Temporal Patterns

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How is water quality in the Western Durham nearshore changing over time?

This is a work in progress! The Province is monitoring certain nearshore regions approximately every five years to try to assess the water quality.

Our results fill in the gaps between the years the Province monitors the area. We are just beginning to be able to determine whether conditions are improving or worsening with time.

Since there have not been a lot of management changes to the watersheds in the timeframe of this program, it is possible the differences we see are due to climatic or biological (e.g. mussel/goby fish) influences.

However, there were some milestones at the Duffin Creek Water Pollution Control Plant (e.g. 2010: Initiation of Stage 3; 2012: Stage 3 complete, Stage 1 and 2 phosphorus treatment upgrades complete; late 2014-2015: parts of the plant were online for Stage 1 and 2 refurbishments). Preliminary water quality analyses suggest that there might be responses observed in nearshore water quality.

The TRCA monitoring program has measured water quality from 2007 to the present. We have completed over 75 regional surveys and 12 maintenance design surveys. Survey designs have changed over time, but there are a number of sampling locations that have remained the same.

A map illustrating the location of sites that are consistent within the entire monitoring program
Figure 1: A map illustrating the location of sites that are consistent within the entire monitoring program. The red dots show the sites that are consistent between 2007 and 2018, and the yellow circles have been sampled from 2007 to the present. Only surface water is collected at the sites closest to the shoreline (at 100 m).

If we look at the patterns over time we can see that:

Total Phosphorus:

  • Total phosphorus concentrations in surface waters are increasing at most sites.
  • Total phosphorus concentrations are decreasing in surface waters by the Duffin Creek Water Pollution Control Plant outfall.
  • Significant total phosphorus concentration increases occurred in surface waters 3 km from shore in the east.
  • Total phosphorus concentrations in bottom waters are increasing significantly at nearly all sites.
  • Total phosphorus concentrations in bottom waters are decreasing by the Duffin Creek Water Pollution Plant outfall.
Total phosphorus concentration trends in surface waters over time
Figure 2: Total phosphorus concentration trends in surface waters over time.
Total phosphorus concentration trends in bottom waters over time
Figure 3: Total phosphorus concentration trends in bottom waters over time.

Soluble Reactive Phosphorus:

  • Soluble reactive phosphorus concentrations in surface waters are increasing at most sites.
  • Soluble reactive phosphorus concentrations are decreasing in surface waters by the Duffin Creek Water Pollution Control Plant outfall and in the east near 2 km.
  • Soluble reactive phosphorus concentrations are increasing in bottom waters at 70% of the sites (3 sites are significant increases).
  • Soluble reactive phosphorus concentrations are decreasing by the outfall to the Duffin Creek Water Pollution Control Plant outfall.
Soluble reactive phosphorus concentration trends in surface waters over time
Figure 4: Soluble reactive phosphorus concentration trends in surface waters over time.
Soluble reactive phosphorus concentration trends in bottom waters over time
Figure 5: Soluble reactive phosphorus concentration trends in bottom waters over time.

Nitrate + Nitrite:

  • Nitrate + nitrite concentrations in surface waters are decreasing significantly at all sites (including by the Duffin Creek Water Pollution Control Plant outfall).
  • Nitrate + nitrite concentrations in bottom waters are decreasing at all sites (including by the Duffin Creek Water Pollution Control Plant outfall).
  • The bottom water decreases in nitrate + nitrite concentrations are significant at ~60% of the sites.
Nitrate + nitrite concentration trends in surface waters over time
Figure 6: Nitrate + nitrite concentration trends in surface waters over time.
Nitrate + nitrite concentration trends in bottom waters over time
Figure 7: Nitrate + nitrite concentration trends in bottom waters over time.

Take Home Messages

  1. While phosphorus concentrations are decreasing by the Duffin Creek Water Pollution Control Plant outfall, they are increasing at most other sites.
  2. Patterns in the surface waters and bottom waters are not always the same.
  3. Nitrate + nitrite concentrations are decreasing at all nearshore sites analyzed.