Chapter 14 – Sediment Chemistry

Release Date: March 2013

The overall health of an aquatic ecosystem is closely linked to its sediment chemistry. Contaminants can settle out of the water column and be deposited on the sediment bed where they accumulate over time. Once a contaminant becomes part of the sediment bed it is, however, not locked away forever. Contaminants can be resuspended in the water column and reintroduced to aquatic food-webs through a variety of means. For example, large storms can stir up sediment, reintroducing buried contaminants to the open water environment. Also, benthic organisms feeding on organic matter in the sediment can accumulate contaminants in their body, returning contaminants to aquatic food-webs. Monitoring sediment chemistry (Figure 1) provides an understanding of where contaminants are accumulating in the watershed and establishes long-term changes in contaminant concentration to aid management decisions.

Figure 1. Sampling sediment in the Credit River Watershed

Sediment Chemistry Monitoring

In 2004 CVC initiated a sediment monitoring program. As part of this program, sediment samples were collected from at least 23 monitoring stations across the watershed since 2004 (Figure 2). Although sediment samples were collected from at least 23 stations every monitoring year the location of some stations changed from year to year and only seven stations have sediment chemistry data for every sampling year.

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

More than 80 chemical parameters are analyzed in each sediment sample and grouped into one of 6 categories:

  1. Polychlorinated Biphenyls (PCBs): Used in electrical equipment such as transformers and capacitors until the 1980s
  2. Phenanthrene (PAHs): Produced during the incomplete combustion of fossil fuels
  3. Organochlorine Pesticides (OC): Including DDT (dichlorodiphenyltrichloroethane) and other common pesticides
  4. Inorganics: e.g. total nitrogen
  5. Petroleum Hydrocarbons: e.g. oil and grease
  6. Metals: e.g. lead, zinc.

The results of chemical analyses are then compared with the Provincial Sediment Quality Guidelines Lowest Effect Level (LEL) (MOE 1993a and 2008). Sediment quality is then summarized using the Sediment Quality Index (SQI), a technique similar to the Water Quality Index (CCME 2001) used to summarize water quality data. The SQI provides an easily understandable method of communicating complex sediment chemistry data by generating a score between 0 and 100, where a score of 0 indicates poor sediment quality and 100 indicates excellent sediment quality (Table 1).

Table 1: Sediment quality index (SQI)

SQI Score

Sediment Quality

95 – 100


80 – 94


65 – 79


45 – 64




Sediment Chemistry Status (2011)

In 2011, sediment quality across the watershed is rated as marginal to poor (Figure 3). Only four of the 23 sampling stations (~17%) were rated as fair or better with a single station receiving a rating of good (Levi Creek in the Lower Watershed). In 2011 no station received a rating of excellent.

In general, the Upper Watershed had better sediment quality than either the Middle or Lower Watershed, however, all physiographic zones showed some degree of impairment in sediment quality. Typical of urban regions PAHs were detected throughout the Credit River Watershed. In the Credit River Watershed, PAHs exceeded federal threshold effect levels in 16 of the 23 monitoring stations. Elevated levels of metals (arsenic, cadmium, chromium, copper, lead and nickel) were also found throughout the watershed with the monitoring stations on Black Creek in the Middle Watershed having the highest concentration of metals. The most severe violations of Provincial Sediment Quality Guidelines were for total organic carbon and total Kjeldahl nitrogen, both which exceeded the LEL threshold at all 23 monitoring stations in 2011. These results highlight the legacy of pollution that can build up over time in sediment at the bottom of streams and lakes.

Figure 3. 2011 sediment quality index (SQI) scores for the 23 monitoring stations in the Upper, Middle and Lower Watershed.

Sediment Chemistry Trends

The length and intermittent nature of the sediment chemistry monitoring record is not suitable for statistical time series analysis at this time.


Analysis of sediment chemistry in the Credit River Watershed suggests that contaminated sediment is found in many regions of the watershed with over 80% of monitoring stations classified as marginal or poor in 2011. Although contaminated sediment is a concern, sediment chemistry alone does not provide an indication of the bioavailability of the contaminants and consequently the potential effects on aquatic organisms. Additional analyses such as benthic community composition or concentrations of contaminants in fish are often needed to properly investigate any ecological or human health risk posed by contaminated sediment. Nevertheless, monitoring sediment chemistry provides a picture of where contaminants are accumulating in the watershed.

In the next chapter we will examine what types of bugs live at the bottom of the Credit River and its tributaries. Many types of bugs are sensitive to pollution and by looking at the types of bugs that live in streams you can gain an understanding of the health of the ecosystem. Are you curious to know what the bugs are telling you about the health of the Credit River Watershed? Be sure to read the next chapter of the Watershed Health Report.

Did you know?

Pollution sensitive benthic macroinvertebrates are increasing in the Credit River Watershed, suggesting that water quality is improving.



Environment Canada

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

MOE (Ontario Ministry of the Environment). 1993a. Guidelines for the Protection and Management of aquatic sediment quality in Ontario.

MOE (Ontario Ministry of the Environment). 2008. Guidelines for Identifying, Assessing and Managing Contaminated Sediments in Ontario: An Integrated Approach.


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