Thursday, December 7, 2023

New research pinpoints problem streams for Greenfield Lake

New researched pinpointed streams which feed excess nutrients into Greenfield Lake, considered an impaired body of water by the state of North Carolina since 2014. (Port City Daily photo / JOHANNA FEREBEE)

EDITOR’S NOTE: Last week, we caught you up on some of the water quality issues Greenfield Lake has been facing for years. New research revealed on Friday provides some potential answers to a decade-long problem.

WILMINGTON —The city is looking to redirect its strategy regarding the health of Greenfield Lake in light of recent findings.

Previously, the city has taken spot-treatment measures to solve water quality issues at the lake. Those included adding grass carp, SolarBees and select herbicides directly to the lake.

Now, the city is turning its efforts to address the cause of the lake’s dire status rather than the effects.

“Our focus is going to be a more holistic approach,” said Derek Pielech, stormwater services manager for the city. “We’re looking at the watershed as a whole instead of the lake itself.”

In 2014, the lake was placed on North Carolina’s 303(d) list, which registers the public body of water as impaired. This was after years of interventionist efforts by the city, which began in 2005.

In cooperation with UNCW professor Dr. Mike Mallin’s biology and marine biology staff, the city is looking to take a different kind of action considering new research his team has produced.

Preliminary findings

Marine science graduate student Nick Iraola presented his yearlong findings on the water quality of the lake from July 2016 to June 2017 on Friday at the North Carolina Lake Management Society fall workshop.

Iraola measured each stream’s nutrient loads, as excess nutrients have caused much of the standing issues the lake is facing today. Dr. Mallin, Dr. Lawrence Cahoon and Dr. Doug Gamble co-authored the presentation.

The 91-acre lake is subject to a 2,551-acre urban watershed. Thirty-seven percent of this watershed is impervious. Five perennial streams feed into the lake directly and one outfall stream ultimately flows into the Cape Fear River. The input streams include Jumping Run, Squash, Clay Bottom, Silver Stream and 2340.

This diagram outlines water sampling stations, tributaries and SolarBee mixers. (Courtesy of UNCW and Dr. Mike Mallin)

The group found that while the city put in some rehabilitation efforts, it failed to address the excessive nutrients that were leaking into the lake.

“Some of this helped, some didn’t,” Iraola said.

They measured nutrient loads per each stream, accounting for both dry and rainy periods. Phosphorus, orthophosphate, ammonium, nitrogen and nitrate levels were the select nutrients focused on for this study.

With stormwater runoff already being on the city’s radar, it was expected that rain events would increase the overall nutrient input into the lake.

“The biometric loading obviously was increased during rain events as opposed to the base flow,” Iraola said.

Input and output

The research produced a mixed bag according to Iraola, but there were uniform trends that stuck out.

In May, Greenfield Lake experienced an algal bloom. During this time, the team recorded a chlorophyll concentration of 1,851 micrograms per liter coming from 2340, the southernmost stream. The North Carolina State Standard is 40 micrograms.

Jumping Run Branch had the highest load of all the five nutrient parameters and also has the largest impervious surface area.

“Jumping Run Branch should be targeted for a BMP (best management practice) due to its high nutrient loads to the system in addition to Silver Stream and Clay Bottom for their high TSS (total suspended solids) loads,” Iraola said.

Curiously, the team found that nitrate actually decreases in load size from input to eventual output, likely by getting uptaken by phytoplankton.

For phosphorus, orthophosphate, ammonium and nitrogen, they noticed an increase in the load from what the streams bring in to the time they reach the outfall, a factor which may ultimately lead to an unknown source.

An unaccounted source

During the study period, this increase in nutrients upon output suggests there may be an unaccounted source of nutrients in the lake.

“More TN (total nitrogen) and TP (total phosphorous) are leaving the lake than the 5 streams combined are bringing in, so that means that there’s probably another source of nutrients that’s making its way into the lake,” Iraloa said.

This unknown source could be the lake itself, Iraola noted, as organic material remineralizes and recycles nutrients back into the overall supply. The source could also be groundwater, an unidentified leaking septic tank, or something else entirely.

Though Iraola’s preliminary findings are not technically published, they are the first steps to identify the source of the problems the lake has been facing.

“I’m recommending that a little bit more research be done to just investigate further what’s really going on there, so what is the source?” Iraola said.

Moving forward

“To think you’re going to go in in a year or two or three or five and think you’re going to make a significant change in water quality, it’s just not how it works,” Pielech said. “I’ll probably be retired before we really start seeing significant water quality changes.”

“There are improvements going on right now, but they’re just so small,” Pielech said. “We’re at the very beginning stages of getting the water quality in the right direction.”

Pielech said the research the UNCW team focused on is targeting the issues from the right angle.

“It’s a good step,” he said. “It’s paramount for us to figure out where we need to concentrate our efforts.”

The city recently added a simple trash collecting system to combat ongoing litter issues. (Port City Daily photo /JOHANNA FEREBEE)

Recently, the city took a simple measure that has made a noticeable impact. A small amount of chain link fence was strung along the upstream spillway of the lake and has functioned as a trash collector, a cheap alternative to some of their past interventionist strategies.

“The amount of trash and debris that’s collected on that thing is directly proportional to how much it rains,” Pielech said.

The city’s goals will be and have always been to keep the lake at a level that is still accessible to recreation and to keep the public informed.

“We have found that the biggest, the most successful thing we can do to improve water quality is education,” Pielech said. “Nothing helps as much as educated citizens.”
Each eighth-grade student in New Hanover County will learn about watersheds and pollution by the Stormwater Services Department at some point this year.

Given that the research Iraola and his research team just presented is still preliminary, a best management practice or definitive plan moving forward is yet to be announced or decided upon.

The outlook is positive and headed in the right direction by all parties involved in the health of the impaired ecosystem.

“Now it’s time to redirect our issue to the larger picture,” Pielech said.

Because the research was done by students, Dr. Mike Mallin has requested that comments or questions about the recent findings be sent directly to him at .

Johanna Ferebee can be reached at or @j__ferebee on Twitter. 

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