Monitoring & Assessing Water Quality (DP IB Environmental Systems & Societies (ESS))
Revision Note
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:
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
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
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
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
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
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
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
Abiotic factor | How abiotic factor is measured |
|---|---|
Dissolved oxygen (DO) | Measured using an oxygen meter equipped with a probe |
pH | pH levels are determined using a pH meter equipped with a probe |
Temperature | Water temperature is assessed using a digital thermometer or a temperature probe |
Nitrate and phosphate concentrations | Measured using test kits, specific to each nutrient These kits use colorimetric tests where the water sample reacts with chemicals, producing a colour change corresponding to the concentration level of the nutrient |
Total suspended solids (TSS) | Measured by filtering a known volume of water through a pre-weighed filter paper, then drying and weighing the paper again The difference in weight represents the mass of TSS collected This can then be converted to a concentration |
Turbidity | Measured using a Secchi disc—a black and white disc lowered into the water The depth at which the disc disappears from sight is recorded This indicates light penetration and turbidity in the water column |
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
Exam Tip
Turbidity and total suspended solids (TSS) are closely related but are not exactly the same thing.
Turbidity focuses on the effect—it measures how light scatters and absorbs due to suspended particles, making the water cloudy. It doesn't tell you the exact amount or type of these particles, just their presence and impact on light.
TSS focuses on the cause—it measures the actual mass of all the suspended particles in a water sample and is given as a concentration e.g. milligrams per litre (mg/L) or parts per million (ppm).
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
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