GROUNDWATER: AN INVISIBLE RESOURCE
Life would not be possible without groundwater.
It accounts for 98% of the world’s unfrozen freshwater reserves and is our main stock of drinking water on Earth. Although little known, groundwater is a fundamental part of the water cycle and is a finite and fragile resource that must be managed sustainably.
On the occasion of World Water Day 2022, which will focus on the subject, Labaronne-Citaf explains what groundwater is, how it is formed and why it is important to adapt our global water management today.
WHAT IS GROUNDWATER?
Groundwater refers to all the water reserves that are found underground.
It is made up of water tables, sheltered in the rocks of the earth’s crust. When these water tables are located at a shallow depth, they are called “phreatic tables“. There are different types of groundwater.
- Unconfined tables, contained in porous and permeable rocks, which are called “aquifers” and in which water can circulate. Aquifers can store large quantities of water. The amount of water stored by a rock depends on the size of its pores and cracks.
- Confined tables or captive tables, where water is trapped between two sets of impermeable rocks. A confined water table is covered – totally or partially – by a layer of impermeable ground. These tables are under pressure.
Layers of aquifers alternate with layers of impermeable rock. This forces the water to follow a natural flow (streams, underground rivers) or to form confined tables.
HOW IS GROUNDWATER FORMED?
Groundwater is part of the water cycle. These water reserves are formed and recharged by rain, snow or hail, which infiltrate the ground.
Water cycle and groundwater formation
For example, when it rains, water is used in different ways:
- A 1st part of the rainwater will run off to the so-called “surface water” (streams, rivers or lakes).
- A 2nd part of this water will be captured by the vegetation and by the soil.
- A 3rd part will evaporate and return to the atmosphere.
- Finally, if precipitation continues, the water can infiltrate the soil deep enough to reach and saturate the aquifers, thus forming a phreatic table.
Once in the aquifers, the groundwater flows slowly below the surface in a gravitational manner and according to the terrain (bypassing impermeable rocks). On its way, it is loaded with mineral salts. Sometimes travelling long distances, it naturally moves towards outlets (low points in the topography), to feed – this time from the bottom – the rivers and bodies of water on the surface.
HOW IS GROUNDWATER RECHARGED?
Water that has returned to the surface via streams (springs, rivers) or bodies of water (lakes, seas, oceans) evaporates and then condenses in the atmosphere until it begins a new cycle, by air.
Underground, when the table water level is lower than the surface water level, the direction of flow is reversed: the surface water will recharge the table water, by a communicating vase effect. This reverse process of “recharge period” is observed in autumn and winter, or during river floods.
What is the role of groundwater?
Groundwater is crucial to the functioning of ecosystems such as wetlands and rivers. By feeding surface waters with fresh water, groundwater interacts with aquatic environments and the species that live in them or come to drink from them. It thus contributes to life in rivers and wetlands, and also plays a role in coastal waters.
THE BIGGEST NATURAL LIQUID FRESHWATER RESERVE
Freshwater accounts for less than 3% of the total water on Earth. Most of this freshwater is in its solid state; and liquid freshwater is almost entirely underground. Groundwater is thus the largest reserve of natural freshwater that we have and use to live.
Freshwater – filtered, purified and enriched with minerals thanks to this underground process – feeds humans and all living species on a daily basis: plants; trees; land, sea and flying animals…
The distribution of freshwater
- 69% of freshwater is stored as ice or snow, in a solid state.
- 30% of freshwater is stored in aquifers.
- 1% of freshwater is stored as liquid surface water in streams, rivers, lakes, etc.
Source: The French Water Information Centre
Groundwater is the main source of drinking water: for example, in France, 62% of drinking water comes from groundwater and only 38% from surface water. Groundwater is generally extracted from the surface through wells or pumping systems. In order to gain access to water, a passage is drilled in the open water table, usually close to the surface, or deeper in a confined water table.
Groundwater therefore provides most of the water we use, whether for:
(farming & livestock)
2/3 of groundwater extractions are used for agricultural irrigation.
Livestock activities also consume a lot of water:
A French survey conducted in 2017 revealed that 40.7% of livestock farmers had a private borehole and used it for watering their animals.
Groundwater is collected by catchment and then sent to treatment units to produce drinking water for human consumption. 1/4 of this water is used for human consumption.
Another part is used for domestic or collective purposes (wells, swimming pools, green spaces, campsites, etc.)
and additional processes
37% of the fresh water taken by industries is underground. It is used for many purposes: manufacturing processes, cooling of machines, production of food or medicines, maintenance and cleaning, etc.
In 2008, 1/10 of groundwater was used for industry or mining.
Why and how to protect groundwater?
Water has been functioning in a closed circuit for billions of years: this is what is known as the water cycle. Because of this cyclical nature, the quality of groundwater is highly dependent on the quality of surface water (rivers, water bodies, etc.) as they constantly interact with each other.
Reconciling human activity and water management
To protect these limited resources, it is necessary to protect them from pollution and limit their use.
In terms of quality (chemical status)
Contributing to the preservation of groundwater means first of all avoiding the use, on the surface, of substances likely to contaminate groundwater by infiltration by limiting their spreading on the surface of the ground.
Care must also be taken with extraction methods. The protection of groundwater depends primarily on the good maintenance of boreholes, which are likely to cause direct contamination, but also on the good management of water withdrawals carried out through them.
In terms of quantity
In order for groundwater to regenerate, the volumes taken from the water table must not exceed the renewal capacity of the resource. Similarly, in order for the water cycle to run smoothly, the water supply to surface ecosystems must be preserved.
Coping with climate change
In order to limit the impacts of climate change on water resources, withdrawals must be better distributed in space and time.
Challenge N°1 : Fight against soil drying
The rise in temperatures, linked to climate change, favours evaporation. As a result, the drying out of soils is increasing. Drier soils limit the infiltration of water: less hydrated and less permeable, water will tend to run off, which prevents the recharging of the water table during rainfall.
⇒ By preserving wetlands and recreating vegetated areas (plants, trees), moist and permeable soil is promoted. The phenomenon of evapotranspiration leads to the creation of clouds, and therefore to rain.
Challenge N°2: Collecting rainwater
Most of the world’s arid zones depend entirely on groundwater. It acts as a buffer in times of drought and is essential for maintaining wetlands. But with increasingly intense rainfall (downpours), there is no time for water to seep into the ground.
⇒ Rainwater harvesting is advantageous for companies and communities. By recovering part of the rainwater, we can save on the cost of extracting and treating groundwater, while building up water reserves to prevent periods of drought. This avoids drawing too much water from the groundwater, but also limits the runoff of rainwater, which carries all the pollution (roads, soil, etc.) to the surface water.
Challenge N°3: Avoiding saline intrusion
With the consequences of climate change and the consumption of groundwater, such as the increase in sea level and the modification of recharge conditions, the alteration of freshwater quality by saline intrusion in coastal areas is a problem that is likely to increase.
⇒ This is the current challenge for scientists. The main ways to avoid contamination of continental waters by salt water are to optimise withdrawals and even to set up artificial recharge in order to maintain a hydraulic barrier.
The protection and sustainable use of groundwater is therefore essential for survival in adapting to climate change and meeting the needs of a growing population.
We need to protect these waters from the overexploitation and pollution that currently threaten them, as we risk depleting these resources, increasing the costs of treatment, and even making it impossible to use them.
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