1. Sources and Types of Water
Water resources exist across surface, subterranean, and atmospheric reservoirs, playing vital roles in ecological balance and human utilization.
Types of Water
- Surface Water: Water found on the Earth's surface in rivers, streams, lakes, ponds, and oceans. It is directly exposed to atmospheric conditions and anthropogenic activities.
- Groundwater: Water located beneath the Earth's surface within soil pore spaces and fractures of rock formations (aquifers). It is recharged through infiltration.
- Atmospheric Water: Moisture stored in the atmosphere as water vapor, clouds, and precipitation.
2. The Hydrological Cycle
The hydrological cycle represents the continuous movement and recycling of water throughout the Earth's atmosphere, surface, and subsurface systems.
Key Hydrological Processes
- Evaporation: The phase conversion of liquid water from surfaces such as oceans, lakes, and soils into water vapor driven by solar energy.
- Evapotranspiration: The combined total of evaporation from land/water surfaces and transpiration from plant stomata into the atmosphere.
- Precipitation: Moisture falling from the atmosphere to the Earth's surface in liquid or solid forms, including rain, snow, sleet, and hail.
- Infiltration: The vertical downward movement of precipitation from the soil surface into the unsaturated soil zone and underlying groundwater aquifers.
- Runoff: The flow of water across the land surface into surface water bodies when precipitation exceeds the soil infiltration capacity.
3. Classification of Water Resources
Water resources are classified based on their salinity, flow dynamics, and geographical characteristics.
| Water Resource Category | Salinity and Characteristics | Ecological and Practical Role |
|---|---|---|
| Oceans | High salinity (average 35,000 mg/L TDS); covers approximately 71% of Earth's surface. | Drives global climate regulation, major oceanic biological life support. |
| Rivers | Lotic (flowing freshwater) systems with high movement and dissolved oxygen. | Primary channel for transport, irrigation, and public water supply. |
| Lakes | Lentic (standing freshwater or saline) inland water bodies occupying terrain depressions. | Water storage, recreational use, localized aquatic habitats. |
| Wetlands | Land areas permanently or seasonally saturated with water (swamps, marshes, bogs). | Natural water filtration, flood control, high biological diversity support. |
4. Physical Properties of Water
Physical parameters describe the physical state, particle presence, and sensory properties of water samples.
Key Physical Parameters
- Temperature: Determines gas solubility, biological activity rates, and chemical reaction rates in water bodies. Higher temperatures reduce gas solubility.
- Colour: Resulting from dissolved organic matter (such as humic substances), plant pigments, or metallic ions like iron and manganese.
- Odour: Produced by biological decomposition, volatile organic compounds, or dissolved gases such as hydrogen sulfide (H2S).
- Total Dissolved Solids (TDS): Organic and inorganic substances smaller than 2.0 µm dissolved in water, primarily cations (Ca2+, Mg2+, Na+) and anions (HCO3-, Cl-, SO42-).
- Total Suspended Solids (TSS): Organic and inorganic particles larger than 2.0 µm suspended in water that do not settle rapidly, contributing to turbidity.
Comparison of TDS and TSS
| Parameter | Total Dissolved Solids (TDS) | Total Suspended Solids (TSS) |
|---|---|---|
| Particle Size | Finer than 2.0 µm (passes through filter) | Coarser than 2.0 µm (retained on filter) |
| Measurement Method | Evaporation of filtered sample / Electrical Conductivity measurement | Filtration followed by drying and gravimetric weighing |
| Environmental Effect | Affects osmotic balance, taste, and ionic strength | Restricts light penetration, clogs fish gills, increases turbidity |
5. Chemical Properties of Water
Chemical parameters indicate solute concentration, buffering capability, oxygen demand, and chemical safety of water systems.
Major Inorganic and Organic Constituents
- Inorganic Constituents: Includes major cations (Ca2+, Mg2+, Na+, K+) and anions (HCO3-, SO42-, Cl-, NO3-) contributing to total salinity, hardness, and buffering capacity.
- Organic Constituents: Comprises natural organic matter (humic and fulvic acids) and synthetic organic matter (pesticides, industrial solvents) that consume oxygen during decay.
Dissolved Gases
Gases dissolve in water in accordance with Henry's Law. Dissolved gases include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and hydrogen sulfide (H2S).
Dissolved Oxygen (DO), BOD, and COD
Dissolved Oxygen (DO): The concentration of free, non-compound oxygen (O2) dissolved in water, essential for the survival of aerobic aquatic life. Healthy aquatic systems maintain DO concentrations above 4.0 mg/L to 6.0 mg/L.
- Biochemical Oxygen Demand (BOD): The quantity of dissolved oxygen needed by aerobic microorganisms to biologically break down organic matter in a water sample, typically measured over 5 days at 20°C (BOD5).
- Chemical Oxygen Demand (COD): The amount of oxygen consumed to chemically oxidize both biodegradable and non-biodegradable organic compounds using a strong chemical oxidant (such as potassium dichromate, K2Cr2O7) under acidic conditions.
Comparison: BOD vs. COD
| Parameter | Biochemical Oxygen Demand (BOD) | Chemical Oxygen Demand (COD) |
|---|---|---|
| Oxidation Mechanism | Biological oxidation by microorganisms | Chemical oxidation using strong chemical reagents |
| Compounds Measured | Biodegradable organic matter only | Biodegradable + non-biodegradable organic matter |
| Test Duration | 5 days (standard BOD5) | 2 to 3 hours |
| Magnitude | Always lower than COD (BOD < COD) | Always higher than BOD (COD > BOD) |
Acidity, Alkalinity, and Electrical Conductivity
- Acidity: The quantitative capacity of water to react with a strong base to a designated pH, caused by dissolved carbonic acid (H2CO3), free mineral acids, or hydrolyzing metallic salts (Fe3+, Al3+).
- Alkalinity: The measure of water's capacity to neutralize acids, primarily driven by bicarbonate (HCO3-), carbonate (CO32-), and hydroxide (OH-) ions.
- Electrical Conductivity (EC): The ability of water to pass an electrical current, directly proportional to the total concentration of dissolved ionic species.
EC ≈ TDS × 0.65 (where EC is in µS/cm and TDS is in mg/L)
Exam Notes and Common Pitfalls
- Common Mistake: Assuming BOD can be greater than COD. Fact: COD is always greater than or equal to BOD because chemical oxidants break down non-biodegradable compounds that microbes cannot digest.
- Important Observation: An increase in water temperature decreases the solubility of Dissolved Oxygen (DO) while simultaneously increasing the metabolic rate of aquatic organisms, creating elevated biological stress.