7 - Water Properties and Water Quality Parameters

Water (H2O) consists of two hydrogen atoms and one oxygen atom, making it a simple molecule. The arrangement of these atoms results in polar covalent bonds, contributing to water's unique properties, including its high surface tension and ability to dissolve many substances.

Water is often referred to as the "universal solvent" because it can dissolve more substances than any other liquid due to its polarity. This property enables it to interact with various ions and small molecules effectively.
Example interactions: Sodium ions (Na+) and Chloride ions (Cl-) readily dissociate in water, highlighting the solvent properties that are critical for biochemical processes.

Water exhibits amphoteric behavior, meaning it can act as both an acid and a base depending on the surrounding conditions.
Dissociation: The equilibrium can be represented by the reaction (H2O ↔ H+ + OH-).
As an acid, water releases hydrogen ions (H+), and as a base, it can absorb H+ ions and produce hydroxide ions (OH-). This feature is vital for maintaining pH balance, especially in aquatic systems where changes in pH can significantly impact water quality and aquatic life. Acid rain, caused by sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions, illustrates how human activities can lower pH levels in natural water bodies.

One of water's most intriguing properties is that ice is less dense than liquid water. This characteristic allows ice to float on liquid water, creating an insulating layer that protects aquatic ecosystems during freezing conditions.
The crystalline structure of ice, formed by hydrogen bonding when water freezes, expands, contrasting with the denser liquid state that is crucial for supporting life in bodies of water.

Cohesion and Adhesion:
- Cohesion: Water molecules exhibit strong cohesive forces due to hydrogen bonding, resulting in surface tension, which allows small organisms to walk on water.
- Adhesion: Water molecules can stick to various surfaces, enhancing its ability to wet and interact with soils and biological tissues, making it a crucial factor in capillary action in plants.
Thermal Properties:
- Water’s high heat capacity allows it to absorb and store large amounts of heat, stabilizing temperatures in organisms and environments.
- The high heat of vaporization of water provides a cooling effect on the surrounding environment, as organisms can release heat through processes like sweating, which helps to regulate body temperature.
- Water’s relatively high boiling point prevents spontaneous vaporization, which is essential for organisms that rely on liquid water for survival.
Transparency: Water's ability to allow light penetration is essential for photosynthesis in aquatic plants and supports aquatic ecosystems.

The composition of global water resources is predominantly saltwater, encompassing approximately 97.5% of the total water available on Earth. Freshwater, which makes up only 2.5%, is distributed as follows:
- 68.9% in glaciers and permanent ice.
- 30.8% in groundwater, which serves as a crucial source for drinking water and irrigation.
- 0.3% in lakes, rivers, and reservoirs, which are crucial for ecosystems and human use.

A mere 0.01% of total water is readily accessible for human consumption and agriculture, emphasizing the importance of water conservation and management efforts in addressing global water scarcity.

Water plays a vital role in ecological systems, contributing to climate moderation, nutrient cycling, and waste treatment and dilution, which helps maintain ecosystem health.
It also provides habitat for a diverse array of species, supporting biodiversity and genetic resources vital for ecological resilience.

A watershed is defined as an area of land that drains all streams and rainfall to a common outlet, which can be lakes, rivers, or oceanic bodies. Effective management of a watershed includes understanding its various components, such as forests, streams, farmlands, urban areas, and wetland systems.

Water residence time varies significantly across different stages of the hydrologic cycle, critical for understanding how long water stays in various reservoirs:
- Atmospheric water: averages about 1.5 weeks.
- River channels: approximately 2 weeks.
- Lakes and reservoirs: around 10 years.
- Oceans and seas: can retain water for up to 4000 years, reflecting the importance of ocean systems in the global water cycle.

The introduction of excessive biodegradable matter into aquatic ecosystems can lead to a significant boost in bacterial growth, which consumes available oxygen.
This scenario often results in algal blooms due to an overabundance of nutrients (particularly nitrogen and phosphorus), leading to oxygen depletion in the water. The consequence of these processes can shift aquatic environments from aerobic to anaerobic conditions, adversely affecting the health of aquatic organisms and overall ecosystem balance.