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What is groundwater?

Groundwater is water that’s found below the surface of the Earth and which forms primarily as a result of precipitation. Groundwater is located within a geological formation called an aquifer, which is permeable and allows a certain amount of water to be stored. 

These bodies of water cover over twenty times the total surface water of the continents and islands, and the largest ones can extend over more than a million square kilometers. This is why they’re a precious resource; according to UNESCO, they are the source of half of all the drinking water in the world. 

How does groundwater form?

Groundwater responds to the water cycle. When precipitation occurs — whether rain or snow — the water that doesn’t evaporate percolates through the ground until it comes to a permeable rock (an aquifer) below the surface. That aquifer absorbs it like a sponge. Its porosity permits both the flow and the extraction of groundwater. 

An aquifer’s structure is classified into the following:

  1. Aeration zone: the space between the surface and the water table —the accumulation of groundwater at a low depth with respect to the ground— where not all the pores absorb water. 
  2. Saturation zone: located just above the impermeable layer of rock, whose pores are completely filled with water.

How are aquifers where groundwater is found classified?

  1. Free: those that do not have an impermeable layer on top, so the water pressure is equal to atmospheric pressure 
  2. Confined: groundwater is between two impermeable layers; the pressure is greater than atmospheric pressure 
  3. Semi-confined: has characteristics between the free and confined varieties; its upper layer is not entirely waterproof, so water can be discharged and refilled. 

Why is groundwater important?

In addition to being easy to find and resistant to drought, groundwater entails the largest reserve of freshwater on Earth.

  • More than 2.5 billion people depend exclusively on groundwater to meet their needs for this resource. 
  • Groundwater plays a fundamental role in the economy, as it’s the most significant resource used for irrigation and the food industry, accounting for 67% of the total water used for irrigation and food production.
  • They make it possible for the flow and level of water in rivers, wetlands, and lakes to be refilled and maintained when there is little rainfall. This flow is vital for plants, animals, and various ecosystems.
  • Given their characteristics of deep underground storage, if wastewater is treated sustainably, aquifers can preserve its quality, making it accessible for human consumption. 

How and why does groundwater become polluted?

Groundwater contamination is mainly due to human activity, whether that’s waste disposal (solid, animal, sewage, sludge deposits, waste from oil or chemical industries, etc.) or agricultural activities, mining, or construction. The leading causes are:

  • Waste filtration through the aeration zone to the saturation zone from domestic wastewater and public landfills.
  • Use of deep injection systems for treating and flushing wastewater that affect water usage for various reasons (poor design or management). 
  • Development of mining and industrial activity, which generates toxic substances that go into the ground during mineral transportation, washing, and storage.
  • Use of fertilizers and pesticides in agricultural activity (especially nitrates and ammonia) that increase salinization and contamination.

While this is a more natural cause, the extraction of water from aquifers in coastal areas has led to the intrusion of saltwater, which means that it can’t be used for human consumption. The erosion of rock formations is yet another cause of natural pollution. 

How is groundwater managed sustainably?

Groundwater extraction currently exceeds its rate of replenishment, which is why we must bring attention to the importance of water resources and develop processes and measures for monitoring, review, and follow-up to manage the extraction of this resource. Measures may include:

  • Setting up protection perimeters.
  • Properly locating solid waste dumps and drains.
  • Developing sustainable systems to eliminate polluting liquid compounds already within the aquifer zones. 
  • Improving irrigation and fertilization systems.

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