Model Answer
0 min readIntroduction
Saline water intrusion, a growing environmental concern, refers to the movement of saline water into freshwater aquifers. This contamination renders the water unsuitable for drinking and irrigation, impacting coastal communities and agricultural lands globally. The problem is exacerbated by factors like over-extraction of groundwater, sea-level rise, and inadequate aquifer recharge. Understanding the sources of this intrusion and the principles governing it, such as the Ghyben-Herzberg relation, is crucial for effective water resource management. This answer will detail the sources of saline water intrusion and explain the Ghyben-Herzberg principle.
Sources of Saline Water Intrusion
Several factors contribute to saline water intrusion in aquifers. These can be broadly categorized as natural and anthropogenic:
- Natural Sources:
- Density Differences: The fundamental driver is the density contrast between freshwater and saltwater. Saltwater, being denser, naturally tends to intrude into freshwater aquifers.
- Geological Structures: Permeable geological formations, such as fractured rocks or highly porous sediments, can facilitate saltwater movement. Faults and fissures can act as conduits.
- Tidal Influence: Tidal fluctuations exert pressure on coastal aquifers, pushing saltwater inland, especially during high tides.
- Upwelling: In certain coastal areas, upwelling of deep saline groundwater can contribute to aquifer contamination.
- Anthropogenic Sources:
- Groundwater Over-extraction: Excessive pumping of groundwater reduces freshwater hydraulic head, allowing saltwater to encroach. This is the most significant anthropogenic factor.
- Canal Construction: Canals connecting inland water bodies to the sea can provide pathways for saltwater intrusion.
- Land Reclamation: Altering coastal landscapes through land reclamation can disrupt natural groundwater flow patterns and exacerbate intrusion.
- Agricultural Practices: Intensive irrigation with saline water can contribute to the build-up of salt in the vadose zone, eventually leading to aquifer contamination.
- Climate Change: Sea-level rise, a consequence of climate change, increases the hydraulic head of saltwater, enhancing its intrusion potential.
Ghyben-Herzberg Relation
The Ghyben-Herzberg relation, proposed independently by Wilhelm Ghyben in 1888 and Alexander Herzberg in 1901, describes the theoretical relationship between the freshwater head above sea level and the depth of saltwater intrusion in a coastal aquifer. It’s based on the principle of hydrostatic equilibrium.
The relation states that for every 1 meter of freshwater head above sea level, approximately 40 meters of saltwater intrusion can occur. This ratio is derived from the density difference between freshwater (approximately 1000 kg/m³) and saltwater (approximately 1025 kg/m³). The ratio of densities is approximately 1.025, leading to the 1:40 ratio.
Mathematically, the relation can be expressed as:
h = 40z
Where:
- h = freshwater head above sea level (in meters)
- z = depth of saltwater intrusion (in meters)
Limitations: The Ghyben-Herzberg relation is a simplified model and assumes several ideal conditions, including a homogeneous and isotropic aquifer, a sharp interface between freshwater and saltwater, and steady-state flow. In reality, these conditions are rarely met. Factors like aquifer heterogeneity, dispersion, and dynamic recharge can significantly influence the actual depth of saltwater intrusion. Nevertheless, it provides a useful first-order approximation for understanding the relationship between freshwater head and saltwater intrusion.
Practical Application: This relation is used to estimate the required freshwater recharge to prevent or mitigate saltwater intrusion. By maintaining a sufficient freshwater head, the intrusion can be controlled. It also helps in designing well placement and pumping strategies to minimize saltwater contamination.
Conclusion
Saline water intrusion is a complex problem with both natural and human-induced causes. Understanding these sources and applying principles like the Ghyben-Herzberg relation are vital for effective coastal aquifer management. Sustainable groundwater management practices, including regulated pumping, artificial recharge, and minimizing saltwater pathways, are crucial to protect freshwater resources in vulnerable coastal regions. Addressing climate change and its impact on sea levels is also paramount in mitigating this growing threat.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.