Introduction

When astrobiologists search for life on planets in distant solar systems they usually look for water. Water is believed to be the key to life, at least for life on our planet. For humans the availability of fresh water plays a critical roll in where our cities are located, how many people the area can support and the availability of food.

One of the most important sources of fresh water is groundwater aquifers. In this article I will explain how this water resource can become overexploited and as a result ruined by seawater intrusion.

What is seawater intrusion & how does it happen

In every costal area there is a risk of seawater intrusion. Seawater intrusion is when saline water, defined as water with a concentration of dissolved salts of more than 1000 mg/L, migrates into the groundwater system. When the fresh water in aquifers gets replaced by saline water the wells becomes useless for drinking and agriculture. The intrusion of salt water can also affect habitats and ecosystems.

Seawater intrusion can occur naturally, usually together with storm surges, the storm presses salt water into the aquifers. But the most common cause of seawater intrusion is overexploitation of the groundwater by humans.

Ghyben-Herzberg relation

In costal regions there is always an intrusion of saline water into the aquifers as can be seen in fig. 1

How far the intrusion goes inland depends on the flow of the groundwater. The Ghyben-Herzberg relation describes the ralation between the hight of the groundwater above the sea level and the depth under it. Simplified it is:

z = 40h. This means we can detect seawater intrusion in costal regions by measuring the hight of the groundwater.

Seawater intrusion due to pumping

The most common source for seawater intrusion is when the pressure in the aquifers is lowered, this causes an up dwelling of saline water into the aquifers.

As you can see from fig. 2, the usage of groundwater causes a depression in the water level and a seawater intrusion. If this continues the well will eventually be filled by saline water and you have to dig a new one.

If the replenishment of groundwater is slow and/or the usage of it is very high seawater intrusion can spread far inland. This has already happened in Florida, where large parts of southern Florida has ground water with a salt concentration of more then 1000 mg/L

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Effects of seawater intrusion

Effects on plants
If saline water intrusion reaches plants roots system, the plant will die of dehydration. Plants rely on osmosis for their water supply, the concept is based on the plants having a higher concentration of salts in their system and thus water moves in from the outside in an attempt to equalize the ion concentration. If however the water in contact with the plants have a higher concentration of salts then the plants themselves have the effect will be the opposite; water from inside the plants will move out of them.
This can cause a shift in the ecosystems towards plants adapted for survival in and around salty water.


Effects on animals



In the same way that plants dehydrate when exposed to saline water, so do animals and humans when they drink it. This is why salty peanuts and crisps are served in bars, to make people drink more. Another problem is kidney failure caused by the high amounts of salt in saline water




Effects on agriculture
As you now know plants react badly to saline water, for farmers this can be a huge problem. In many agricultural areas rain water alone is not enough to sustain the crops. Irrigation and pumping up groundwater is the most commonly used solution to this. But if the groundwater which is used is salty the crops will die and the soil might even become useless.

Dealing with the problem

The availability of fresh water is a very important resource, so when the resource is threatened governments are prepared to spend a lot of time and money addressing the problem. Preventive and managing action can be grouped into three groups: legislative, scientific monitoring and assessment and engineering techniques.

Legislative actions

The government decrees new wells further inland, less pumping from existing wells and laws of water conservation techniques for home owners and companies. This will allow the groundwater to replenish and press the saline water back to it's original boundaries.

Scientific monitoring and assessment

Usually the government in charge of the public water supply has a number of monitoring wells, these act as an early warning system for detecting saline water intrusion or other kinds of pollutants. It is closely related to the legislative actions of a government.

Engineering techniques

A lot of new techniques are under development to prevent saline water intrusion, and sometimes restore groundwater quality.

• Aquifer storage and recovery. This technique is basically pumping excess water down into aquifers during rainy seasons and then pumping it back up again when the need is higher. This should prevent the depletion of existing water supplies during dry season.
• Artificial groundwater. Producing groundwater can be done in several ways, the most common way is to pump water into them from inland, this will keep the pressure up in the costal regions and prevent seawater intrusion.
• Revers osmosis. This is a membrane filtration technique method used to produce fresh water from saline or brackish water. By applying an external pressure one is able to reverse the effect of osmosis.
• Controlled drainage. By controlling canals and other drainage structures one can prevent seawater intrusion.

Prevention of a canal becoming a conveyer belt for seawater intrusion

References & Contacts

Ground water in fresh water-salt water environments of the atlantic coast. - Paul M. Barlow, USGS 2005 ( http://pubs.usgs.gov/circ/2003/circ1262/#heading152526608 )

Water Marks. - Louisiana Costal Wetland Planning, Protection and Restoration. 1996
( http://lacoast.gov/new/Data/WaterMarks/watermarks_1996_fall.pdf )

Contact: Niklas Båmstedt
niklas.bamstedt@gmail.com