What Is an Aquifer? Not a Lake, Not a River, Not a Cave [older version]

⚠️ A CORRECTED VERSION OF THIS VIDEO IS AVAILABLE Some figures in this cut had a layout fault. Everything else is unchanged. Watch this instead:    • What Is an Aquifer? Not a Lake, Not a Rive...   — — — Ask anyone to picture an aquifer and they draw a cave or an underground river. That picture is wrong almost everywhere on Earth, and correcting it carefully is where hydrogeology starts. This video covers: water in the pore space, and why porosity alone never makes an aquifer; the aquifer / aquitard / aquiclude / aquifuge family; the unsaturated zone, the capillary fringe and the exact definition of the water table; unconfined versus confined aquifers and the potentiometric surface; artesian and flowing artesian wells; why specific yield and storativity differ by a thousand times, and what that does to the cone of depression; perched and leaky aquifers; and then the four geological families that actually make aquifers — sand and gravel, sandstone, limestone and dolomite, and fractured crystalline and volcanic rock. Ends with transmissivity and how to read a real site. CHAPTERS 0:00 There is no cave down there 1:04 What is down there: pore space, not caves 2:05 Porosity is not permeability 3:20 Hydraulic head, gradient and Darcy's law 4:41 Unconfined, confined, artesian, perched 7:47 Specific yield vs storativity 9:48 Which rocks make good aquifers 12:37 Reading a site: K, thickness, transmissivity 13:49 Ordinary rock, well connected NEXT IN THIS SERIES Porosity vs Permeability: Why a Rock Won't Give Up Its Water    • Porosity vs Permeability: Why a Rock Won't...   SUBTITLES English, Español, हिन्दी, 日本語, 한국어 and 中文（简体） — choose one from the CC button. SOURCES FOR THE NUMBERS ON SCREEN *Freeze, R. A. & Cherry, J. A. (1979),* *Groundwater*, Prentice-Hall — Table 2.2 (hydraulic conductivity ranges) and Table 2.4 (porosity ranges). These are the ranges plotted in the comparison figure, covering gravel, clean sand, silt, clay, karst limestone, sandstone, permeable basalt, and fractured and unfractured granite. Specific yield 0.01–0.30 and storativity 10⁻⁵–10⁻³ are the standard textbook ranges for unconfined and confined aquifers respectively. The aquifer definition follows the standard form: a saturated geological unit permeable enough to yield significant quantities of water to wells and springs under ordinary hydraulic gradients. NOTES ON WHAT IS SCHEMATIC All nine analysis figures (`figures/a1`–`a9`) were drawn for this video by `make_diagrams.py`. The cross-sections are schematic; only the property ranges in `a9-ranges.png` plot published values. The litres-per-square-metre figures in `a6-storage.png` are computed from the mid-points of the quoted ranges and are labeled as such on screen. IMAGE CREDITS (WIKIMEDIA COMMONS) | File | Subject | License | |---|---|---| | Bore Hole | a borehole | CC BY-SA 4.0 | | Artesian well Virttaa | a flowing artesian well, Finland | CC BY-SA 3.0 | | Gravel extraction site in Muurame | glaciofluvial sand and gravel, Finland | CC BY-SA 3.0 | | Bentheimer-Sandstein | Bentheim Sandstone | Public domain | | Angitis source, outside | karst spring from limestone, Greece | CC BY-SA 4.0 | | Daepo Jusangjeolli Cliff 01 | columnar basalt, Jeju | CC0 | Full metadata in `credits.json`. #hydrogeology #groundwater #aquifer