- Coastal aquifer - Paul Barlow, USGS
Saline intrusion is the flow of sea water into fresh water coastal areas such as wetlands or aquifers and it can pollute drinking water supplies or severely disrupt ecosystems. The process is often caused by the actions of humans, with excessive extraction of groundwater being the leading cause of salinification in coastal aquifers. Saline intrusion has affected aquifers across the world and is likely to become an increasing problem as sea level rises.
What Causes Saline Intrusion?
In a coastal aquifer fresh groundwater overlies saline water because the fresh water is less dense. This makes it easy for humans to extract the fresh water, which is replenished by rain and flow from inland aquifers. However, if we extract fresh water more rapidly than it is replenished, more saline water is drawn into the aquifer and the boundary between salt and fresh water moves upward, leading to salinification of the fresh water supply.
Although groundwater extraction is responsible for much of the saline intrusion seen across the world, it can arise from other activities such as the lowering of the water table by drainage canals and the breakdown of natural coastal barriers that served to protect vulnerable areas. Storm surges and rising sea level are also cause for concern.
A storm surge is a temporary increase in sea level, potentially by several metres, that can result from the drop in air pressure associated with an intense storm. Storm surges can inundate miles of coastline and the flooding can be exacerbated by wind blowing the water onshore. Sea water can submerge, and flow into, coastal wells or boreholes, contaminating the aquifer.
Increasing sea level is also making coastal aquifers more vulnerable. At present, sea level is rising by about 2mm/year and it is estimated that this will increase in the coming decades. Current predictions expect sea level to rise by approximately 1m by 2100, which will make storm surges increasingly dangerous and increase the vulnerability of coastal fresh water supplies.
Where Has Saline Intrusion Occurred?
Saline intrusion is affecting aquifers all over the world, typically in areas where humans have been pumping groundwater or building drainage canals. According to a case study by Solinst “Pump/Recharge Rate Affects Saltwater Intrusion” (accessed August 21 2010), saline intrusion is a growing problem in North Africa, the Middle East, the Mediterranean, China and Mexico. There have also been reports of saline intrusion in Australia.
Aquifers along both the Atlantic and Pacific seaboards and the Gulf Coast of the USA are experiencing saline intrusion, while many European aquifers have also been affected. For instance, there are particular problems along the Mediterranean coast, with an estimated 60% of Spanish coastal aquifers experiencing saline intrusion. On the North Sea coast, flooding of the coastal plain has led to aquifer salinification.
What Happens Next?
Saline intrusion can lead to well abandonment if salt concentrations in the water increase above the limits considered safe for drinking water. In some cases, salinification may be localised and most of the aquifer remains a suitable source of fresh water, but this is not always the case. In one New Jersey county more than 100 wells had to be abandoned because of saline intrusion.
However, if salt levels in the aquifer are monitored and vulnerability to saline intrusion is identified before large regions of the aquifer have been contaminated, water extraction can be managed in such a way as to allow fresh water levels in the coastal aquifer to recover.
Managing Saline Intrusion
For instance, many procedures have been developed along the Atlantic seaboard of the USA to deal with the widespread saline intrusion problem. A crucial aspect of these water management strategies has been the development of scientific monitoring procedures that can identify when a water source may be at risk from saline intrusion. Early identification of a potential problem assists in the planning of water management procedures, such as those described below.
A common solution is to reduce pumping from coastal wells or move the wells inland, thereby permitting the replenishment of fresh water in the coastal areas of the aquifer. An alternative to this is the artificial recharge of the aquifer, whereby fresh water is pumped into the rock, forcing out the saline water.
Legislation restricting water pumping and encouraging water conservation have also been found to be effective, as have aquifer storage and recovery systems, whereby excess water collected during the wet season is pumped into wells and stored in preparation for the dry season. Desalination systems are also in use in some areas and are also proving effective as technology improves and costs decrease.
The advantage of desalination technologies is that the salinification of a water source no longer renders that water unusable. This opens up the possibility of using saline groundwater sources in future years, although this has not been investigated fully and exploitation of such water bodies would require extensive research and development.
Ideally, the problem would best be managed by sustainable use of water resources so that saline intrusion did not occur in the first place. However, we only became aware of the problem once salinification had already occurred, so the best we can do now is adopt sensible water management strategies and try to minimise the damage. Unfortunately, this is likely to become more difficult as demand for water continues to increase and sea level continues to rise.
Sources
Paul M. Barlow, "Ground Water in Freshwater-Saltwater Environments of the Atlantic Coast", USGS Circular 1262, last modified September 01 2005, accessed August 21 2010
Geoscience Australia, "Saline Intrusion", OzCoasts online article, 2010, accessed August 21 2010
University of New South Wales “Potential impacts of sea-level rise and climate change on coastal aquifers” July 19 2010, accessed August 21 2010
Margaret Wallace - Hello, my name is Mags Wallace. I did an MESci in geophysics and Geology from Liverpool University, followed by a PhD in Physical ...
