 djoles@startribune.com Residents in 5 northern Ramsey County communities - Shoreview, Vadnais Heights, Little Canada, Arden Hills and Roseville - are up in arms over-disruptive train noise as a result of increased rail traffic between the Twin Cities and western North Dakota. Cities are taking steps, sometimes expensive](https://arc.stimg.co/startribunemedia/4OMAC6N73AVKOGKESXBQFGSFMQ.jpg?&w=1080)
Stormwater ponds are not little lakes. They are polluted by design and require a significant public and private expense to maintain. All this, and they do not function as well as we might think.
Urban lands produce more polluted runoff compared with native lands. Runoff — that amount of rainfall and snowmelt that does not filter into the ground — occurs in higher volumes and at faster rates following urbanization. In the process, runoff waters pick up and carry many kinds of pollution, notably phosphorus, that impact lakes. Stormwater ponds are a method to slow the runoff and trap some of this pollution.
The article "Cities awash in pond gripes" (Dec. 14) lamented the fact that these ponds are smelly eyesores and expensive to keep clean. But this is normal and expected. When the ponds are designed specifically to trap pollution, of course they become polluted. Sometimes, city officials try to deflect the blame back to homeowners, suggesting they not mow to the ponds' edge — "the best thing you can have around the pond is vegetation."
However, there is no evidence that vegetation surrounding stormwater ponds will mitigate their poor condition, although a mix of taller native vegetation can provide a more inviting view and a more welcoming environment for wildlife.
The important question is: Are stormwater ponds doing the job of protecting our lakes? The answer is: Not as much as we would hope. Are stormwater ponds better than nothing? Yes. But how much better?
The one kind of pollution in stormwater that is critical for lake quality is phosphorus — but stormwater ponds mitigate phosphorus the least of all kinds of pollution. With phosphorus pollution, stormwater ponds underperform because they are almost always improperly built and inadequately maintained. The design standards for these ponds are also compromised when they must be shoehorned into development sites. In addition, the design standards are based on rainfall events typical of what occurred up to about the 1980s. Now, rainfall patterns have changed and storm events are more intense, meaning the ponds retain less phosphorus.
Properly designed ponds require regular maintenance. As the article pointed out, costs in the tens of thousands of dollars are common — per pond. Some cities are doing a good job of maintenance, but many are not.
What's the upshot? Mostly, we don't know, because the actual performance of ponds is never measured. It's hoped that the ponds remove phosphorus and other pollution, but this is most likely not the case. In 30 years of conducting studies where I have measured pond outflow for phosphorus, it's common that phosphorus exceeds the design level by up to fourfold. In the metro area, the investment in ponds is enormous. There are thousands of ponds requiring tens of millions of dollars annually for maintenance. Yet I'm not aware of a single case where a downstream lake has improved water quality following urbanization that includes the required ponding. At best, lakes are degrading more slowly.
