The unseen contamination
Beneath the city of Livorno, an invisible environmental problem is unfolding. The coastal aquifer system, a source of freshwater stored in subterranean layers of sand and gravel, is being progressively contaminated by saltwater from the Mediterranean Sea. This process, known as seawater intrusion, began accelerating after the Second World War. The primary cause was the increased demand for freshwater needed for urban expansion and industrial development. This led to excessive pumping (overpumping) of groundwater from the aquifer.
The hydrogeology of the Livornese coastal plain is complex, consisting of a multi-layered aquifer system formed from Pleistocene and Holocene alluvial and marine sediments. These deposits of sand and gravel have high permeability, allowing water to flow through them easily. Under natural conditions, the higher pressure of the freshwater in the aquifer keeps the denser saltwater from the sea at bay, forming a stable interface. However, heavy extraction of freshwater lowers the water table and reduces this pressure. This allows the saltwater to advance inland, creating a wedge-shaped intrusion that contaminates wells. Since the 1950s, this saltwater wedge has pushed up to 3 kilometers inland, rendering many wells unusable for drinking or agriculture.
Tracking the underground front
Monitoring this subterranean saltwater front is a continuous effort. Scientists and water management authorities track the intrusion primarily by measuring chloride concentrations in a network of monitoring wells. Chloride is a reliable, non-reactive tracer for seawater. While freshwater typically has chloride levels below 200 mg/L, seawater contains about 19,000 mg/L. Concentrations above 250 mg/L are generally considered undrinkable. Regular sampling from wells at various depths and distances from the coast allows for the three-dimensional mapping of the saltwater interface and its movement over time.
The data shows a direct correlation between periods of high water demand and the advancement of the saltwater front. The problem is not unique to Livorno; many coastal aquifers in Tuscany and around the Mediterranean are experiencing similar issues due to overexploitation. In some nearby areas, over-pumping has lowered the water table to sea level as far as 7 kilometers from the coast, worsening the intrusion. The geological structure of the Livorno area, part of the Northern Apennines orogenic system, includes formations of sandstone and limestone that influence how and where the saltwater moves underground. Efforts to manage the problem include seeking alternative water sources and implementing managed aquifer recharge (MAR) projects, where excess river water is used to replenish the aquifer and push the saltwater back toward the sea.