Aquifer
An aquifer is a permeable geological formation—composed of rock, unconsolidated sediment, or soil—that stores and transmits sufficient groundwater to supply economically useful quantities to wells and springs. Aquifers are classified as confined (bounded above and below by impermeable aquitards, creating artesian pressure) or unconfined (having a free water table as the upper boundary). The world's major aquifer systems, including the Ogallala Aquifer of North America and the Arabian Aquifer System, are critical freshwater resources, but many are being depleted faster than natural recharge rates due to intensive agricultural and urban water extraction.
Key Formula
T = K × b
LaTeX: T = K \cdot b
| Symbol | Meaning | Unit |
|---|---|---|
| T | Transmissivity (ease of water transmission) | m²/s |
| K | Hydraulic conductivity | m/s |
| b | Saturated thickness of the aquifer | m |
Worked Example
Problem
A confined aquifer has a hydraulic conductivity of K = 5×10⁻⁴ m/s and a saturated thickness of b = 40 m. Calculate its transmissivity.
Solution
Step 1: Identify values — K = 5×10⁻⁴ m/s, b = 40 m. Step 2: Apply the transmissivity formula — T = K × b. Step 3: T = 5×10⁻⁴ × 40 = 2×10⁻² m²/s. Step 4: Convert to m²/day — T = 0.02 × 86,400 = 1,728 m²/day.
Answer
Transmissivity T = 0.02 m²/s = 1,728 m²/day — a highly productive aquifer suitable for large-scale water supply
Major Global Aquifer Systems and Their Status
| Aquifer | Location | Type | Area (km²) | Status |
|---|---|---|---|---|
| Ogallala (High Plains) | Central USA | Unconfined | 450,000 | Critically over-extracted; declining 0.3–1 m/yr |
| Arabian Aquifer System | Arabian Peninsula | Confined (fossil) | 1,200,000 | Non-renewable; rapid depletion |
| Guaraní Aquifer | South America | Confined | 1,200,000 | Large, relatively intact freshwater reserve |
| Indus Basin Aquifer | Pakistan/India | Unconfined | 1,000,000 | Severely stressed; groundwater crisis |
| Great Artesian Basin | Australia | Confined | 1,700,000 | Largest artesian basin globally; recharge very slow |
| North China Plain | China | Unconfined | 400,000 | Heavily over-exploited; land subsidence occurring |
Interactive Tools
USGS Water-Resources Investigations — Aquifer Basics
Official USGS educational resource on principal aquifer types, properties, and major US aquifer systems.
Open ToolWolfram Alpha — Aquifer Transmissivity
Calculate transmissivity and other hydrogeological parameters from hydraulic conductivity and aquifer thickness.
Open ToolKhan Academy — Groundwater and Aquifers
Article and exercises explaining confined vs. unconfined aquifers, artesian conditions, and depletion issues.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Groundwater
Groundwater is subsurface water that occupies the pore spaces, fractures, and voids within saturated zones of soil and rock, existing below the water table in the phreatic (saturated) zone. It constitutes approximately 30.1% of Earth's freshwater and is recharged by precipitation that infiltrates through the unsaturated vadose zone, and discharged naturally through springs, streams, and wetlands or artificially through wells. Groundwater plays a critical role in sustaining ecosystems, supplying drinking water to over 2 billion people globally, and supporting agriculture through irrigation, making sustainable groundwater management essential in the face of climate variability and rising demand.
Mineral
A mineral is a naturally occurring, inorganic, crystalline solid with a definite chemical composition and characteristic physical properties. Minerals are the building blocks of rocks and are classified by their crystal structure, hardness, luster, cleavage, and color. They are essential to industry, construction, and biological processes, with over 5,000 known mineral species recognized by the International Mineralogical Association.
Soil Formation
Soil formation (pedogenesis) is the process by which parent rock material is transformed into soil through the combined effects of weathering, biological activity, organic matter accumulation, and the movement of water and dissolved substances through the soil profile. The CLORPT model identifies five key soil-forming factors: climate, organisms, relief (topography), parent material, and time. The result is a layered soil profile with distinct horizons—O, A, B, C, and R—each reflecting the degree of weathering, organic content, and mineral alteration at different depths.
From Latin "aqua" (water) and "ferre" (to carry, to bear), literally meaning "water-bearer" or "water-carrier." The term was introduced into scientific hydrogeology in the late 19th century as systematic groundwater studies developed in response to growing demand for reliable water supplies during industrialization. Henry Darcy's 1856 work on water flow through porous media laid the quantitative foundation for aquifer science.