Stormwater from impervious surfaces is collected in storm drains and enters Lake Ontario through direct storm drain channels such as the one below.
Why do we need to treat stormwater?
In the area near Frisco Road, hard impervious surfaces like parking lots, buildings, and roads, prevent stormwater from entering the ground. Instead, stormwater flows on top of the ground until it reaches a low point. The channel you see by Frisco Road is the lowest point in the local area so the stormwater collects here and flows directly out into Lake Ontario.
TRCA has measured nutrients (like phosphorus and nitrogen), E.coli (bacteria), and sediments in Lake Ontario between Pickering and Ajax since 2007 (trca.ca/nearshore). In the lake water, we can see the influence of this stormwater drain with high concentrations of nutrients where the drain enters the lake.
We need to manage stormwater that enters Lake Ontario because:
- Stormwater can impact swimming beaches by transporting E.coli and sediments to the lake, and
- Stormwater can add phosphorus to Lake Ontario, which is a nutrient source for algae.
What are we doing about it?
TRCA was able to transform this storm drain channel into a pilot treatment system with funding from the Ministry of Environment and Climate Change. There are two main components to this treatment system: 1) water control structures to create desired flow patterns and 2) wetland vegetation in the region. The system was designed to treat waters from small frequent storms or the first batch of water that comes in a big storm.
The water control structure is used to create a baseflow and also to achieve a desired flow through the channel. Prior to the construction of the treatment system, water flowed out of the storm channel in pulses with the storms. This sent pulses of nutrients to the lake. By adding water control structures, water is able to flow through the channel at a reduced speed over a longer time frame; this creates a situation where water can be “treated” as it passes through a series of filters, allowing the settling of suspended solids, the uptake of nutrients, and longer exposure to UV light. The importance of these are described in more detail below.
Natural processes occur within a wetland environment which help to improve water quality. After the water control structure slows down and holds some of the waters flowing thru the channel, the sediments have a chance to settle to the bottom of the channel and the nutrients can be used by the wetland plants in the area. As the plants grow, they will hold that settled sediment in place. Not only can the nutrients be taken up by plants, but they can also be absorbed by wetland soils and transformed by microorganisms. Sometimes microorganisms transform nutrients into forms which are inactive and remove them from the water. When the bacteria are held back, UV light helps inactivate E.coli so there is a reduction in bacteria entering the lake.
How did we do this?
TRCA was able to transform this storm drain channel into a treatment system by first excavating the current channel.
Once the channel was excavated, TRCA created a graded filter. A graded filter has layers of different sized gravel (coarse to fine) which act as a filter underneath the channel bottom. It is good at helping remove suspended sediments and also with reducing bacteria.
Baseflow was created using a perforated PVC pipe which transports water through a water level control structure. This structure allows us to change the amount of water held behind a series of check dams. The check dams were constructed out of a clay mound covered with large rocks (6 to 10 inches) to slow the transport of water that flows to the lake during storms. Water does not flow well through clay, so it acts as a good barrier. The large rocks on top of the clay mounds protect the clay from being eroded and prevent the transport of a lot of the sediment down the channel to the lake.
Within this system, the check dams and graded filter have a dual purpose. Not only do they help slow and filter the water, but they have also been installed to help make conditions favourable for wetland plant growth.
Will this treat all rain events?
Most of the rain events that we have are small and they can occur frequently. This pilot treatment system is designed to operate and treat the stormwater from small frequent storms that are less than 12.7 mm (or ½ inch). Big storms are not as common, and we don’t have the space to contain and treat all the water that comes with them. These big storms can bring a lot of nutrients to the lake. In the future, we aim to look at treating the stormwater from the large rain events upstream of this channel.
How do we know the pilot treatment system is working?
To see if this design is working, Environment Canada has funded a monitoring program. We are sampling the water in this storm drain channel after storms to see if some of the nutrients and sediments are staying in the channel. We are also investigating what happens to E. coli in the channel. The York-Durham Regional Environmental Lab, an accredited lab, are analyzing the samples. From all of this monitoring, we will be able to estimate the amount of nutrients and sediments entering the lake; we will also be able to see if the treatment design works at reducing the amount of nutrients entering the lake. If it does, we plan on using this type of treatment system to treat storm water in other parts of of the city as well.
Preliminary results suggest that average nutrient and E.coli (bacteria) concentrations are decreasing between the top of the storm drain channel and the bottom of the storm drain channel before the stormwater enters the lake. In the following graphs, the upstream location is where the stormwater enters the channel, the bridge is a midpoint location, and the downstream location is the stormwater before it enters the lake.
Average concentrations are sometimes higher in the lake than the downstream site, and this tells us that if the amount of water coming from the storm drain is low, there could be additional sources of nutrients and suspended solids influencing the lake in this area. Individual samples show us at times the downstream location has higher concentrations than the lake and this tells us that the treatment system cannot treat larger rain events which can influence lake water quality. This reinforces the need to implement stormwater reductions in the community around this area.
Next Steps
Working collaboratively with Partners in Project Green and Durham Sustainability/Durham Partners in Project Green, future efforts will focus on outreach to local businesses to promote and facilitate the implementation of new stormwater technologies in the industrial business park that drains into this stormwater channel. By installing Low Impact Development best management practices, on a retrofit basis, we hope to achieve further reductions in nutrient, bacteria and sediments entering Lake Ontario.