Definition
Water quality
Water quality is a measure of the chemical, physical, and biological characteristics of water, determining its suitability for human consumption, agriculture, industry, and aquatic ecosystems.
- Water quality refers to the chemical, physical, and biological characteristics of water that determine its suitability for supporting life and human use.
- Monitoring these parameters helps identify pollution sources, track ecosystem health, and guide effective management and restoration strategies.
Methods of Assessing Water Quality
Direct (Chemical and Physical) Measurements
1. Dissolved Oxygen (DO)
- Measures how much oxygen is dissolved in water,essential for aquatic life.
- Low DO indicates organic pollution, eutrophication, or thermal pollution.
- Measured using oxygen probes or the Winkler titration method.
Tip
- 75% saturation = healthy water
- <10% = severe pollution (e.g., sewage contamination)
Note
A river with turbulent water (like near waterfalls) usually has high DO, while stagnant or polluted areas have low DO.
2. pH
- Indicates the acidity or alkalinity of water.
- Normal freshwater ranges from 6 to 8.
- Deviations suggest acid rain, industrial effluent, or alkaline runoff.
- Measured using pH probes, litmus paper, or pH meters.
Example
- A stream near a coal plant may show pH 4–5 due to acid mine drainage, which releases sulfuric acid into waterways.
- pH < 5.5 can cause fish eggs to fail and mobilize aluminum, which is toxic to gills.
3. Temperature
- Affects dissolved oxygen, metabolic rates, and species distribution.
- Thermal pollution from power plants or industries can reduce oxygen solubility.
- Measured using a digital thermometer or a temperature probe.
Note
When the temperature rises, oxygen solubility falls.
4. Turbidity
Definition
Turbidity
Turbidity measures water transparency based on suspended particles (sediments, plankton, pollutants).
- Measures the clarity of water, affected by sediments, organic matter, and algae.
- High turbidity reduces sunlight penetration and affects photosynthesis.
- Measured using a Secchi disc (depth visibility test) or a turbidity tube.
Note
A Secchi disk visibility of less than 0.5 m indicates heavy sedimentation or algal bloom.
Example
After heavy rainfall, river turbidity rises due to runoff and soil erosion, impacting aquatic plants.
5. Nutrient Concentrations (Nitrates and Phosphates)
- High concentrations indicate agricultural runoff or sewage discharge.
- Promote eutrophication, leading to algal blooms and oxygen depletion.
- Measured using colorimetric test kits or test strips.
Note
Typical Ranges:
- Nitrate <5 mg/dm³ = clean
- Nitrate >15 mg/dm³ = polluted
- Phosphate <5 mg/dm³ = clean
- Phosphate >15 mg/dm³ = polluted
Example
The Baltic Sea suffers recurrent algal blooms due to excessive nutrient discharge from nearby farmland.
6. Ammonia
- Indicates sewage pollution or organic decomposition.
- Levels above 1 mg/dm³ are toxic to fish.
- Measured using test kits with color-change reagents.
7. Heavy Metals and Salinity
- Metals such as lead, mercury, and arsenic enter from industrial discharge or mining.
- Salinity changes may indicate seawater intrusion or evaporation in closed systems.
- Measured using electronic sensors or chemical test kits.
Example
Lead contamination in Flint, Michigan caused widespread health crises due to corroded pipes.
Indirect Measurement of Water Quality (Biological Indicators)
- Assess organisms living in the water as indicators of quality.
- Sensitive species (mayfly larvae, stonefly nymphs) indicate clean water.
- Pollution-tolerant species (Tubifex worms, bloodworms) suggest organic pollution.
- Biological surveys give insight into long-term ecosystem health.
Tip
- Mayfly nymphs and stonefly larvae indicate clean, oxygen-rich water.
- Their absence signals deteriorating conditions.
Analogy
- Think of biological indicators like “natural sensors”.
- If sensitive species vanish, pollution is likely increasing.
Water Quality Index (WQI)
- Combines several physical, chemical, and biological parameters into a single value to indicate the overall health of a water body.
- Scaled from 0–100 (100 = excellent quality).
- Used to communicate results easily to the public and policymakers.
Example
- A WQI of 85–100 = Excellent (safe for drinking and recreation)
- WQI below 50 = Poor (unsuitable for aquatic life and human use)
Biochemical Oxygen Demand (BOD) as an Indicator of Water Quality
Definition
Biochemical Oxygen Demand (BOD)
Biochemical Oxygen Demand (BOD) is a key water quality parameter that measures the amount of dissolved oxygen required by microorganisms to break down organic material in water.
- Biochemical Oxygen Demand (BOD) measures the amount of oxygen required by aerobic microorganisms to decompose organic matter in water.
- It is an indirect measure of organic pollution.
- The more organic matter, the higher the oxygen demand.
Note
- High BOD = High organic pollution → Low dissolved oxygen → Poor water quality.
- Low BOD = Clean water with minimal organic matter.
How BOD is Measured
- Collect a water sample and measure its initial dissolved oxygen (DO₁).
- Store the sample in the dark at 20°C for 5 days to prevent photosynthesis.
- Measure the final dissolved oxygen (DO₂).
- BOD = DO₁ − DO₂ (mg O₂ per litre).
Example
If a sample starts at 8 mg/L of DO and drops to 2 mg/L after 5 days,
BOD = 6 mg/L, indicating high organic load.
Interpreting BOD Values
| BOD (mg/L) | Interpretation |
|---|---|
| 1-3 | Clean water, high oxygen levels |
| 3-6 | Moderate pollution, some organic matter |
| 6-10 | Poor quality, high organic load |
| >10 | Heavily polluted, low oxygen, possible anaerobic conditions |
Factors Affecting BOD
- Temperature: Higher temperature increases microbial activity and oxygen use.
- Organic Matter: More waste = more oxygen demand.
- Flow Rate: Stagnant water has higher BOD due to reduced aeration.
- Microbial Population: More decomposers = higher oxygen use.
Relation Between BOD and Aquatic Life
- High BOD leads to low dissolved oxygen, creating hypoxic conditions.
- Sensitive species (like trout) die first, while tolerant species (like worms) survive.
- Long-term high BOD can shift ecosystem balance toward pollution-tolerant organisms.
Example
- Ganges River, India: Sections near cities have BOD levels above 10 mg/L, indicating high organic pollution from sewage discharge and industrial waste.
- Thames River, UK: Historically, the Thames had high BOD levels due to untreated sewage, but wastewater treatment improvements have reduced BOD to less than 3 mg/L in most areas.
Self review
- Explain why dissolved oxygen levels are used as an indicator of water quality.
- Describe how high nitrate and phosphate concentrations affect aquatic ecosystems.
- How does temperature influence dissolved oxygen levels in water?
- Outline the steps used to measure biochemical oxygen demand (BOD).
- How can the Water Quality Index (WQI) simplify the communication of environmental data?
- How can high BOD levels impact aquatic life and human water use?
