Thermal pollution

When we think of how stormwater impacts lakes, rivers and streams, we tend to think of sediment and other contaminants that degrade water quality. We rarely think of how elevated runoff temperatures, known as thermal pollution, can greatly alter the temperature regime of natural water bodies. On hot days, impervious surfaces, such as parking lots and roofs, absorb and emit heat.  When it rains, heated runoff that flows over these surfaces can potentially increase the temperature of receiving waters. Increased temperatures can negatively impact coldwater fish populations by interfering with spawning and migration patterns, decreasing dissolved oxygen levels and even resulting in fish kills.

Stormwater Best Management Practices have an effect on runoff temperature, and the University of New Hampshire's Stormwater Centre has been researching the effectiveness of various BMPs in regulating thermal pollution from stormwater runoff.

They found that surface systems, such as retention ponds, that are exposed to direct sunlight provide little no reduction of high runoff temperatures and can potentially increase already elevated summer runoff temperatures. Various infiltration and filtration systems were best able to moderate runoff temperatures by thermal exchange with cooler subsurface materials. Systems with a large subsurface area had the greatest ability to buffer runoff temperatures. The full report titled 'Examination of Thermal Impacts from Stormwater Best Management Practices' can be found here. 

Brook Trout. (Photo: Source)

Permeable Pavement

Permeable pavement allows precipitation to slowly soak into the ground, thereby reducing runoff from driveways, roads and parking lots. There are two main types of permeable pavement – permeable pavers (also known as interlocking concrete blocks) and porous pavement. These BMPs can be incorporated into designs for new developments, or used as a retrofit option for existing homes and buildings. 

Permeable Pavers consist of impervious concrete blocks or tiles that allow water to infiltrate between blocks in voids filled with gravel or grass.  

Permeable Paver (photo: bit.ly/be1x3a)

Parking space at the Ecology Action Centre

Porous Asphalt or Concrete consists of standard asphalt or concrete mixes with the finer aggregates removed resulting in pores within the pavement that allows water to permeate through the surface. 

A parking lot on the Dalhousie University campus uses porous pavement  to reduce stormwater runoff. The lot was graded allowing runoff to flow towards the strip porous pavement area at one end of the lot. There is a clear stone base under the permeable area with a perforated PVC pipe leading to catch basin in the corner of the lot.  Water that infiltrates through the pavement has a chance to soak into the ground before being picked up by the perforated pipe. A strip of permeable pavement on a graded lot works well, therefore repaving the entire parking lot is not necessary. Some concern exists about how porous pavements can withstand Nova Scotia's harsh winter conditions, however this was put in during the Fall of 2010, and there has been no damage reported to date.

Strip on porous pavement on a Dalhousie University parking lot (width of the car)

Regular asphalt on the left, porous pavement on the right

The City of Vancouver is using an alternative approach to reduce imperviousness on residential side streets called ‘Country Lane Treatment’. This design incorporates a soil and grass covered plastic mat between two parallel concrete driving strips. Permeable pavers were used to connect driveways to the driving strips. Click here to see more examples of road treatments from Vancouver. 

'Country Lane' in Vancouver (Photos: http://bit.ly/A4s9Os)

More information on permeable pavements and other stormwater management technologies can be found on the Sustainable Technologies Evaluation Program (STEP) website, an initiative led by the Toronto and Region Conservation Authority.