Earth’s Hydrosphere: Composition, Processes, and Contamination
Life on Earth directly depends on water. Civilizations have disappeared, and many ecosystems have been affected by changes in water quantity and quality.
The ocean represents 97.2% of the hydrosphere, while ice in the polar caps accounts for 2.1%. Groundwater and water in lakes and rivers account for 0.6%. The remaining 0.1% is saline (potable) water found in lakes or groundwater.
The Hydrosphere
The hydrosphere is in close contact with the Earth’s lithosphere; therefore, their properties are related. Many lithosphere compounds dissolve in water and become part of the hydrosphere.
The Hydrologic Cycle
Ocean water evaporates and is transported by winds to land, where it falls as rain or snow. This infiltrates the soil, replenishes groundwater, and flows back to the ocean. The amount of water exchanged is measured in billions of liters per day.
Chemical Reactions in the Hydrosphere
Compounds and ions in natural waters originate from the atmosphere, lithosphere, and biosphere. Chemical reactions at the lake bottom (in contact with sediment) differ from those at the surface (in contact with the atmosphere).
Aerobic Biodegradation
Much of the dissolved oxygen is consumed during aerobic biodegradation or respiration by aquatic life. While the oxygen solubility in water is approximately 8.3 milligrams per liter, natural waters generally contain lower concentrations because atmospheric oxygen incorporation is slow in slow, non-turbulent water.
Anaerobic Biodegradation
Sewage and waste from meat, food, or paper processing consume oxygen. When large amounts are discharged into natural waters, carbon dioxide increases while oxygen decreases. This allows anaerobic bacteria to thrive, causing changes in water properties, such as acidity.
Hydrosphere Contaminants
Twentieth-century agricultural and industrial growth has contaminated the hydrosphere with various pollutants. These can be grouped as:
Inorganic Contaminants
Main metallic contaminants include cadmium, lead, and mercury, usually in the form of cations from mining and battery production.
Organic Contaminants
Pesticides and herbicides can cause serious problems in animals and humans, even at low concentrations. Polychlorinated biphenyls (PCBs) are another example. PCBs have low water solubility but are highly soluble in nonpolar substances, allowing them to spread globally via air and water.
Summary
- Saltwater: 97.2%
- Ice caps: 2.1%
- Freshwater: 0.1%
Understanding the hydrosphere’s composition, processes, and contamination is crucial for environmental protection and sustainable resource management.