrastructure, pipes channels, [etc.]. They have to be larger and more efficient and that by default […] does not help water quality. So we have to also […] take water quality and into our minds when we're looking at design alternatives.”

[11:44] MM: “My laboratory has been sampling Greenfield Lake for many, many years. […] And as part of the project that's ongoing right now, my particular team is sampling the surface waters that are coming in and hopefully, when the engineering solutions are done, those surface waters will be much cleaner. A colleague […] is sampling the sediments of the lake and of the incoming streams and of the surrounding wetlands. […] What we're looking for there is the nutrients that are stored in the sediments and […] microbial processes.”

[12:34] MM: “The key word here is denitrification. What that means is the polluting nutrients that run into the water, generally in the form of nitrate or ammonia, when they're in excess, you want to get them out. […] This process of denitrification is done by bacteria and it's done within the roots and rhizomes of wetland plants. The nitrate comes in there, the bacteria […] turn [it] into nitrogen gas, which is harmless [and] as part of our atmospheric boom […] goes out of the system. […] The goal would be to try to first figure out how much denitrification is going on in these branches coming into the lake, and then figure out how we can encourage this, either by retaining the water longer in nearby wetlands, or selectively planting proper aquatic vegetation.”

[13:49] FR: “One project that is going on now is [turning] wet pond, owned by the city, into essentially a wetland. We will incorporate more shallow shelf areas for vegetation to be planted, just to do that job of denitrification and to settle out solids. […] The wet ponds tend to fill with sediment and they tend to become almost anoxic […], low oxygen in the summer, and they actually become dangerous in a public health hazard […].”

Rating: 💧💧

🎙️ Full Episode: Apple | Spotify
🕰️ 26 min | 🗓️ 03/20/2021
✅ Time saved: 24 min

Additional Links:
Cape Fear River Watch