Monitoring & Assessing Water Quality (DP IB Environmental Systems & Societies (ESS))

Monitoring & Assessing Water Quality

• Water quality is the measurement of chemical, physical and biological characteristics of water

  • Chemical characteristics include: levels of dissolved substances like minerals, pollutants and nutrients

  • Physical characteristics include water clarity, temperature and turbidity (cloudiness)

  • Biological characteristics include the presence of microorganisms (e.g. bacteria) and invasive species

• Water quality is highly variable and is often measured using a water quality index (WQI)

  • Scientists use various tests to measure different water quality parameters

  • A water quality index is then calculated

  • This combines multiple measurements into a single value or score

  • This provides an assessment of the overall water quality of a particular water body

  • This index helps in easily communicating the quality of the water body to the public and policymakers

    • E.g. indicating whether water quality is good, acceptable, or poor for various uses such as drinking, recreation and aquatic organisms

  • A high WQI indicates good water quality

Water quality parameters

• Some of the different water quality parameters that can be used are:

1. Dissolved oxygen (DO)

   • Measures the amount of oxygen dissolved in water

   • Sufficient oxygen levels are important for the survival of aquatic organisms

   • Low dissolved oxygen can lead to hypoxia

     • This can suffocate or kill aquatic life

2. pH

   • Measures the acidity or alkalinity of water

   • pH impacts the survival, growth and reproduction of aquatic organisms

   • Unusual pH levels can indicate pollution, acidification, or other environmental changes

3. Temperature

   • Measures the degree of heat or coldness of water

   • Temperature affects the metabolic rates, behaviour and distribution of aquatic organisms

   • Abnormal temperature fluctuations can stress or kill aquatic life

4. Nitrates and phosphates

   • Measuring nitrates and phosphates assesses nutrient pollution in water

   • High nutrient levels can lead to eutrophication

   • Monitoring nutrient concentrations helps manage nutrient inputs and prevent water quality degradation

5. Metals

   • Testing for metals, such as mercury, lead, cadmium, or arsenic assesses contamination levels

   • Metals can accumulate in aquatic organisms

     • This poses risks to their health and the health of organisms in higher trophic levels

   • Monitoring metal concentrations helps identify pollution sources and evaluate potential ecological impacts

6. Total suspended solids (TSS)

   • TSS is the concentration of solid particles suspended in water

   • High levels of TSS can decrease water quality by blocking sunlight

     • This reduces photosynthesis in aquatic plants and disrupts aquatic food chains

   • Suspended solids can also smother the gills or breathing apparatus of aquatic invertebrates and fish

   • High TSS can be a sign of erosion, wastewater discharge, or runoff from urban and agricultural areas, leading to habitat degradation

7. Turbidity

   • Turbidity measures the clarity or cloudiness of water

     • This is affected by suspended particles

   • High turbidity can reduce light penetration

     • This reduces photosynthesis in aquatic plants and visibility for predators and prey

   • High turbidity can indicate soil erosion or urban, agricultural or industrial run-off

Measuring key abiotic factors in aquatic systems

• These parameters provide valuable information about the health and condition of aquatic ecosystems

• It is crucial to compare readings from various locations, such as upstream and downstream of a sewage outlet or factory, to assess any potential impacts on the ecosystem

• Monitoring and analysing these parameters at regular intervals helps scientists, environmental agencies and policymakers to:

  • Understand the overall water quality

  • Identify potential issues

  • Implement appropriate management strategies to protect and restore aquatic ecosystems

Biochemical Oxygen Demand

• Biochemical oxygen demand (BOD) is a measure of the amount of dissolved oxygen required to break down the organic material in a given volume of water through aerobic biological activity

• Aerobic organisms rely on oxygen for respiration

• When there is a higher abundance of organisms or an increased rate of respiration, more oxygen is consumed

• This means that the biochemical oxygen demand (BOD) is influenced by:

  • The quantity of aerobic organisms present in the water

  • The rate at which these organisms respire

• BOD can be used as an indirect measure to evaluate:

  • The amount of organic matter within a sample

  • The pollution levels in water

    • The introduction of organic pollutants, such as sewage, leads to an increase in the population of organisms that feed on and break down the pollutants

    • This, in turn, results in increased BOD values

    • Certain species, such as bloodworms and Tubifex worms, show tolerance to organic pollution and the associated low oxygen levels

    • On the other hand, mayfly nymphs and stonefly larvae are typically only found in clean-water environments

Example of how BOD is used to indirectly measure the amount of organic matter within a sample

• Higher BOD values indicate a larger amount of organic matter present in the water sample

  • This is because more oxygen is needed for its decomposition

• By measuring the decrease in dissolved oxygen levels over a specific incubation period, BOD provides an estimate of the organic load or pollution level in the water

• BOD values are typically expressed in milligrams of oxygen consumed per litre of water (mg/L) or as a percentage of the initial dissolved oxygen level

• The BOD test involves:

  • Collecting a water sample in a closed container

  • Measuring the initial dissolved oxygen concentration

  • Re-measuring the dissolved oxygen concentration after a specific incubation period (usually 5 days) at a constant temperature (usually 20℃)

• For example:

  • A water sample has an initial dissolved oxygen concentration of 8 mg/L

  • After 5 days, the dissolved oxygen concentration decreases to 2 mg/L

  • The BOD value would be calculated as 8 mg/L - 2 mg/L = 6 mg/L

    • As the dissolved oxygen levels have decreased substantially, this indicates that the sample has a relatively high organic load