Saltwater intrusion – the fear for the water well
Saltwater intrusion into the well is the fear of many well owners 😱, as it risks destroying the well for a long time to come. The risk is even greater during periods when groundwater levels are lower or much lower than normal – which right now (May 2022) is unfortunately the case for almost all of Sweden. Moreover, groundwater is not normally replenished until the fall (see previous newsletter on groundwater).
To avoid saltwater intrusion, it is important to keep track of your well; how quickly it recovers after use and how the groundwater level is affected by local conditions, precipitation and use.
Aqvify helps you to use your well in a sustainable way!
We asked Anders Nordström, Senior Lecturer at Stockholm University, to explain in more detail why saltwater intrusion into wells occurs, where it is most likely to occur, whether it is actionable and how to avoid it.
Anders is the author of the book ‘Drinking water – our most important foodstuff’ and a pioneer in the field.
He is also one of the longest users of Aqvify, something we are very happy and proud of!
Over to Anders!
How does saltwater intrusion occur in your well?
Normally, groundwater is sweet and has been formed by precipitation infiltrating the soil and then percolating (sinking) by gravity into the pores of the soil layers and further into the cracks of the rock or, for sedimentary rocks, into the pores. Groundwater fills all pores or cracks below the water table.
In the vicinity of sea shores, it can be understood that the extraction of groundwater from a dug or drilled well close to the shore can lead to the penetration of salty sea water into the well.
However, perhaps the most common is the risk of encountering fossil (relict) groundwater, i.e. an old salty groundwater that was formed when the area of your well was below sea level in a salty sea. This happened during the melting of the last ice age. The ground surface was then much lower than today because the thick ice pressed the rock down. Large areas of Sweden were below sea level (below the so-called marine limit, i.e. in the salty sea). After the melting of the ice, the land began to rise – land uplift.
Large parts of the coastal areas and central Sweden were below sea level for a long time and during this time the salty water penetrated and formed salty groundwater. Salty groundwater has a higher density (is heavier) than freshwater. With the help of land uplift, much of Sweden’s land area has been raised above sea level, and this is where sweet groundwater is formed by rain and snowmelt. The sweet groundwater floats like a cushion on top of the salty relict or fossil groundwater.
Salt water in the groundwater source
The salt in the sea and salty groundwater consists largely of chloride (Cl) and sodium (Na). Sea salt water can enter the cracks and pores of the rock in various ways to form salty groundwater. The chloride content of groundwater is most commonly referred to. The power threshold for chloride is about 200-300 mg/l Cl and for sodium 50-200 mg/l Na. Natural groundwater has 1-20 mg/l Cl. Groundwater with more than 50 mg/ Cl is called saline affected. 100 mg/l Cl is the Swedish National Food Agency’s limit value for technical remarks on drinking water due to the risk of corrosion in pipes. The limit value applies to both municipal and private water wells.
In nature, there is saltwater intrusion from the sea 0-100 m from the shore. As described above, relict groundwater exists in large parts of the country. In addition, chlorides are normally added in very small quantities with precipitation, but also from the wind from the sea and towards the coasts. Coastal areas thus receive more salt than inland areas. This does not normally cause a problem in groundwater quality. The salting of winter roads can also increase the chloride content of groundwater in nearby wells.
In coastal areas, such as the Stockholm archipelago, groundwater withdrawals can be so large that salty groundwater suddenly appears in the well.
Why does this happen? Can the salt water disappear again?
Why does salt water enter? Can the salt water disappear again?
Near the sea, the water table is normally (when there is no abstraction) higher than sea level, preventing the salty seawater from entering and mixing with the freshwater. This makes it possible to find freshwater even close to the shore. Most wells in coastal areas are in the bedrock, i.e. groundwater in fractures is used. Due to the higher density of salt water, a fresh/salt water boundary occurs at a certain depth. The fresh water floats like a cushion on the salty groundwater.
When groundwater is extracted, the water table drops and a so-called sinking funnel is formed with the tip facing downwards. The difference in density between fresh and salt water means that the boundary layer rises many times more than the fall in the groundwater surface.
There is then a risk that the withdrawal from the well takes place below the freshwater layer and thus produces salt water instead. It is therefore very easy to get salt water in deep wells when water withdrawals in your own or your neighbors’ wells increase so that the water table drops – and especially if the groundwater is already low due to low precipitation during the period of the year that replenishes the deposits.
The groundwater level can also be lowered so much that the sea penetrates into empty cracks. This occurs most easily in the coastal area up to about 100 m from the shore. This also produces saline groundwater. Because the movement of groundwater in the fractures of the bedrock is so slow, it will not be possible to force out the newly intruded salt water or salty fossil groundwater until perhaps 10-30 years from now in a rock-drilled well 60-80 m deep. Large variations in recovery time exist.
Who is affected?
Holiday home owners are perhaps the main victims of saline groundwater in areas below the marine boundary and particularly in coastal regions. They have increased their annual water consumption through increased water use for showers, toilets, washing machines and more days of use of the property per year, as well as creating a high density of holiday homes, which can lead to groundwater abstraction being greater than the new formation of groundwater, resulting in groundwater subsidence. This changes the position of the boundary between fresh and salt water in the aquifer. This can lead to saltwater intrusion in some wells.
Even those who have been using freshwater from the bedrock very moderately for a long time can be affected by neighbors’ increased withdrawals. The sinkhole created in a neighbor’s well (at a distance of about 50-100 m) affects your well. In Figure 3, the upper image shows a granitic rock where the fractures are in contact with each other. The lower picture shows how they affect each other. In this case, the one with the largest drawdown gets salt water first. The other well has not yet experienced saltwater intrusion.
What can be done to avoid saltwater intrusion?
Everyone should observe the position of the water table in their well and not allow it to fall below the previous annual minimum level. In years that start with a lower than normal groundwater level (such as May-June 2022!), extra care must be taken with water use.
If you have a chloride level higher than 100 mg, you should immediately REDUCE your water withdrawal from the well. If you are lucky, the chloride level will drop, but it may take time (years). For the holiday home, the use of toilets, washing machines and showers may have to be prioritized. You can also significantly reduce water withdrawal by using low-flow nozzles on your taps.
If the well is already affected, it may also help to raise the pump so that the water withdrawal is above the threshold and possibly use methods to reduce the depth of the well.
IF YOU ARE ALWAYS CAREFUL WITH THE WITHDRAWALS FROM THE GROUNDWATER WELL, I HOPE YOU WILL AVOID SALTWATER PROBLEMS.
Do you want a more detailed explanation of the problem of saltwater intrusion? Then read the book (in Swedish) “Dricksvatten – vårt viktigaste livsmedel.“
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