When seawater moves underground: coastal aquifer salinisation threatens freshwater supplies
Published on 1 October 2026 at 01:57 GMT
By Editorial Team SDG6
Coastal aquifer salinisation can leave a community surrounded by water yet short of usable freshwater. As seawater enters underground reserves, drinking-water wells can become unsuitable for supply and irrigation can carry damaging salts into farmland. Rising seas add to pressures from drought and groundwater extraction, affecting reserves that coastal settlements draw on when surface supplies run low.
Aquifers hold water in the pores and fractures of rock and sediment. Near a coast, freshwater flowing towards the sea meets denser saltwater. The boundary is a mixing zone rather than an impermeable wall. As the U.S. Geological Survey explains, pumping can weaken the seaward flow and draw saltwater inland. Saline water can also move upwards towards a pumping well from deeper layers, a process known as upconing.
Three pressures on the same reserve
Sea-level rise changes the balance between the sea and groundwater, while inundation can introduce salt from above. Drought can reduce replenishment from rainfall and increase reliance on wells when rivers and reservoirs provide less water. Where withdrawals lower freshwater levels, these pressures can combine. Their relative importance varies with geology, recharge, the position of wells and the local relationship between groundwater levels and the sea.
The 2022 United Nations World Water Development Report identifies intensively pumped coastal aquifers, including those associated with Jakarta and Tripoli, as settings where extraction can amplify saline intrusion beyond the effect of sea-level rise alone. This is particularly consequential for expanding cities that depend on groundwater: growing demand can increase pressure on the same reserve used to bridge shortages. Where additional demand is met by wells, urban growth changes the balance between extraction and replenishment.
Salinisation is not proof that modern seawater has just arrived. The U.S. Geological Survey describes several routes, including leakage from saline canals and movement between aquifers. Establishing where the salt comes from changes the interpretation of a deteriorating well. A coastline on a map does not show the depth, pathways or extent of saline groundwater beneath it.
Drinking water and farmland
For a water supplier, salinity can reduce the usable output of an otherwise productive well. Severe intrusion can force abandonment. The distinction between water quantity and water quality matters: an aquifer may still contain abundant water while offering less freshwater for households. In regions with limited alternative supplies, losing a well also reduces flexibility during the next dry period. That connects the issue directly to SDG 6 (Clean Water and Sanitation), through the reliability and quality of drinking-water supplies.
Farming faces a related but different problem. Guidance from the Food and Agriculture Organization of the United Nations explains that repeated irrigation adds salts to soil. Crops take up water while much of the salt remains, making it harder for roots to extract moisture. Yields can fall even where soil is not visibly dry. The outcome depends on the crop, irrigation-water quality, salt accumulation and drainage.
Salt-tolerant crops and leaching can help manage some conditions, according to that guidance. Leaching carries salts below the root zone but uses additional water and depends on drainage. In a water-scarce coastal district, this creates a practical tension: maintaining production with poorer-quality water can require more water to control soil salinity. Adapting cultivation does not itself remove saltwater from the aquifer supplying the farm.
Detecting movement before wells are lost
Monitoring connects underground change to operational decisions. Measurements of groundwater levels, chloride and electrical conductivity, an indicator of dissolved salts, help track changes over time. Wells at different locations and depths, combined with geophysical surveys, can reveal movement that a single supply-well sample misses. The U.S. Geological Survey describes monitoring networks as early-warning systems, while groundwater models help test possible pumping and recharge scenarios.
Reducing coastal pumping or moving withdrawals inland can allow freshwater levels to recover and restrain intrusion. Neither measure guarantees immediate restoration. The U.S. Geological Survey notes that flushing contaminated groundwater can take years or decades, although this is not a recovery timetable for every saline aquifer. Recovery depends on aquifer conditions and continuing stresses. Desalination or blending can make some saline water usable, but treating extracted water is distinct from restoring the freshwater reserve underground.
Recharge in practice
Managed aquifer recharge deliberately replenishes underground storage through infiltration or injection. Its potential is visible in California, where the Orange County Water District operates the Talbert and Alamitos seawater intrusion barriers. The district combines recharge with groundwater monitoring and uses several water sources, including stormwater, river water, imported water and recycled water. Its percolation basins allow water to enter the aquifer through the ground.
The district reports a network of more than 400 monitoring wells. This example illustrates the infrastructure behind aquifer protection: water collection, recharge facilities, sampling and continuing management work together. It also shows why a recharge scheme is more than an injection well. Its operation depends on available replenishment water and knowledge of the basin receiving it.
For coastal authorities, management choices depend on local pumping rates, the availability of replenishment water and the costs of changing supply arrangements. Recharge and withdrawals both affect groundwater levels and the movement of saline water. The U.S. Geological Survey describes monitoring as a basis for assessing those changes and managing supplies before saltwater reaches freshwater wells.
Written by a human author, edited with AI assistance.
Further information:
U.S. Geological Survey, “Saltwater Intrusion”, explains pumping, freshwater–saltwater mixing and pathways into coastal wells.
U.S. Geological Survey, “Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast”, documents monitoring, pumping controls, recharge, treatment and slow groundwater flushing.
UNESCO, “The United Nations World Water Development Report 2022: Groundwater: Making the invisible visible”, provides context on coastal pumping, climate pressures and groundwater dependence.
Food and Agriculture Organization of the United Nations, “Water quality for agriculture”, chapter 2, explains crop water stress, salt accumulation and leaching.
Orange County Water District, “Groundwater management”, describes recharge sources, monitoring wells and seawater intrusion barriers.
