Groundwater Flashcards
1
Q
porosity
A
- free space in the rock
- Depends on grain size and sorting
- Eg. In conglomerates, there are large spaces/pores; in shales you’ll have small pores
- Poorly sorted rocks have smaller porosity because small grains fill in the gaps between large grains
- Metamorphic rocks like gneiss and igneous rocks like granite have no/low porosity -> crystals fill the spaces that would have been pores
2
Q
primary vs. secondary porosity
A
- Primary porosity: present when a rock forms
- Secondary porosity: open space formed after a rock forms, due to:
- Joints, fractures (eg. Granite)
- Solution (eg. Limestone)
3
Q
permeability
A
- how easily water can move through the rocks
- Depends on pore size and connections between pores
- Ie. Basalt has large pores, but they aren’t connected, so low permeability
- Surface tension causes water to stick to the surfaces of grains, so it will only flow through the larger pore spaces
4
Q
surface tension on wet soil particles
A
- Hygroscopic: absorbed (on inside of grain) or adsorbed (stuck on surface) water
- Capillary water: held to the soil particle by surface tension
- Gravitational water: flows downwards under influence of gravity
5
Q
water cycle
A
- percolation/collection (groundwater)
- evaporation (due to sun)
- condensation (in clouds)
- precipitation (rain)
… and cycle repeats
6
Q
acquifer
A
- body of permeable rock or sediment, saturated with groundwater and through which the groundwater moves
- form perched water table in pockets of clay in heterogenous materials
7
Q
good acquifers
A
sand, gravel, sandstone, limestone (in humid climates only)
8
Q
water table
A
- level below which rock or sediment is saturated with water
- Mimics topography
- Level varies with the season
9
Q
how groundwater flows in acquifers
A
- Flow is from high elevation to low elevation and from high pressure to low pressure (downslope and down pressure gradient)
- Rate of flow depends on both porosity and permeability
- Note seasonal variations in the water table in homogenous rock -> wet periods = swollen rivers, dry periods = shallow rivers, dried-up rivers
10
Q
acquitards
A
- rock or sediment that makes a poor aquifer (not permeable)
- Ex: mudstone, shale, most igneous and metamorphic rocks
11
Q
wells
A
- Controlled by permeability -> you want wells in highly permeable rocks
- Controlled by secondary porosity -> you want your well to intersect lots of water-bearing fractures
- Effect of pumping on the water table -> drawdown of water creates a cone of depression of the land
- Confined aquifer (in heterogenous sedimentary rocks)
- Artesian wells: flow without pumping due to pressure created by impermeable confining layer
12
Q
geological activity and groundwater (3 kinds of activity)
A
- Cementation: groundwater binds sediments together (ex. Sand vs. Sandstone)
- Replacement: replacing a substance with another one (ex. Petrified wood)
- Solution: dissolving carbonate rock (usually in humid climates), creating caves and karstic landscapes
13
Q
groundwater as a resource
A
- Domestic, industry, agriculture uses it as a resource
- Is it a renewable resource?
- Must consider time scale (how long it takes to renew), and supply vs. Demand
- The deeper the water is in the aquifer, the longer it takes to renew
14
Q
recharge vs. discharge area
A
- Recharge area = ground surface where water soaks into the ground
- Discharge area = where the water table is at the ground surface and water flows out of the ground
15
Q
problems when demand is greater than supply
A
- Lower water table = deeper wells, more money
- Land subsidence = flooding, structural damage
- Loss of recharge area due to urbanization
- Saltwater intrusion (In near coastal regions, saltwater intrusion makes well water non-potable)
- Pollution (toxic waste, sewage, pesticides, herbicides, fertilizer, heat)
