Coastal Water Security: 3 Things to Know and 3 Must-do’s About Saltwater Intrusion

By Ziyang Cai 22 September, 2026

Saltwater intrusion not only causes coastal water insecurity during typhoons, but also poses long term risks – CWR's Cai demonstrates how this threat unfolds and how we can prepare for it

When saltwater intrudes deep into the river & passes the freshwater intake point, a city's freshwater supply may be cut for days
Saltwater intrusion can make groundwater saline, which is difficult to restore once contaminated; imperative to avoid falling into “groundwater depletion – land subsidence – saltwater intrusion” cycle
Saltwater can release heavy metals from pipes or soil into the water; actions based on understanding of accelerating risks needed

When we think of saltwater intrusion, we often picture seawater overflowing onto the land driven by storm surges from typhoons – a dramatic, visible threat. But saltwater intrusion is largely an insidious threat… a constant, creeping process, driven not only by typhoons but also by sea level rise, rivers running dry, depleted groundwater and sinking land. And since all these climate impacts are only accelerating, saltwater intrusion is here to stay.

Why worry? Because saltwater intrusion eats into our freshwater supply, which threatens the water security of coastal areas. AND it is not something that will happen in the distant future… it’s happening now. Read on to see how saltwater intrusion is quietly reshaping global water security – and check out what we can do to prepare…

1. Saltwater intrusion can impact drinking water supply

In 2022, a severe saltwater intrusion event struck Shanghai’s Qingcaosha Reservoir (青草沙水庫) – a critical water infrastructure serving 13 million people, or 52% of the city’s population. The freshwater intake was disrupted for over 90 days, with 43 consecutive days during which no fresh water could be drawn from the Yangtze River.

Drought, tide & SLR together caused a saltwater wedge, cutting off its freshwater supply

The cause was a severe drought in the Yangtze basin that greatly reduced upstream flows, weakening the river’s ability to push back seawater. This, compounded with astronomical tides and sea-level rise, allowed a saltwater wedge to advance far upstream, eventually surpassing the reservoir’s intake point and cutting off its freshwater supply.

This disruption to freshwater intake sounds serious, but those who live in Shanghai were barely aware because the Shanghai government immediately switched to the Jinze Reservoir & Chenxing Reservoir, both of which had been pre-filled with fresh water, enabled by early warnings from the monitoring system about the approaching saltwater intrusion. Without backup water sources and the interconnecting pipelines between reservoirs, this could have become a public crisis.

Saltwater intrusion is not limited to the Yangtze River Delta. It also occurs seasonally in several of China’s most economically vital regions, including the Yellow River Delta and the Pearl River Delta.

A classic case is Macao, located at the Pearl River estuary. Long plagued by saltwater intrusion, Macao has moved its water intake points upstream 3 times over the decades:

  1. from the early intakes in Wanzai (near the coast) to the Guangchang Pumping Station (built in 1988);
  2. then upstream by 21 km to the Pinggang Pumping Station (2006); and
  3. again further upstream by another 7.4 km to the Zhuzhoutou Pumping Station (2010).

But good adaptation plans & monitoring can ensure continuous freshwater supply

The cost of the relocations is high: the move from Guangchang to Pinggang alone cost CNY856mn. And as saltwater intrusion persists with fast rising seas, the Pearl River Water Resources Commission has established a three-tier “saltwater suppression with freshwater release” mechanism. By strategically releasing water from upstream reservoirs in a coordinated cascade with the help of its digital twin, this system can temporarily push the saltwater wedge downstream of the intakes, creating a critical window for freshwater extraction.

Over in New Orleans, drought conditions in 2023 reduced the flow of the Mississippi River causing saltwater intrusion which advanced approximately 70 miles upstream from the Gulf of Mexico. This prompted both state and federal governments to declare a state of emergency because if saltwater entered New Orleans’ old water piping systems, lead could be leached into the drinking water supply. In response, the U.S. Army Corps of Engineers had to raise an underwater sill on the riverbed (a structure they had built just a year before) to keep the saltwater out.

2. Saltwater intrusion is devastating coastal aquifers

Over-extraction of groundwater in coastal cities can accelerate saltwater intrusion. When groundwater is extracted at a faster rate than can be replenished, aquifer level drops and seawater creeps in. The lower the level, the faster saltwater intrudes. This problem is pervasive… NASA has warned that by 2100, SLR and reduced groundwater recharge will lead to saltwater intrusion in 77% of the world’s coastal areas.

Over-extraction = sinking land = higher SLR than elsewhere

What’s even worse is that when aquifers below shrink, the land above sinks. This situation is more evident in river deltas with soft soil. Indeed, research shows that for 18 of the 40 deltas (the Nile, Po, Vistula, Ceyhan, Rioni, Brahmani, Mahanadi, Ganges-Brahmaputra, Godavari, Chao Phraya, Mekong, Red River, Ciliwung, Brantas, Amazon, Parana, Pearl and Yellow River), the average rate of local land subsidence is greater than the rate of regional geocentric SLR. A vicious cycle forms because higher sea levels bring greater pressure causing saltwater to intrude at a faster rate.

…which causes a vicious cycle that is difficult to break

According to veteran hydrologist U Myint Thein, around 60% of Yangon’s groundwater is already saline yet 60% of Yangon residents rely on groundwater for their daily water needs. These numbers highlight Yangon’s dilemma. As shallow groundwater gets saltier and less usable, factories and wealthy households are tapping deeper aquifers that are not yet saline. This traps them in a vicious cycle of “groundwater depletion – land subsidence – saltwater intrusion”.

Other cities on river alluvial fans in Southeast Asia, such as Jakarta and Bangkok, are also suffering from different levels of saltwater intrusion due to severe land subsidence and excessive groundwater extraction. Tianjin has the same problem. Over-extraction of groundwater has resulted in saline aquifers with saline layers reaching up to 210m deep. Today, the area of saline groundwater in Tianjin exceeds 9,000km2, accounting for over 75% of the city’s total area.

This is a global issue, with Italy’s Po’s salinity disrupting farmland. The Po River drying up in recent years means that the sea is now seeping in causing farmland to become saline. This is bad as once seawater seeps into aquifers, it can only be flushed out with lots of freshwater.

We wrote about this in Rivers are Running Dry Today – that was back in 2022… fast forward to 2026… this year the Po’s river flow velocity dropped by more than half – from ~​1,000m3/s to below 300m3/s – too weak to push the salt water out.

3. Saltwater intrusion can cause heavy metal pollution in drinking water

Saltwater intrusion not only affects drinking water supply by disrupting freshwater intake, but it can also dislodge heavy metals from the soil/clay at the delta and pollute groundwater supplies.

Saline water can more easily wash out heavy metals

Studies targeting specific deltas has confirmed this phenomenon. Focusing on the periodic seawater inundation of soils in the Pearl River Delta, researchers have verified that high-salinity wetting-drying cycles significantly activate and enhance the bioavailability of soil heavy metals.

A study of the Gulf of Thailand shows that groundwater resources are polluted by seawater intrusion and heavy metals. Highlights from the study shows that seawater intrusion accelerates the release of heavy metals in aquifers and that seawater-groundwater interaction makes a significant contribution to heavy metal pollution in coastal waters.

Ignore at your peril! 3 must-do’s for coastal water security…

As climate impacts are only accelerating, saltwater intrusion is here to stay and likely accelerating… So, for coastal cities that haven’t started thinking about saltwater intrusion, here are 3 must-do’s for coastal water security…

  1. Include saltwater intrusion in coastal adaptation plans! Consider it holistically with water security as well as food & energy security. Have a systemic & transformative approach that factors in pollution as well as rising chronic risks…
  2. Understand that chronic risks like sea level rise & rivers running dry are accelerating. In the future, the frequency of saltwater intrusion would only increase – a 2025 Nature article reveals that on average, sea-level rise contributes ~2x more to increasing future salt intrusion than reduced river discharge under an SSP3-7.0 scenario. It is therefore important to understand the extent to which sea level rise is accelerating driven by rapid ice melt. As over 99% of the evaluated impact assessments have handled sea-level and land elevation data inadequately, it is likely that we’ve materially under-estimated the impacts – see how fast ice science can evolve in in just one year.
  3. Avoid falling into the vicious cycle of “groundwater depletion – land subsidence – saltwater intrusion”. Because it is almost impossible to “undo” saline aquifers, it is best to avoid overextraction of groundwater and/or reverse the fall in groundwater levels. Careful water management (which should include alternate water sources) must be considered together with town planning – dense new buildings not only increase the demand for water but also weighs down the land causing subsidence.

All this needs money – lots of it. For ideas as to how this can be financed, please see CWR’s new white paper – “Financing resilience at scale: Water conservancy REITs & water asset/revenue backed securities”. Produced in collaboration with HSBC & HKGFA, the paper highlights distinctive financial product innovations that can mobilise private capital to unlock trillions of dollars needed to finance water resilience at scale. Join us!

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Author: Ziyang Cai
Ziyang focuses on China’s climate policies and contributes to CWR’s monthly newsletter management and publication. She is passionate about building resilience in the water-energy-climate nexus. Ziyang holds a Master’s degree in public policy from the Hong Kong University of Science & Technology.
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