Recent headlines about the City of Mound releasing sewage into lake Minnetonka sent shivers to our community of lake lovers. Sewage in lakes – that stinks!
But what is the real extent of this problem? Let’s put some perspective on this.
Due to heavy rains overwhelming the City of Mound’s sewage collection system, some untreated wastewater bypassed the treatment system and was discharged (after filtration) into the City’s storm sewers, which in turn empty into area lakes, including Lake Minnetonka. This occurred for about two days, May 31st and June 1st.
No one likes the idea of discharging untreated human waste into our lakes. But was this incident as horrible as many fear? I don’t think so.
I am not saying this should happen with regularity. From all accounts, the City of Mound acted prudently and responsibly given the unfortunate situation. In the context of lake pollution, this was not such a big deal. Why do I say this?
First of all, our sewage collection systems are leaky. Sewage leaks out of the pipes and into surface and groundwater regularly. Sewage overflows from leaking, aging or mal-fitting pipes can enter storm sewers and get into the environment. I am not sure how prevalent this is around Lake Minnetonka, but it is a recognized problem in the nation.
Secondly, let’s look at the numbers. I have not found any measurement of the sewage that was discharged, but let’s assume it was from all of Mound’s 9,052 residents and was totally untreated. Using that figure, with respect to E. coli, a bacterial indicator of fecal contamination, I estimate that 180 billion organisms were discharged over these two days. This may sound like a large amount, but sections of Painter’s Creek, which enters Jennings Bay on Lake Minnetonka, average about 22 billion organisms per day or 8,000 billion organisms per year – 45 times more than the my estimate that the City of Mound may have discharged. And, Jennings Bay is not the only source of E. coli contamination to Lake Minnetonka. Other sources include the other creeks and streams plus animal wastes.
Warm-blooded animals have the same pathogenic contamination in their feces as humans. For the most part, these wastes are not collected or treated — rather, they wash off in heavy rains. It is likely the contamination from our pets (as well as raccoons, geese and other wildlife) and livestock feedlots was far greater than was discharged by Mound. Many beaches are closed due to non-human fecal contamination following heavy rains. We should note that Mound’s beaches were tested at acceptable levels of contamination less than a week after the discharges.
Many of us recall the legendary algae blooms on Lake Minnetonka in the 1960s. At that time, there were six wastewater treatment plants discharging sewage into the lake. Excess phosphorus from these discharges caused the algae blooms. Since that time, the wastewater has been diverted elsewhere (where it is treated and discharged into the Minnesota River) and Lake Minnetonka’s algae blooms have been substantially mitigated. Because of this history, we are keenly aware of phosphorus sources into Lake Minnetonka (and many other lakes).
So, what impact did Mound’s discharges have at this time?
I estimate that Mound’s recent discharges totaled about 88 pounds of phosphorus. Again, not acceptable if it’s avoidable. But here is some perspective. Six Mile Creek, which enters Halsted’s Bay, delivers 6,171 pounds of phosphorus per year – every year. Other sources of phosphorus to Lake Minnetonka raise the total to over 14,000 pounds per year. So Mound’s discharge is almost literally a drop in the lake.
The Clean Water Act demands that much of this excess phosphorus be mitigated. The plan to do that (called a TMDL for “total maximum daily load”) finds that an “allowable” phosphorus load to Lake Minnetonka’s four impaired Bays (Jennings, Halsteds, Stubbs and West Arm) still exceeds 5,000 pounds per year – more than 50 times what Mound discharged. In other words, even after cleaning up these phosphorus sources (at a cost of tens of millions of dollars), there will still be far more phosphorus entering Lake Minnetonka than Mound’s discharge.
Why don’t we get excited about this? Probably because we cannot see it, and it doesn’t make headlines in local media. Or because runoff portrays a cleaner image than poop.
Overall, our society has done a good job in managing human waste. Our nation’s lakes and rivers are much improved. Like any mechanical system, there are imperfections, but in context, these are minor. Mound’s discharge, while unfortunate, is a small matter. We have bigger fish to fry.
So yes, sewage does stink. But perhaps not as much as other things now going into our lakes.