A Miocene inheritance
The entire Jaffna Peninsula sits atop a thick layer of fossil-rich limestone, the sole source of fresh water for its population of over 600,000 people. This geological formation, known as the Jaffna Limestone, was deposited between 15.5 and 23.5 million years ago, during the late Oligocene to middle Miocene epochs. At that time, a shallow tropical sea covered the area, and the limestone is composed of the remains of countless marine organisms like corals, foraminifera, and mollusks. The formation records the Miocene Climatic Optimum, a period of global warmth when sea levels were significantly higher.
This is not a solid block of rock. Over millions of years, rainwater, which is naturally slightly acidic, has percolated through cracks and fissures, dissolving the calcium carbonate in the rock. This process has created a complex underground network of solution cavities and channels, a karstic system. This subterranean labyrinth, with a thickness exceeding 35 meters in some areas, now is a vast, natural sponge, storing the monsoon rains that are the only source of recharge for the region. The limestone itself has almost no inherent purification ability; once pollutants enter this cavernous system, they can spread rapidly throughout the aquifer.
Floating on salt
The hydrogeology of the Jaffna Peninsula is a delicate balancing act. The fresh water from rainfall, being less dense, does not simply fill the limestone cavities. Instead, it forms a convex layer, or lens, that floats on top of denser saline water that has seeped in from the surrounding Indian Ocean. This phenomenon is a Ghyben-Herzberg lens. The principle dictates that for every meter the freshwater table is above sea level, the freshwater lens extends approximately 40 meters below sea level.
The thickness of this important freshwater lens is greatest in the center of the peninsula and thins out towards the coasts. The entire system is dependent on the northeast monsoon, which brings the bulk of the region's 1,290 mm of average annual rainfall from October to January. Only about 30-32% of this rainfall actually recharges the aquifer due to runoff and high evaporation rates.
For centuries, the population has accessed this water via thousands of large, open-dug wells. Increasing demand for agriculture and domestic use has led to over-extraction. Pumping too much water lowers the freshwater table, causing the underlying saltwater to rise in a process called up-coning, leading to the contamination of wells. In some coastal villages, over 80% of wells have become too saline for human consumption, with salinity levels exceeding 900 parts per million. This saltwater intrusion, combined with nitrate pollution from fertilizers, threatens the long-term viability of this ancient water source.