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Short answer

Groundwater is water that fills the spaces in soil and rock below the water table. Aquifers, the underground layers that hold usable amounts of it, store about a hundred times more fresh water than all the world's rivers and freshwater lakes. [S1][S2]

Key ideas

• Water table: the top of the saturated zone. Dig a hole at the beach and the water you see is at the water table. [S2]
• Unsaturated zone: the ground above it, which holds air and some water. [S1]
• Confining layers: dense rock such as granite or shale that water has a hard time passing through. [S1]

Claims

• id: C1 claim: There is about a hundred times more fresh water in the ground than in rivers and lakes. source: S1 evidence: "about a hundred times more fresh water in the ground than is in all the"
• id: C2 claim: The ground is saturated below the water table. source: S1 evidence: "how the ground is saturated below the water table"
• id: C3 claim: The upper surface of the zone of saturation is the water table. source: S2 evidence: "The upper surface of this zone of saturation is called the water table."
• id: C4 claim: Usable groundwater is found in aquifers. source: S2 evidence: "it is only found in usable quantities in certain places underground — aquifers"

Sources

• [S1] Groundwater: What is Groundwater?. USGS. https://www.usgs.gov/special-topics/water-science-school/science/groundwater-what-groundwater . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Aquifers and Groundwater. USGS. https://www.usgs.gov/special-topics/water-science-school/science/aquifers-and-groundwater . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_19khkrnz,
  author = {{iinventedlight}},
  title = {{What is groundwater?}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/groundwater-basics/},
  note = {Permanent ID iil:note:19khkrnz. License: CC-BY-4.0.},
}

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