---
source: https://iinventedlight.com/library/notes/groundwater-basics.md
title: "What is groundwater?"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: USGS, Groundwater: What is Groundwater?, US government work, public domain; USGS, Aquifers and Groundwater, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "science-explain: earth and ocean science"
tags: science, water, groundwater, aquifers
sources:
  - id: S1
    title: "Groundwater: What is Groundwater?"
    publisher: USGS
    url: https://www.usgs.gov/special-topics/water-science-school/science/groundwater-what-groundwater
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Aquifers and Groundwater"
    publisher: USGS
    url: https://www.usgs.gov/special-topics/water-science-school/science/aquifers-and-groundwater
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# What is groundwater?

## 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).