Salinity is rising in coastal areas and hurting farmlands

The slow march of seawater across riverbanks and through underground aquifers may not grab headlines like a flood or wildfire. Yet, in many coastal regions, especially in river deltas, salinity is advancing with relentless force.

Salinization – the intrusion of saltwater into freshwater systems – is reshaping landscapes, threatening crops, displacing families, and challenging the resilience of communities.

This creeping threat stems from rising seas, dwindling river flows, and intensifying storms. The changes are quiet but far-reaching.

One of the most vivid and instructive examples comes from the Bengal Delta in Bangladesh, where scientists have tracked salinity changes for nearly two decades. Their work is more than a regional study; it’s a glimpse into the future of low-lying coasts everywhere.

Salinity rises across Bangladesh

The Bengal Delta, formed by the mighty Ganges, Brahmaputra, and Meghna Rivers, is home to over 150 million people.

Its vast wetlands and intricate network of rivers, creeks, and estuaries support rice paddies, mangrove forests, and some of the densest rural populations on Earth. But as global temperatures rise, this fragile ecosystem is being transformed.

Salinity has become an especially serious concern here. Ocean tides, which already reach deep inland through the delta’s tidal creeks, are now carrying more salt.

Researchers found that since the mid-2000s, salinity levels have climbed steadily, especially in the western delta near Khulna, where levels exceed 25,000 µS/cm. In comparison, the eastern stretches near the Meghna River maintain much lower levels below 3,000 µS/cm.

Two decades of growing salinity

The study, conducted by researchers from the University of Portsmouth, Dhaka University, and Curtin University, used data from 54 monitoring stations from 2000 to 2017.

The team discovered an average annual rise in river salinity of 76 µS/cm, with some sites recording increases above 800 µS/cm per year. These rates are not uniform. The western delta shows the fastest and most intense changes, while the east remains relatively buffered.

But the most striking observation was the pattern of stepwise increases. Around 2007, salinity jumped noticeably and remained elevated. This sudden rise coincided with major cyclonic events, including Cyclone Sidr.

The data showed that storm surges, compounded by rising sea levels and shifting offshore dynamics, played a central role in pushing saltwater inland and keeping it there.

Why delta salinity is rising faster

To decode these changes, the researchers proposed a new conceptual model: the Offshore Controlled Estuarine and Aquifer Nexus (OCEAN) framework.

This model explains how offshore bathymetry, particularly steep underwater slopes like the Swatch-of-No-Ground, interacts with tides and sedimentation to trap salt in estuarine and groundwater systems.

The steeper gradient in the west dissipates tidal energy, leading to slower water flushing and more salt entrapment. Embankments and polders – human-made dikes – exacerbate the issue by limiting the movement of freshwater.

Dense networks of tidal creeks in these low-lying zones act as storage areas for saline water, which eventually seeps into shallow aquifers.

“What we’re seeing in the Bengal Delta is not just a local crisis, it’s a signal of what’s coming for low-lying coastal areas around the world,” Dr. Mohammad Hoque explained.

“Salinity is rising faster and reaching farther inland than many people realise, and it’s happening quietly with major consequences for water security, agriculture, and livelihoods. This study helps us understand the mechanics behind it, and underscores the urgency for coordinated, global action.”

Reduced river flow makes salinity worse 

Seasonal changes in river discharge used to help flush salt from the delta during monsoons. However, upstream water withdrawals, reduced rainfall, and sediment buildup are weakening this natural defense.

The experts found that salinity levels surged when river flows dropped, especially in the dry months. In the Gorai River system, for example, reduced freshwater flow correlated closely with rising salinity at sites like SW241 and SW244.

The fluvial dominance of the eastern delta helps maintain lower salinity. The eastern zone receives stronger flows from the Ganges and Brahmaputra Rivers, which dilutes incoming salt. In contrast, the western zone, with fewer freshwater inputs, is at greater risk.

Cyclones transfer seawater to rivers

Cyclonic storms have become increasingly destructive, not only due to wind and rain but because they carry seawater deep into freshwater regions. During La Niña years, which are linked to intensified cyclonic activity in the Bay of Bengal, salinity spikes have been particularly severe.

Between 2006 and 2009, the study recorded a pronounced rise in salinity across the delta. This period coincided with Cyclones Akash, Sidr, Bijli, and Aila.

The geomorphological disruptions caused by these storms – such as scouring of riverbeds and changes in sediment flow – may have altered the very shape of channels, making them more vulnerable to future saltwater intrusions.

Fertile lands turning into barren fields

Rising salinity in rivers is not just a water issue. It is also a soil issue. The researchers used satellite imagery to observe trends in soil salinity near monitoring stations.

The team found that areas like SW24, SW258, and SW259 experienced increased soil salinity between 2000 and 2020, especially after 2007. As river water became saltier, the soil nearby became less fertile.

This affects farmers first. High salt levels reduce crop yields, damage irrigation systems, and push families toward debt or migration. The mangrove-rich Sundarbans region, which buffers the coast from storms, also faces threats.

With higher salinity, its delicate ecosystem may struggle to survive, further exposing inland areas to climate extremes.

What Bangladesh’s crisis means

“While the focus is on Bangladesh, the study’s implications are global,” noted Dr Sean Feist, a co-author of the study.

“Coastal regions from the Mekong Delta in Vietnam to Louisiana’s wetlands in the United States face similar pressures. As sea levels continue to rise, the risk of agricultural land turning salty, drinking water becoming undrinkable, and shallow groundwater becoming permanently brackish grows ever more serious.”

The patterns seen in Bangladesh are also emerging in the Mekong and Pearl River Deltas. Model projections show that even a modest 30 cm rise in sea level could significantly increase salinisation in these areas. This study offers a roadmap to understand and manage similar risks elsewhere.

Long-term planning is key

The researchers call for long-term monitoring, not just short-term surveys. Data from 1965 to 2017 helped them detect slow-building patterns that would have been missed otherwise.

Using robust statistical methods like the modified Mann-Kendall test and robust linear models, they were able to identify critical change points, especially around 2007.

Solutions require more than engineering fixes. Short-term projects like dredging rivers or building embankments may help temporarily but won’t reverse the underlying trend.

The study stresses the need for integrated planning – across river basins, national boundaries, and environmental systems.

A call to protect the future

“Ultimately, this study highlights that the creeping salinisation of deltas is a slow-moving but deeply disruptive force,” noted Dr. Ashraf Dewan.

“Without urgent investment in salt-tolerant agriculture, better water storage, and strategic planning across entire river basins, the disruptive impacts of salinity are likely to intensify.”

“The Bengal Delta is on the frontline of climate change, but it is not alone. The patterns observed here are emerging in many of the world’s great coastal regions. What happens next depends on how quickly we respond.”

The time for action is now. While saltwater rises quietly, its effects are loud and lasting. Without decisive, collective, and long-term interventions, the fertile deltas that nourish millions may turn into barren landscapes. The Bengal Delta’s story is a warning – but it’s also a guide, if we choose to listen.

The study is published in the journal Ecological Indicators.

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