What have we learned about nutrient sources in the Western Durham nearshore environment?
Nutrients in the nearshore environment can come from landscapes, watersheds, and watercourses connected to the Great Lakes by hydrology (e.g., surface runoff, rivers, creeks, groundwater, etc.) and outfalls to the lake (e.g. wastewater treatment plant outfalls, storm drains, etc).
There are a number of sources, which fall into two broad categories: natural and anthropogenic (human-induced).
Nitrogen and phosphorus are two important nutrients to nearshore environments; sources of nitrogen and/or phosphorus might include:
- freshwater runoff which passes over geologic formations rich in these nutrients (low)
- marshes and creeks
- decomposing organic matter and wildlife waste that gets flushed into rivers and streams
- wastewater treatment plants
- leaky septic tanks
- industrial wastewater
- storm drains
- acid rain
- fertilizer runoff from agricultural
- residential and urban areas
- atmospheric deposition
- shoreline erosion
- sediment re-suspension
TRCA has measured the concentrations of nutrients from some of the potential nutrient sources affecting Ajax and Pickering.
Concentrations are standardized measurements which take into account the mass of the nutrient and the volume of water it is in by the equation:
Concentration = mass of nutrient / volume of water.
Often concentrations are measured in milligrams per litre (mg/L) or in the case of the Western Durham nearshore – 1000 times smaller: micrograms per litre (µg/L).
TRCA found large concentration ranges exist in many potential nutrient sources, and these concentrations can be much greater than those observed in the lake.
These concentration ranges are shown below in a series of diagrams called “boxplots”, which are useful because they illustrate changes in data ranges.
If you are not familiar with boxplots, please see below to understand the meaning of the boundaries in the diagrams.
On the above diagram is a solid dark horizontal line which indicates Maximum DL (detection limit). A detection limit is the minimum concentration that the lab will report. If the detection limit is 2, and the lab measures 1.9, then they will say that the concentration is < 2. Having data with concentrations below a detection limit does not mean that the lab that measured the result is of poor quality. Only specialized government and university labs can measure extremely low nutrient concentrations and these labs are not commercial.
In the Ajax and Pickering nearshore area, there are many data points that are non-detects, or below measurable concentrations. As a result, we have used techniques and analytical tools which are created for these types of data. Some of these methods are recommended by the United States Environmental Protection Agency.
The specialized boxplots below are different than normal boxplots and they take into account the non-detect data. The distributions of data below the maximum detection limit are modeled, and therefore lie between 0 and the maximum reported detection limit.
From the following images, it is clear that the range in concentrations observed in the streams, storm drains, and marshes can be higher than the concentrations from the lake!
E. Coli
In the lake, the range in E. Coli concentrations is large in the samples located less than 1 km from the shoreline, and overlaps the concentrations found in water sources originating on land. General observations include:
- At distances more than 1 km from the shore, 50% of the E coli measurements are at or below detection.
- Concentrations by the drinking water intake are below detection illustrating we have a safe supply of drinking water.
- Median concentrations decrease from land to lake.
Phosphorus
There is such a large difference between the phosphorus concentrations we see from water coming from land in comparison to the lake that we need to plot the data on a log scale (each group of numbers increases by 10 times on the nutrient axis). We notice for phosphorus concentrations:
- total phosphorus from creeks, storm drains, and marshes is much higher than in the lake
- there are higher total phosphorus concentrations closer to the shoreline than towards open waters
- median concentrations in the lake falls below the phosphorus objective recommended by the International Joint Commissions for open waters
NO TOTAL PHOSPHORUS OBJECTIVES EXIST FOR SHORELINE OR NEARSHORE ENVIRONMENTS.
There is a lot of total phosphorus in the shoreline regions that could be available for biological growth if it is in a form that plants and algae can consume.
Soluble Reactive Phosphorus
Similar to total phosphorus, there is a large range in the soluble reactive phosphorus concentrations we see from land based water in comparison to the lake. Since many samples in the lake fall below the lab detection limit, the results below the detection limit are modeled.
With soluble reactive phosphorus, we notice:
- Median concentrations decrease from land based water sources to open lake.
- Detection limits for the lake samples changed over the years from 4 (in 2007) to 2 (in 2008) to 0.5 (in 2015) µg/L
- Median concentrations are between 1 and 2 µg/L, but there are still many samples that are below 0.5 µg/L.
NO NEARSHORE WATER QUALITY OBJECTIVE EXISTS FOR SOLUBLE REACTIVE PHOSPHORUS.
Nitrogen
Similar to total phosphorus, there is a large range in the nitrate + nitrite concentrations we see from land based water in comparison to the lake. One difference is that median concentrations remain similar between the different locations, except for the marshes.
However, it is important to realize that these are concentrations and not loads.
Nutrient Loads
Concentrations are variable depending on changing sources, and climate. Loads take into account the amount of water coming into the lake with a particular concentration. A load is the total mass of the nutrient entering the lake from a particular source.
Let’s consider two examples to understand how loads and concentrations are related. We should first understand concentrations. If we have a mass of a nutrient that is 1 microgram, and we put that mass into 1 litre of water, we will have a concentration of 1 microgram per litre (or 1 µg/L). Similarly, if we put 50 micrograms of the nutrient into 1 litre of water, we will have a concentration of 50 µg/L. Concentration is equal to the mass of the nutrient divided by the volume of water it is in. Often concentrations are measured in milligrams per litre (mg/L) or 1000 times smaller: micrograms per litre (µg/L).
Example 1:
For our first example relating concentrations to loads, let’s say that the amount of water coming into the lake from a creek (discharge) is constant and is 1000 litres. If we change the concentration of the nutrient in that water, different total amounts (or masses) of the nutrient will enter the lake:
a) Low nutrient concentration of 1 µg/L x 1000 L = 1,000 µg of nutrient load
b) High nutrient concentration of 50 µg/L x 1000 L = 50,000 µg of nutrient load
If the volume of water entering the lake is the same, a larger total mass of nutrient will enter the lake with the higher nutrient concentration in (a). Considering the picture above, we would have 50,000 little yellow dots versus 1000 little yellow dots.
Example 2:
For the second example, let’s change the amount of water entering the lake – something that might happen between a wet day and a dry day.
In this case, we will keep the concentration of the nutrient the same at 1 µg/L.
c) Dry: 1,000 L x 1 µg/L nutrient concentration = 1,000 µg of nutrient
d) Wet: 10, 000 L x 1 µg/L nutrient concentration = 10,000 µg of nutrient
This would be 10 times the dots above.
In this example, if the nutrient concentration remains the same, then more water entering the lake means more total nutrients will enter the lake.
The relationship between the amount of water entering the lake and the concentration of the nutrients in the water becomes very important to understanding the nutrients we see in the nearshore.
Unfortunately, it is very difficult to measure the nutrient loads from each of the sources entering the nearshore region. The changes in concentrations and the changes in the volume of water entering the lake from each source which occurs over time would need to be understood. This would require many of samples, and be very costly.
Some scientists estimate nutrient loads from known major sources by making assumptions. Some of these assumptions might be that concentrations stay the same over certain periods of time, concentrations only vary over a certain range, or that the amount of water or discharge is a certain value.
Efforts are underway by TRCA, Ganaraska Region Conservation Authority (GRCA), Durham Region, Region of Peel, Toronto Water, Ministry of the Environment, Conservation and Parks, Ministry of Agriculture, Food and Rural Affairs, and Environment and Climate Change Canada to collect nutrient samples during storm events.
These measurements will be used to calculate nutrient loads from the watersheds across Western Lake Ontario including in Ajax and Pickering. Ongoing projects with academics are also helping us figure out the nutrients coming from storm sewer-sheds in Ajax and Pickering.
The results of our water quality project will be shared with Great Lakes scientists to assist them in their efforts to understand factors influencing the growth of nuisance algae. Hopefully the ongoing research by Environment and Climate Change Canada and the Ontario Ministry of the Environment, Conservation and Parks will lead to local management recommendations.
Ajax and Pickering Nutrient Sources
TRCA works with the Ontario Ministry of the Environment, Conservation and Parks and Environment and Climate Change Canada to monitor and model watersheds in our water quality study area to estimate nutrient and suspended solid loads.
This program is independent of TRCA’s nearshore water quality surveys and the Municipal Class Environmental Assessment completed for the Duffin Creek Water Pollution Control Plant.
Water samples were collected in Duffins Creek and Carruthers Creek during a number of storms in in 2007, 2008 and 2009 using special equipment that collects samples throughout the resulting runoff event. The weather sampling results were combined with routine monitoring data for the watersheds to estimate nutrient loads on a daily, annual, and seasonal basis.
This research on Duffins Creek Total Phosphorus loads underwent extensive peer review and was published in scientific journals (e.g. Booty et al., 2013; Leon et al., 2013).
Initial results were summarized for 2008 in a technical report and in presentations prepared for the Credit Toronto Central Lake Ontario Source Protection Study (Bowen and Booty 2012 a,b). Results were extrapolated for Lake Ontario and compared with watersheds on the United States side of Lake Ontario.
For the Duffins Creek watershed using averaging algorithms, ratio estimators, event mean concentration (EMC) and regression based methods the 2007, 2008 and 2009 ranges for total phosphorus loads are 6.2-30, 22.3-78 and 19-242 tonnes of P, respectively.
Taking some uncertainties in estimation methods into account, the lower and upper bounds of total phosphorus loads for 2007, 2008 and 2009 were 13 and 17, 57 and 73 and 69 and 92 tonnes of total phosphorus per year, respectively.
Loads are currently being revisited and new estimates will be calculated based on the collaborative project described above with GRCA, the Regions, and the governments.
The maximum amount of total phosphorus released by the Duffin Pollution Control Plant through the outfall to the nearshore is regulated by the Provincial government. A maximum of 311 kg of total phosphorus per day, which is equal to 113.5 tonnes of total phosphorus per year is permitted (CH2MHILL, 2013).
In 2020, the average daily load was less than one-third of the permitted load. Limits are not expected to be reached until 2034.
There are also large storm drains nearby which empty water from land to the nearshore. One of these drains carries water from an area which spans Highway 401 to south to Lake Ontario, through industrial and agricultural lands.
Although we have concentrations available for this large storm sewer, we do not have discharge data and cannot assess the relative importance of this drain to the lake.
Currently, work is underway on a collaborative project with the University of Waterloo to examine flows in and nutrient loads from some of the storm drains in the Western Durham area.
Take Home Messages
- Nutrient concentrations and ranges from land based sources are greater than concentrations and ranges in the lake.
- Land based sources can contribute a lot of the nutrient loading to lakes, however, more work is needed to accurately understand the land based loading to the lake by Ajax and Pickering.
- There are no nearshore phosphorus water quality objectives recommended by the Ontario Ministry of the Environment, Conservation and Parks or the International Joint Commission, only open-water objectives.