Groundwater layers
- unconfined aquifers
- impermeable rock (aquiclude)
- confined aquifer
- impermeable rock (aquiclude)
Aquifers
-porous rock units that have enough permeability to supply water at a rate useful to humans
Aquiclude
A unit with permeability low enough to form a barrier to flow
Unconfined aquifer
- has a water table everywhere
- upper surface of aquifer is water table
Confined aquifer
Bounded above by an aquiclude
Wells in unconfined aquifers
-if rate of withdrawal (pumping) exceeds rate of local groundwater flow, can get cone of depression in the water table surrounding the well
Wells in confined (artesian) aquifers
- water table only in recharge area
- water within aquifer is under pressure
- water level in lower well rises due to hydraulic head
Solution and mineral precipitation from groundwater
- solution by groundwater
- caves, sinkholes, and karst
- deposition from groundwater
- stalagmites, stalactites
- cement
Solution by groundwater
- depends on water chemistry, especially pH, groundwater may dissolve minerals
- CO2 dissolved in rain water makes it slightly acidic
- calcite (main constituent of limestone) is particularly soluble in acidic water
Caves, sinkholes, and karst
- groundwater solution creates distinctive landscape features in limestone and other soluble rock types
- caves are very large pore spaces formed by solution
- sinkholes form where the roof of a cave collapses
- karst is characterized by numerous sinkholes and/or pillars of rock isolated by solution
Deposition from groundwater
-dissolved mineral ions may be deposited in response to:
- Evaporation of groundwater in caves
- Other changes in chemistry
Stalagmites, stalactites
- mineral deposits (typically calcite) form in air filled portions of caves
- stalactites resemble icicles hanging from ceiling
- stalagmites are corresponding pillars growing from cave floor
Cement
- more abundant than stalagmites and stalactites
- are mineral deposited in small pore spaces
- cement may convert:
- Loose hand into hard sandstone
- An aquifer into an aquiclude
- A potential oil reservoir into a non porous non reservoir
Huge economic significance
Overland flow and stream flow
- rainfall and melt-water flow down slope under gravity
- initial sheet-like overland flow
- erosion creates channels in which flow is concentrated
Stream flow
- base flow (fed by groundwater)
- no base flow=ephemeral
- with base flow=perennial
- storm flow (overland flow into channel from precipitation)
Characteristics of stream systems
- streams are organized in river systems
- small streams (tributaries) typically merge downstream
- area drained by a major river and its tributaries is a drainage basin
- drainage basins are separated by drainage divides
