Earth science
Groundwater
Porosity, permeability, water table, Darcy, aquifers and aquitards.
Basics
Porosity
Porosity is the fraction of rock or soil that is void space. Sand is porous and well connected; clay may be highly porous yet slow because pores are tiny. Effective porosity counts only the connected part that can flow. Compaction and cementation shrink voids. Storage lives in porosity; flow lives in connectivity.
Permeability and the water table
Permeability is how readily a medium lets fluid through. The water table is where pore pressure equals atmospheric at the top of the saturated zone. Below is saturated; above is unsaturated (soil water). A well’s water level tracks that surface. Drought and pumping lower it and can dry springs.
Darcy’s law
Darcy sets specific discharge q proportional to the hydraulic head gradient: q = −K ∇h. K is hydraulic conductivity (medium and fluid). The minus means flow from high head to low. It underpins lab permeameters and well-field analysis. Strong turbulence or fractures leave the simple Darcy regime.
Aquifers and aquitards
An aquifer stores and transmits useful water. Unconfined aquifers have an open water table; confined ones sit between aquitards and may be under pressure above atmospheric. An aquitard leaks slowly; a near-impervious bed is sometimes called an aquiclude. Contamination follows the flow. Pumping rules aim to limit depletion and subsidence.
Formulas
Porosity
Between 0 and 1; also quoted as percent.
Symbols
-
V_voidvoid volume -
V_totalbulk volume
Darcy
One-dimensional. q is specific discharge.
Symbols
-
Khydraulic conductivity -
hhydraulic head
Hydraulic head (sketch)
Elevation plus pressure head; velocity head usually tiny.
Symbols
-
zelevation -
ppore pressure
Key table
| Sand vs clay | sand: connected pores, high K; clay: high n but low K |
|---|---|
| Artesian well | if the potentiometric surface is above ground, water rises on its own |
| Recharge | rain, rivers, irrigation seep down and refill aquifers |
In this field
Rocks and plate tectonics
Rock cycle, earthquakes, volcanoes, plate boundaries.
Atmosphere and weather
Layers, wind, clouds, fronts.
Oceans, climate, time
Currents, carbon, climate vs weather, relative and absolute age.
Minerals and water
Minerals and Mohs, water cycle, groundwater, soil.
Hazards and resources
Volcanoes, slides, storms; fuels and ores.
Geologic time and carbon
Relative and absolute ages, the carbon cycle.
Weathering, soil, and the water cycle
Weathering, erosion, soil profiles, and the water cycle.
Earth’s magnetic field
The dynamo, the dipole, inclination and declination, paleomagnetism and seafloor stripes.
Glaciers and sea level
Glacier budget, isostasy, sea level, and glacial deposits.
Seismic waves
P, S, and surface waves, travel-time curves, magnitude, and Earth’s interior sketch.