What is groundwater and why is the level to which it is contaminated such a concern?
Groundwater is the natural water stored beneath the Earth's surface and it is essential because it supplies a third of the world's population. As with other of our planet's elements, it also finds itself under threat from contamination. In fact, a recent study undertaken at the Albert-Ludwigs University in Freiburg (Germany) warns this threat could be greater than at first thought.
Groundwater, increasingly under threat from contamination, supplies half the planet with drinking water.
Groundwater is a vital resource as it provides almost half of all the world's drinking water, is an essential reserve during periods of drought and contributes to the sustaining of numerous ecosystems. However, a recent study headed up by Andreas Hartmann, Professor of Water Resources at the Albert-Ludwigs University, confirms that the level of contamination of these waters could be worse than at first thought.
What is groundwater
As its very name indicates, groundwater is that which is found beneath surface, and is one of the key component parts of the water cycle. Whenever it rains or snows, part of the water filters down through the ground until it reaches a layer of impermeable rock, filling its pores and fissures and remaining stored there in aquifers.
According to UNESCO, UN organisation — promoter of SDG 6: Clean Water and Sanitation —, this natural resource provides multiple benefits, some of which are well worth highlighting:
- Groundwater provides almost half of all the world's drinking water and around 43 % of all water used for irrigation.
- In the region of 2.5 billion people depend exclusively on aquifers for meeting their daily water needs, and this is why responsible consumption is so important.
- It represents an essential reserve for the populations of arid regions by providing them with a source of drinking water, even during long periods of draught.
- It contributes to the sustaining of many types of ecosystem, on which different human populations and species depend.
Types of groundwater
Aquifers are classified differently in accordance with their characteristics:
According to their hydraulic behaviour:
- Unconfined: these are not covered by an impermeable layer and, therefore, the uppermost limit of the saturated zone (water table) is at atmospheric pressure.
- Confined: these are found between two impermeable layers and, as a result, are subjected to a higher than atmospheric pressure.
- Semi-confined: their uppermost part is up against a semi-permeable terrain, so their characteristics are somewhere between unconfined and confined.
According to their texture:
- Porous: the water is contained between non-consolidated materials, such as gravels or sands, which absorb the water like a sponge.
- Fractured: the water is contained between rock formations with fractures (or fissures) and cracks that enable internal circulation.
How groundwater are formed
The water from precipitations, after infiltering down through the ground, continues to descend through what is known as the aeration zone or non-saturated layer until it accumulates in the saturation or saturated layer. Its uppermost limit is the water table, while the lowest limit can occasionally be as deep down as ten kilometres. This can be seen in greater detail in the following infographic:
According to the study External link, opens in new window. undertaken by the Albert-Ludwigs University mentioned at the top of this article, groundwater contamination could be worse than at first thought. For example, up to 50 % of the contaminants studied — such as the pesticide glyphosate — reached the groundwater without passing through any filtering process whatever, defying earlier assumptions. As a result, the level of glyphosate in the groundwater could be up to 19 times greater than the maximum concentration permitted in Europe.
Generally speaking, groundwater contamination occurs from three different sources:
Due to ground contamination, by way of the accumulation of solid residues on the surface (for example, a landfill site located above an aquifer) whose soluble components are dissolved by rainwater (leaching) and make their way down to the aquifer.
Propagation from the non-saturated zone
One of the most common cases is that caused by the purification of domestic waste water when these are filtered via the aeration zone before reaching the saturation zone.
Propagation from the saturated zone
When deep injection wells are used for evacuating waste water and they are not suitably designed, located or managed, they can cause the presence of contaminants in the aquifer.
Likewise, the main sources of groundwater contamination are:
Urban and domestic contamination
This is divided into solid waste (surface propagation) and liquid waste (subterranean propagation from the non-saturated zone). As we have seen above, the leaching of the soluble materials from a landfill site or the filtration from the sanitation networks are habitual sources of contamination.
Agricultural and livestock contamination
This is the main cause of water contamination due to the leaching of the products used by farmers and cattle breeders — fertilisers, pesticides and plant-protection products —, to intensive manure disposal spreading or irrigation, which increase the salinisation and contamination by nitrates of the aquifers.
Industrial and mining contamination
Industrial contamination is mainly due to the dumping of solid waste in landfill sites and of liquids in surface watercourses, as well as to the loss of substances during transportation and storage. In the case of mining, the toxic sources are the slag heaps and the run off of the mineral washing processes.
How to combat the groundwater pollution
In addition to sustainable management, groundwater contamination is combated on two fronts: preventive methods, the purpose of which is to prevent the contamination by anticipating the problem; and remedial methods, which are applied once the water has become contaminated. These are the main courses of action:
- Preventive methods: establishing protective boundaries, choosing suitable locations for landfill sites and drainage installations, treating liquid contaminants, rationalising irrigation and nitrogen-based fertilisation, controlling of pumping in nearby boreholes close to the coast in order to prevent the filtration of seawater, etc.
- Remedial methods: the formation of impermeable subterranean and depression barriers between the aquifer and the source of contamination or the interception and extraction of the contaminant by pumping (a method whose efficacy has been put in doubt), among others.