Coastal areas are the most densely populated geographical zones globally because the land is generally flat, fertile, and close to river deltas that wash rich silt onto coastal plains. Urbanization and agricultural and industrial development of coastal zones have further complicated these environments as large numbers of people require equally large amounts of freshwater.
Saltwater intrusion is a common groundwater problem in coastal areas due to excessive extraction of underground water from aquifers. It causes land subsidence, encroachment of the sea into coastal plains, reduction of available arable land for farming, and freshwater shortages for coastal populations.
When coastal aquifers are depleted due to over-pumping, they are susceptible to invasion by seawater, raising their salinity. This can be further aggravated by drought and rising sea levels. Saltwater intrusion, as it’s technically called, upsets the delicate balance between fresh water and the sea. This article explores the reasons for saltwater intrusion into groundwater, why it’s such a problem, and what can be done about it.
Saltwater Intrusion: A Common Groundwater Problem in Coastal Areas
The irony is that while coastal areas are close to the vast expanses of the world’s oceans, people, plants, and animals still need fresh water to live there. They need water to hydrate their livestock and irrigate crops as well as drinking water. The fertility of coastland has proven to be its Achilles heel in the past because crops grew so well there that farmers extracted too much groundwater for irrigation.
It is estimated that more than one billion people living in coastal regions rely on fresh underground water, and that number is rapidly increasing. In major cities and agricultural capitals on the coast, the demands on groundwater now exceed its long-term sustainability in many places, and the sea is gradually pushing deeper inward.
Desalination of seawater is an expensive, time-consuming process that is not viable for securing groundwater in the long term. To make matters worse, some coastal regions are not linked to inland water systems, which means they are hydrologically isolated and utterly dependent on coastal aquifers. If these aquifers turn too salty, the highly productive coastal farming areas will fail with economic implications for entire countries that depend on those crops.
Problems Caused By Over Extraction Of Groundwater
Coastal plains are flat, low-lying areas, just slightly above or at sea level and below the topsoil, consisting mainly of sand and fine sediments that originated from the sea bed. This land is not suitable for urban and industrial development as it doesn’t drain well and is highly susceptible to compaction. Over-extraction of groundwater causes the earth to subside below sea level, increasing the risk of flooding.
It also increases the salinity of the groundwater which, in low-lying areas, leads to the seepage of brackish water onto the land surface from underground. The process is insidious and may escape notice for many years. However, the long-term problems it creates are significant and far-reaching for coastal ecosystems, rivers, estuaries, and plains.
- Groundwater can no longer safely be used to irrigate crops because the increased saltiness of the water and soil kills them.
- Residents of low-lying coastal areas become more vulnerable to floods as the land subsides.
- Drinking water becomes scarce and more costly.
- Buildings on coastal land are more likely to crack and deteriorate as the stability of their foundations is affected.
- The saltiness of the groundwater also erodes steel reinforcements, brick, and concrete in these buildings to the point that they can collapse.
- The increased salinity of groundwater can persist for decades after flooding by the sea.
When combined with increasing demand for water in upper water catchment areas, rising sea levels, and changes in the climate, the problem is only getting worse. The oceans are slowly pushing further inland, taking back the low-lying coastal areas. Agriculture has become more complex and expensive, making food more costly to produce.
Coastal Groundwater Is Threatened Worldwide
The groundwater problem in coastal areas is not restricted to only a few places worldwide. It occurs in many regions, including the US, African countries, Asian countries, and Europe. Efforts to manage groundwater have only begun relatively recently in many of them. Political, socio-economic, demographic, and climate changes negatively affect the sustainability of coastal groundwater in many countries.
Saltwater intrusion is a complex ecological problem with no simple solutions. The leading cause is the excessive extraction of groundwater by resident populations. Examples of countries with saltwater intrusion challenges include Pakistan, Tanzania, Kenya, Mozambique, China, Greece, Spain, Israel, Italy, India, California, and Northern China.
The protection of the underground aquifers is becoming more challenging because of the increased frequency of extreme weather conditions. Storm surges and flooding of the land increase the chance of seawater intrusion, and people have now realized that it’s essential to protect shorelines to preserve the groundwater.
Only three percent of the world’s water is fresh, and most of it is inaccessible because it’s locked up in glaciers, snow, permafrost, and other geographical features. A scarcity of potable water from coastal aquifers increases the risk of infectious diseases such as cholera, dysentery, hepatitis A, typhoid, and diarrhea. In children under five years, this diarrhea is often fatal.
Some climate modeling indicates that precipitation is expected to decline in subtropical regions, including many of the world’s coastlines. It is likely to increase in the Arctic, Antarctic, and areas near the equator. While precipitation modeling is not an exact science, it is advisable to hope for the best but prepare for the worst.
Solutions For Saltwater Intrusion In Coastal Areas
Solutions for problems of saltwater intrusion in coastal areas require a mix of techniques, including –
- Improved land management
- Education of coastal farmers on the use and conservation of water
- Restrictions on construction in coastal areas
- Reduction of groundwater pumping rates and relocation of pumping wells away from the coastline
- Changing crop patterns to avoid crops needing a lot of water, such as sugarcane and rice
- Encouraging more efficient methods of crop irrigation such as drip irrigation and nocturnal irrigation
- Increasing public awareness by encouraging the use of recycled water by households, and the industrial and agricultural sectors
- Reducing water extraction from aquifers in wet years when there’s plenty of rainfall
Other methods include the erection of physical subsurface barriers along the seashore, such as sheet piles, slurry walls, and concrete, and the erection of physical surface barriers designed to artificially extend the coastline seaward. In India, the restoration of small dams to collect water in the rainy season helped restore surface water. In tropical areas, restoring coastal mangrove swamps by replanting mangrove trees has also been shown to protect coastal areas from flooding.
The recharging of aquifers with treated wastewater, desalinated seawater, and desalinated brackish water are hydraulic barriers that can be used to prevent saltwater intrusion. However, these methods can be costly to implement.
Coastal environmental management is crucial to prevent saltwater intrusion into groundwater in coastal areas and the many problems it creates. Coastal aquifers are increasingly under threat due to excessive water extraction for agricultural and industrial purposes and the growth of coastal populations. With sea levels rising, increasing frequency of extreme weather events, and declining yearly rainfall in some areas, the problem is now more urgent than ever.
- Mastroccio N and Colombiani N ‘The Issue of Groundwater Salinization In Coastal Areas of the Mediterranean Region: A Review’ https://www.mdpi.com/2073-4441/13/1/90
- Hussain M.s. et al. ‘Management of Seawater Intrusion in Coastal Aquifers: A Review’ https://www.mdpi.com/2073-4441/11/12/2467