Geology for Global Development


Saltwater intrusion: causes, impacts and mitigation

In many countries, access to clean and safe to drink water is often taken for granted: the simple act of turning a tap gives us access to a precious resource. In today’s post,Bárbara Zambelli Azevedo, discusses how over population of coastal areas and a changing climate is putting ready access to freshwater supplies under threat. 

Water is always moving downwards, finding its way until it gets to the sea. The same happens with groundwater. In coastal areas, where fresh groundwater from inland meets saline groundwater an interesting dynamic occurs. As salt water is slightly denser than freshwater, it intrudes into aquifers, forming a saline wedge below the freshwater. This boundary is not fixed, it shows seasonal variations and daily tidal fluctuations. It means that this interface of mixed salinity can shift inland during dry periods, when the freshwater supply decreases, or seaward during wetter months, when the contrary happens.

Freshwater and saltwater interaction. Credit: The National Environmental Education and Training Foundation (NEEF).

Once saline groundwater is found where fresh groundwater was previously, a process known as saltwater intrusion or saline intrusion happens. Even though it is a natural process, it can be influenced by human activities. Moreover, it can become an issue if saltwater gets far enough inland that it reaches freshwater resources, such as wells.

According to the UN report, about 40% of world’s population live within 100km from the coastline or in deltaic areas. A common source of drinking water for those coastal communities is pumped groundwater. If the demand for water is higher than its supply, as can often occur in densely populated coastal areas, the water pumped will have an increased salt content. As a result of overpumping, the groundwater source gets contaminated with too much saltwater, being improper for human consumption.

With climate change, according to the IPCC Assesment Reports, we can expect  sea-level to rise, more frequent extreme weather events, coastal erosion, changing precipitation patterns and warmer temperatures. All of these factors combined with the a increased demand for freshwater, as a result of global population growth, could boost the risk of saltwater intrusion.

Shanghai – an example of densely-populated coastal city. By Urashimataro (Own work) [CC BY-SA 3.0 ],via Wikimedia Commons.

Although small quantities of salt are important for regulating the fluid balance of the human body, WHO advises that consuming higher quantities of salt than recommended can be associated with adverse health effects, such as hypertension and stroke. In this manner, reducing salt consumption can have a positive effect in public health, helping to achieve SDG 3.

With the aim of preserving fresh groundwater resources for coastal communities at present and in the future, dealing with the threat of saline intrusion is becoming more and more important.

Therefore, to be able to mitigate the problem, first of all, it needs to be better understood. This can be done by characterising, modelling and monitoring aquifers, assessing the impact and then drawing solutions. Currently there are many mitigation strategies being designed worldwide. In Canada, for example, the adaptation options rely on monitoring and assessment, regulation and engineering. In the UK, on the other hand, the simpler solution adopted is reducing or rearranging the patterns of groundwater abstraction according to the season. In Lebanon, a fresh-keeper well was developed as an efficient, feasible, profitable and economically attractive way to provide localised solution for salination.

Every case should be analysed according to its own characteristics and key management strategies adopted to ensure that everyone has access to clean and safe water until 2030 – SDG6.

Water Series (1): The Quantity and Quality of Groundwater

The water available in or near your home can vary dramatically over short distances. In Manchester, there is a robust supply of fresh water from the Lake District, whereas in London (only 200 miles away) the water has passed through limestone, leaving it with a cloudy taste and causing limescale build-up. Signs up on the London underground at the moment are encouraging people to save water by taking the “4 minute shower challenge” and this summer we have had a series of localised droughts and floods. Food prices are expected to rise because there was too much rain this summer, leading to widespread crop failure. Even in the UK, where we have plenty of year-round rainfall, controlling the quantity and quality of water is an expensive and precarious business.

It was in London that the connection was first made between water and health. John Snow noticed that the cholera outbreak in Soho was being caused by a contaminated water supply from the broad street well. In the UK there is now a secure and safe water supply. However, the water available to people around the world is much more variable. Over two million deaths a year are caused by poor water hygiene – equivalent to AIDS or malaria.

The primary control on precipitation (water that falls as rainfall, sleet or snow) is the large-scale convection cells in the atmosphere, which vary systematically with latitude – are you in a tropical zone or a desert zone? Groundwater levels, however, follow more complex patterns. Groundwater maps of Africa produced by a team at UCL show surprising levels of groundwater in unexpected places, such as deep beneath the sahara desert. The primary control on the quality of water is often geological – what rock and sediment does the water pass through between the source and the point of access?

NASA’s landsat educational archives: Latitudinal bands of tropics and deserts across the globe are driven by large scale atmospheric circulation cells.

In developing countries projects often have to work on a local scale, because there is no centralised water supply. Lack of access to water often has a disproportionate impact on women, who are normally expected to walk long distances to collect water from uncontaminated wells. Babies and small children are then the most vulnerable to health problems if the water supply is contaminated. Provision of clean water  is the single most important factor in reducing infant mortality.

Clean groundwater is being extracted from a deep borehole in Ethiopia – giving local communities a better chance of staying healthy. (c) Geology for Global Development 2012

Surface water is more susceptible to contamination from bacteria, but groundwater is more susceptible to heavy metal contamination. Two of the most worrying contaminants are Fluoride and Arsenic, and we will discuss each of these in depth in future blog articles. GfGD has discussed problems relating to water supply in the past, such as our winning entry in last years blog competition, and Donald John MacAllister’s guest blog sharing his practical experience in Bangladesh. Look out for more on our ‘water series’ over the coming weeks.