chapter 11 Flashcards
(18 cards)
What is the water composition on Earth?
96% saline water 4% fresh water divided into: - 2.97% glaciers and polar ice - 1.05% underground water - 0.009% lakes and rivers - 0.0001 biosphere
What is the primary source of groundwater?
Rain
List the factors that affect infiltration of ground with water.
a) intensity of the rain
b) steepness of the slope
c) nature of soil (porous or not)
d) type and amount of vegetation (holds water)
Define water table.
Limit between saturated and unsaturated zones. Water infiltrates, penetrates downwards through the unsaturated zone then into the saturated zone.
Distinguish between saturated and unsaturated zones, and define their boundary.
Unsaturated zone: portion of soil and rock in the subsurface in which open spaces are filled with air and water (vadose zone)
Saturated zone: zone in which pore spaces are completely filled with water
Water table: boundary btw them. It’s the upper limit of the zone of saturation.
Factors affecting the shape of the water table:
Variations in rainfall
Variations in permeability in the aquifer
Topography
How are streams and lakes formed?
When water table intersects with surface topography at low elevations, and streams and lakes occupy areas where water table is above land surface.
Define and describe porosity.
Porosity is the percentage of the total volume of rock or sediment that is pore space, (the voids or openings in sediment or bedrock).
Porosity depends on:
a. Size: coarse grains-high porosity, fine grain-low porosity
b. Shape of grains (angular-lower porosity, rounded-higher porosity)
c. Packing: less packing provides higher porosity
d. Degree of sorting: well sorted material have higher porosity than non-sorted.
e. Cementing materials: porosity is reduced in sediments of various sizes as finer particles fill openings
Primary porosity: pore space between grains (during rock deposition)
Secondary porosity: develops due to fractures & fissures
Example of low porosity: limestone, shale, cemented sandstone
Example of high porosity: gravel, clay
Define permeability.
Permeability is the ability to transmit water (allow water to move through). It is the interconnection between pores.
Example: clay (high porosity) mineral in shale has high ability to store water, but due to its small pore spaces (low permeability), the water is unable to move
Differentiate between aquifers and aquicludes
Aquifers are layers of highly permeable rocks (ex. sandstone) with relatively high porosity that can store and transmit large quantities of groundwater.
Aquicludes are low permeability beds that are able to store water (high porosity) but do not let water flow through. (ex. shale)
What are the types of aquifers?
Confined aquifer is bounded above and below by impermeable layers such as shale or clay. Such impermeable layers (like salt or shale) are called known as aquiclude.
Unconfined aquifer is one that is not separated from the zone of aeration by an aquiclude.
Perched aquifer occurs when as water is percolating downward, a portion of it intercepted by the aquiclude, creating localized zone of saturation.
How are streams related to groundwater?
- Effluent streams: its flow is maintained by a steady supply of groundwater.
- Influent streams: streams in dry regions that provide water to the water table (by downward seepage from the stream).
Define water wells.
- Are the most common device for removing groundwater.
- When the water is over-pumped from a well, it produces a depression in the water table known as a cone of depression occurring around the well.
- Over-pumping causes depression in the water table at a large area sometimes causing nearby wells to become dry.
- Artesian well: When drilling water wells, if ground water rises naturally in a well above the level where it was initially found:
1. Water occurs in an inclined aquifer that has one end exposed at the surface where it can recharge water.
2. That the aquifer is overlain & underlain by two aquicludes to prevent the water from escaping and water will rise under its own pressure.
What happens to the water table over the seasons?
Low water table in the summer (no rain)
High water table in the winter (rain)
Describe the geologic work of groundwater.
Primary work is dissolution dissolution of soluble rock such as limestone that underlie the Earth’s surface.
Limestone (insoluble in pure water) can be dissolved by water containing weak carbonic acid (H2CO3) as most natural water dissolves carbon dioxide from air & from decaying plants.
CO2 + H2O => H2CO3 carbonic acid
The carbonic acid reacts with calcite to form calcium bicarbonat (a soluble material found in solution), forming limestone caverns.
CaCO3 + H.HCO3 => 2(HCO3)- + Ca++
Ca bicarbonate (soluble material)
Chemical composition: Ca(HCO3)2
These features are formed below the water table in zone of saturation where groundwater follows bedding planes, joints that dissolve limestone slowly, to form cavities & caverns, and deposit it as dripstones such as stalactites, stalagmites, and calcite columns
All the above features are related to the Karst Topography; an irregular terrain punctuated with many depressions (sinkholes) that form when roof of a cavern collapses under its own weight.
Define karst topography
irregular terrain formed due to differential dissolution of calcite. Different types of karst features: Doline and Karren
What is the depositional work of groundwater?
In the vandose zone (unstaturated zone), caclite cacan precipitate again to form: Cave features: Speleothems e.g., Stalactites & Stalagmites.
2(HCO3)- + Ca2+ → CaCO3 + H2O + CO2
Example: Afqa cave
What is geothermal energy and where can we obtain it?
It is the natural steam (from natural geysers) used for power generation.
The underground supplies of superheated steam come from:
1. Hot springs: Natural rising of hot water, that was heated as a result of groundwater circulation at great depths. The other source of heat is also cooling of igneous (magmatic) rocks.
- Geysers: are intermittent hot springs, where water is ejected at various intervals (e.g., the Old Faithful geyser in Yellowstone, national Park, US, that erupts once each hour).
