Is the Harpeth River Past the "Tipping Point" for Production of Blue-Green Algae?
What's Growing in the Harpeth River?
Dr. Clifford W. Randall, PhD, DIST.M. ASCE, HON.M. AAEES, C. P. Lunsford Professor Emeritus, of the Department of Civil and Environmental Engineering at Virginia Tech University, one of the nation’s leading experts on sewer plant operation, had the following to say about the state of the Harpeth River in November 2016:
The primary threat to water quality in the Harpeth River is from excessive growth of algae, and especially the excessive growth of blue-green algae, aka cyanobacteria.
It was confirmed in 2015 that at least one genus of cyanobacteria is already growing in the River…[C]onditions in the River have already passed the N to P ratio ‘tipping point’ at which cyanobacteria will flourish. To continue with the current manner of nutrient removal at the City of Franklin treatment plant represents a form of cyanobacteia ‘Russian Roulette’.
Any increase in P[hosporus] discharges will push the N[itrogen] to P[hosphorus] ratio even further beyond the ‘tipping point’ and increase the probability of highly toxic cyanobacteria domination in the Harpeth River.
A year earlier, in 2015, Dr. Randall noted that:
There are already signs of the shift from green to blue-green algae in the Harpeth River in the vicinity of Franklin and the COF STP [City of Franklin Sewage Treatment Plant]. The algal growths shown in the photos of Item 2, above, are characteristic of blue-green algae, and a sample from those growths sent to JoAnn Burkholder at North Carolina State was found to contain a blue-green called Nostoc. Nostoc forms a mildly toxic compound, but it indicates that current conditions in the Harpeth favor blue-green growth, and there is the potential for the growth of much more toxic forms to grow such as Microsystis, the most common filamentous form of high toxicity, Selenastrum, and Anabaena, the most toxic form.
The River Has a Sewage Signature
Dr. JoAnn Burkholder, PhD, Professor and Director, North Carolina State University Center for Applied Aquatic Ecology, and a leading national expert on aquatic algae, said in November 2016 that:
“The WRF [Franklin sewer plant] discharge dominates not only the river flow, but also the N[itrogen] and P[hosphorus] entering the river during low-flow periods, based on…City of Franklin river monitoring data and effluent data in monthly reports to TDEC…
High biomass of algae, which are fueled by the excessive nutrients and organic matter in the treated effluent, [also] cause or contribute to low DO [Dissolved Oxygen] in violation of the state standard.
[Under t]he draft permit … both N and P supplies will still be extreme in comparison to what the natural algal assemblage needs. The … high N and P supplies, added in unhealthy proportions, will increasingly encourage noxious algal overgrowth when other conditions … are conducive ….” The shift is so extreme that the river has a “sewage signature”….
[I]t is the excess of N and P loads that is at issue in present-day waters….In the Harpeth River…managers’ mistaken view that N rather than P is “limiting algal growth” is analogous to the following situation: A man sits down to have dinner at a restaurant. The server apologetically informs the man that 200 steaks are available for him to eat, but only 150 potatoes. Which will the man run out of first, steaks or potatoes? This question is nonsensical. Obviously, one person cannot consume 200 steaks or 150 potatoes at a dinner – the supplies of each are so high that they are at saturating (non-limiting) levels.”
Yet, TDEC has designed the draft permit for the WRF … based on the irrational premise that “N is limiting” in the Harpeth River. The draft permit reflects no understanding by the writers of the critical importance of N:P stoichiometric balance in aquatic ecosystems, or of the fact that N:P ratios can only be used to interpret nutrient limitation when N or P are in limited supply (that is, limitation should only be invoked when something is limiting).”
Franklin’s Sewage Discharges are Degrading the State Scenic Harpeth River in Nashville (and Beyond)
Dr. Burkholder also said that discharges from the City of Franklin’s sewer plant are having significant adverse impacts on the State Scenic River in Davidson County (Nashville):“There is no question but that the Franklin WRF, under its present permit, is degrading the water quality and aquatic habitat of the Harpeth Scenic River Complex….
Excessive nitrate from the Franklin WRF is contaminating this Scenic River.
A major portion of the Harpeth River’s flow is Franklin WRF effluent. According to calculations from USGS’s gage 0.9 river miles downstream of the Franklin WRF, in October 2016 approximately 55% of the river’s flow is comprised of effluent from the Franklin WRF. Approximately 22.9 river miles downstream, at Bellevue, approximately ~28% of the river flow consists of treated effluent from the Franklin WRF….
There is compelling evidence that the excessive NOx and phosphate from the Franklin WRF is degrading the Harpeth Scenic River Complex…There is additional evidence that excessive nutrient contamination is already fueling downstream algal overgrowth [in the Bellevue/Nashville area].”
Although the City of Franklin has reduced its phosphorus discharges as a result of the permit comment and appeal process initiated by Harpeth Conservancy, the river is still on the State’s list of impaired waters and a pollution reduction study and plan must be completed.