During these Take a Stake in the Lakes Days, it might be a good idea to look at local efforts to protect and restore Lake Wingra and the wetlands that surround it. Of special interest to this blog is the impact of storm water runoff on the Lake and the opportunities and constraints it places on ecological restoration of the urban natural resources of the Lake Wingra Watershed.
Each year on average, Lake Wingra receives 470 million gallons of storm water runoff. That’s right, nearly one-half billion gallons of storm water flows off roof tops, driveways, streets and parking lots and runs into Lake Wingra, each year.
A storm water detention pond as pictured above represents a typical strategy for dealing with runoff relying upon traditional storm water management as practiced in Madison, WI. However, although a scum layer is a typical outcome of storm water detention, the annual appearance of this thick and persistent layer is a bit unusual. What’s going on here?
Known variously as Marion Dunn Pond (after the former Arboretum super-volunteer who led prairie planting efforts near the site on Monroe Street in the early 1980’s) or Pond #5, this storm water detention pond serves one of fourteen sub storm watersheds in the Lake Wingra Watershed. Pond #5 receives about 58 acre feet–or about 180,000 gallons–of storm water each year from portions of the Nakoma, Westmorland, and Dudgeon-Monroe neighborhoods. (An acre-foot is the amount of water needed to cover an acre of land in water one foot deep.)
This pond was originally built in the early 1980’s and then dredged and reconstructed in 2004. It was designed to treat runoff to achieve a 40% reduction in Total Suspended Solids (such things as soil, street sand, and road salt that otherwise might enter Lake Wingra. But, since neither the City of Madison Engineering, UW-Madison, nor the Wisconsin Department of Natural Resources (DNR) conduct sampling and testing to decide if the pond is meeting its goals of capturing sediment and preventing its entry into Lake Wingra, or not, the effectiveness of the storm water pond is anyone’s guess.
The pond scum is likely the result of high nutrient and sediment loads, so in the sense that these solids are captured, the pond may be performing a service by preventing a part of them from flowing into West Marsh and Lake Wingra. But questions remain:
What is unique about this sub storm watershed that produces such a thick scum layer on the pond?
What is going on up-stream in the neighborhoods to produce such nutrient and sediment loads?
What are the up-stream sources of nutrient and sediment runoff?
Are there heavy metal pollutants in the runoff?
Is the situation being monitored by municipal and university authorities; are they even aware of it?
Is there a public health risk from the pond scum itself or from the up-stream conditions that have created this situation?
Something in the pond itself or in the sub watershed changed in 2004. This is because in the years prior to reconstruction in 2004 there was never a scum layer on the pond but, the scum layer first appeared in 2004 and has reappeared each year since. Here is some speculation about the causes.
Since Scummy Pond (aka Pond #5) does not flush perhaps there was a failure of engineering or construction to prevent the outflow of water at the bottom end?
During dredging of the Scummy Pond in December, 2003 were sediments and the underlying soil layer was stirred up to such an extent that the disturbance has fed the annual scum layer?
Has there been a change in land use upstream in the sub watershed?