Aquifers, wells and radial seepage

Question 1

aquifier

Answer 1

saturated rock or soil layers that are a source of groundwater

Question 2

aquifer types

Answer 2

• unconfined
• confined
• semi-confined (leaky)
• perched

Question 3

Aquitard

Answer 3

a permeable layer with a much smaller k than an aquifer

Question 4

Aquiclude

Answer 4

an impermeable layer - no water passes

Question 4

Unconfined aquifer

Answer 4

• water table defines upper boundary of aquifer and where water will rise to in well
• exposed to atmosphere at surface of water

Question 5

Confined aquifer

Answer 5

• Groundwater isolated from atmosphere by aquiclude/aquitard at well location
• Pressure higher than atmospheric at top of aquifer — Piezometric surface is elevation where water will rise to in well and determine is a pump or valve is needed

Question 6

Perched aquifer

Answer 6

small discontinuous layer
- Location of discontinuous water table
- Exposed to atmosphere at surface
- Unsaturated conditions below

Question 7

Leaky aquifer

Answer 7

combination of confined and unconfined
- Intermediate between confined and unconfined
- Movement of water through aquitards

Question 8

well pumping

Answer 8

Creates radial seepage towards well. max drawdown at location on well - cone of depression around well - reduces water level/ piezometric surface.

Question 9

Pumping test assumptions for confined and unconfined aquifers

Answer 9

1) The well fully penetrates the water-bearing stratum
2) The soil mass is homogenous and isotropic (unlikely average assumed)
3) Darcys law is valid
4) The seepage is radial towards the well (vertical component ignored)
5) At steady state condition the hydraulic gradient is equal to the slope of the drawdown curve

Question 10

Observation wells

Answer 10

use to measure water height from base of the aquifer at two radii from centre of well –> can then accurately estimate k

Question 11

R - radius of influence

Answer 11

is the maximum radius where ground water level is effected

Question 12

what is the key difference in confined and unconfined aquifers

Answer 12

in confined aquifers, H does not change (distance away from the well doesn’t change area).
Unconfined aquifer - A=2pirh
Confined aquifer - A=2pirH

Question 13

confined aquifer recharge

Answer 13

• Groundwater enters at a source area in order to feed/recharge confined aquifer
• At that location is unconfined becomes confined when head is greater than atmospheric at top boundary

Question 14

well pumping effects

Answer 14

• Removal of water can change structure of aquifer (reduce void space, settlement of ground surface above, natural system needs to not have a net loss of volume)
• Ground settlement effects (cracking of structures, change in ground elevation)

Question 15

other well applications

Answer 15

construction dewatering - overlapping zones of influence can bring down overall water table