Chapter 12 – Water Chemistry

Release Date: March 2013

Water is one of our most valuable natural resources. We drink water to live and eat fish from streams and lakes. We use water for recreation, in agriculture and in industrial processes. Water chemistry monitoring enables us to protect the quality of water for both human use and to maintain healthy ecosystems.

Similar to groundwater quality, human activities can negatively impact surface water quality in a number of ways. Contaminants such as excess nutrients, pesticides, metals, pharmaceutical products and bacteria can all enter streams as a result of human actions. At certain concentrations these contaminants can have negative effects on both ecosystem and human health. It is therefore important to monitor water chemistry (Figure 1) to help ensure the health and safety of our aquatic systems and detect emerging trends to inform management actions.

Figure 1. Sampling the Credit River for water chemistry analysis.

Water Chemistry Monitoring

Credit Valley Conservation’s (CVC) water chemistry monitoring network consists of 23 stations located across the watershed: 16 on the Credit River, six on tributaries and one on a stream flowing directly into Lake Ontario (Figure 2). Sixteen of these 23 stations are also part of the Ministry of Environment’s Provincial Water Quality Monitoring Network (PWQMN). Water chemistry samples are collected from all monitoring stations on a monthly basis (except December) and analyzed for chemical and biological indicators of water quality. In addition to the monthly monitoring program, CVC operates four real-time water quality stations in the Credit River Watershed that provide instantaneous information on any changes to water quality.

To evaluate status and trends in water chemistry CVC focuses on nine parameters (Table 1) that were identified as parameters of concern in the Credit River Water Quality Strategy (CVC 2003) and Interim Watershed Characterization Report for the Credit River Watershed (CVC 2007). These parameters represent stressors from both urban and rural land. Water quality objectives for these parameters are based on Provincial Water Quality Objectives (PWQO; MOE 1994) and Canadian Council of Ministers of the Environment (CCME; CCME 2001) guidelines for the protection of aquatic life.

Table 1. Water chemistry parameters of concern and their guideline threshold.




20 μg/L


75 μg/L


120 mg/L


5 μg/L


300 μg/L


2.93 mg/L


30 μg/L

Total suspended solids

25 mg/L


20 μg/L


Based on the nine parameters of concern, water quality is summarized using the Water Quality Index (WQI; CCME 2001). The WQI was developed to provide an easily understandable method of communicating complex water chemistry data to a general audience. The WQI calculation generates a score between 0 and 100, where 0 indicates poor water quality and 100 indicates excellent water quality (CCME 2001; Table 2).

Table 2. Classification of water quality based on WQI score (CCME 2001)

WQI Score

Water Quality

95 – 100


80 – 94


65 – 79


45 – 64





Figure 2. Location and 2011 status of water chemistry monitoring stations in the Credit River Watershed.

Water Chemistry Status (2011)

Water chemistry status in 2011 ranged from good to poor (Figures 2 and 3). Although the overall pattern of WQI scores in 2011 is similar to the range of values observed during 2002-2010, the 2011 values for the Credit River stations were often outside (both higher and lower) the 2002-2010 range of WQI values (Figure 3). This indicates that some stations experienced their best water quality over the monitoring period in 2011 whereas many others had their lowest water quality.

The Credit River starts with good water chemistry at Island Lake in the Upper Watershed, however the WQI score quickly declines to marginal in response to high concentrations of iron, aluminium, chloride and phosphorus. The high iron concentration is likely the result of naturally iron rich groundwater inputs. Elevated aluminium, chloride and phosphorus concentrations are possibly being driven by urban runoff (aluminium), road salt applications (chloride) and input from the Orangeville Water Pollution Control Plant (chloride and phosphorus). The WQI score of the Credit River then increases to fair as groundwater fed tributaries with good water quality contribute to the Credit River.

Water quality subsequently declines through the Middle and Lower Watershed. In the Middle Watershed chloride, phosphorus and aluminium concentrations regularly exceed water quality guidelines in Black and Silver Creeks which carry discharge from Water Pollution Control Plants in Acton and Georgetown respectively. These tributaries then flow into the Credit River reducing the water quality of the river.

In the Lower Watershed, Fletcher’s Creek has the lowest water quality of all monitoring stations and in 2011 is the only station to be classified as having poor water quality. Concentrations of chloride, phosphorus, aluminium, copper and iron exceed water quality guidelines in Fletcher’s Creek. As Fletcher’s Creek flows into the Credit River, the water quality of the Credit River is further reduced. The final monitoring station is located in Sheridan Creek. Sheridan Creek does not flow into the Credit River but rather enters Rattray Marsh before flowing into Lake Ontario. The concentration of chloride in Sheridan Creek is very high (about 7 times the water quality guideline) and phosphorus, aluminium and copper also exceed water quality guidelines giving this station marginal water quality.

Figure 3. 2011 Water quality index (WQI) scores compared to the range of WQI scores between 2002 and 2010 for the 23 monitoring stations in the Upper, Middle and Lower Watershed. Boxplots show the minimum (lower whisker), lower quartile, median, upper quartile and maximum (upper whisker) of the range of WQI scores from 2002 to 2010.

Water Chemistry Trends

Although water chemistry has been variable over the monitoring period (Figure 3), most of the monitoring stations (20 out of 23) had no statistically significant trend in water chemistry over the monitoring period (Table A1). Three monitoring stations (Orangeville Dam and Melville Dam in the Upper Watershed and Fletcher’s Creek in the Lower Watershed) did however have a statistically significant trend of increasing water quality over the monitoring period, suggesting that water quality has been improving at these stations (Figure 4). It is important to note that the water chemistry monitoring record is just beginning to approach sufficient length for statistical trend analysis and as new data are collected a greater understanding of the long-term trends in water quality will be obtained. For example, 18 of the 23 stations have a pattern of increasing WQI scores (Table A1) and although most of these records are not statistically significant further monitoring will clarify if this trend of improving water quality is widespread across the watershed.

Figure 4. Statistically significant trends in water quality index (WQI) scores.


Water chemistry in the Credit River Watershed varies from good to poor with low water quality stations generally receiving much of their water from Water Pollution Control Plants and urban runoff, which have higher concentrations of contaminants. Although early, there appears to be a general trend of improving water quality at some stations and further monitoring will clarify if this trend is evident watershed wide. It is important to remember, however, that the monitoring record covers approximately the last decade and although trends of improving water quality are encouraging, water quality at some stations may still be well below historical levels and provincial objectives and federal guidelines. The monitoring data suggests improving water quality in the Credit River Watershed. Continued management, informed by monitoring data, will help to meet the goal of a healthy and sustainable Credit River ecosystem.

CVC measures the water temperature of the Credit River and it’s tributaries. How is climate altering water temperature of the Credit River and it’s tributaries and what does this mean for the plants and animals that live in the river? Read the next chapter of the Watershed Health Report to find out.

Did you know?

Cold water holds more oxygen than warmwater.



CCME (Canadian Council of Ministers of the Environment). 1999, Updated 2002. Canadian Water Quality Guidelines for the Protection of Aquatic Life, Protocol for the Derivation of Water Quality Guidelines for the Protection of Aquatic Life.

CCME (Canadian Council of Ministers of the Environment). 2001. CCME Water Quality Index.

CCME (Canadian Council of Ministers of the Environment). 2011. Scientific Criteria Document for the Development of the Canadian Water Quality Guidelines for the Protection of Aquatic Life, CHLORIDE ION, PN 1460.

MOE (Ontario Ministry of the Environment). 1994, Reprinted 1999. Water Management Policies, Guidelines, Provincial Water Quality Objectives of the Ministry of Environment and Energy.

CVC (Credit Valley Conservation Authority). 2003. Credit River Water Quality Strategy.

CVC (Credit Valley Conservation Authority). 2007. Interim Watershed Characterization Report for the Credit River Watershed.

CVC (Credit Valley Conservation Authority). 2012. Integrated Watershed Monitoring Program: Ten-Year Review (1999-2008).

Table A1. Mann-Kendall analysis of water quality index (WQI) versus year. A positive tau value indicates that water quality is improving whereas a negative tau value indicates water quality is declining. Statistically significant (p<0.05) trends are shown in bold.

Station name

Sampling years



Upper Watershed




Credit River at Orangeville Dam




Credit River downstream of Mill Creek




Credit River at Hwy 10 North




Credit River at Hwy 10 South




Credit River at Melville Dam




Shaw’s Creek at Bruce Trail




Credit River at Beechgrove Sideroad




Credit River downstream of Hwy 24




West Credit River at Winston Churchill Blvd




Middle Watershed




East Credit River upstream of Hwy 10




Credit River at McLaughlin Rd




Credit River at Terra Cotta




Credit River at Glen Williams




Black Creek at Acton Waste Water Treatment Plant




Black Creek downstream of Third Line




Silver Creek downstream of Mountainview Rd




Silver Creek upstream of Hwy 7




Lower Watershed




Credit River downstream of Hwy 7




Credit River upstream of Old Derry Rd




Fletcher’s Creek downstream of Steeles Ave




Credit River at Erindale Park




Credit River at Mississauga Golf and Country Club




Sheridan Creek at Rattray Marsh






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