Published: Jul 21, 2012 07:00 PM
Modified: Jul 21, 2012 05:14 PM
Durhams stormwater folks seek citizen feedback this summer on a stormwater facility at the Duke Diet and Fitness Center. Their presentation describes and promotes a policy choice, not an explanation of the available policy choices nor the science behind the needs. In this first of two columns, I will cover the basic problems of stormwater. On Wednesday, Part 2 will give a short discussion of the options in a broader light. The Basic Problem:
Urban areas have lots of impervious surfaces: parking lots, roads, houses, sheds, parked cars, etc. When its not raining, pollutants settle on these surfaces; these pollutants include nutrients, oils, heavy metals, salt, pesticides and herbicides. More pollution settles out in urban areas than rural ones, and most biologically available nitrogen (in our area) comes from burning fossil fuels cars, trucks, coal-fired power plants, but emissions controls help reduce those loads. Even tire and brake wear add significant amounts of various other pollutants. Storms wash these pollutants off of the impervious surfaces, and the highest pollution concentrations in the stormwater are found in the first flush, or the initial stormwater flow off of a site.
In addition to pollutants, stormwater rushing off of impervious surfaces leads to high flows downstream, like the sudden rush of water from a toilet, and leads to a phenomenon called the urban stream syndrome. High, flashy flows deeply erode streams, not only carrying the eroded sediments downstream, but the resulting stream profile acts almost like a drainage ditch during the dry periods. These "ditches" drain groundwater near the stream, which dries out the stream-side vegetation (the riparian zone), and reduces the nutrient processing abilities of these urban streams. Leaky sewer pipes previously submerged below the water table drew water into the pipes, but a lowered water table can instead let its contents leak into the dry soils.
To deal with pollutants and flashy flows, "stormwater control measures (SCMs)" try to intercept and contain the first one inch of rainfall to promote settling, evaporation, photodegradation, and bioretention of the contained pollutants. Slowly releasing this water also reduces peak flows and helps preserve stream ecosystems that process nutrients, as well as emptying out the SCM in preparation for the next storm. Of course, these SCMs have to be built carefully to not wash away when 6 inch storms come along.
As gardeners know, plants desperately want more nutrients, and algae in lakes are no different. We learned long ago that nitrogen in lakes make algae flourish, but algae-eating animals cant keep up, and when all that algae dies, bacteria break it down and use up all of the oxygen in the water. That lack of oxygen makes fish die, the water stinky, and leads to all sorts of other problems. Weve added nutrient regulations to deal with the problem of nutrients.
The Falls Lake nutrient reductions are clear: we must reduce the nutrients flowing into the lake. We are "graded" on the nutrients in the streamwater right where it enters the lake, so as far as nutrients go at that spot, a reduction of 100 pounds anywhere upstream is a reduction of 100 pounds. Our stormwater services department seeks places upstream where the most reductions can be done for the least amount of taxpayer money. Downtown is likely a disproportionately high source of the nutrients, but its also an extremely expensive place to reduce them. So, just from the perspective of nutrients, there is a reasonable argument that we should make the reductions elsewhere.
Stormwater volume is a different matter. In undeveloped watersheds, rain falls across the entire watershed, evaporates or helps plants and trees grow, or soaks into the ground. Thats a process that naturally holds back water throughout the watershed until it seeps from groundwater to streamwater. In urban areas, all that water quickly flushes downstream because impervious surfaces prevent it from soaking into the ground. We need to hold that stormwater back or sacrifice all of the natural, downstream nutrient processing capabilities. We could hold back that volume with one big facility way downstream like the DDFC site, or lots of smaller facilities farther upstream using a variety of control measures.
Will Wilson is an associate professor of biology at Duke, author of "Constructed Climates: A Primer on Urban Environments" and is writing a book on stormwater. Contact him at firstname.lastname@example.org