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practice > Groundwater > Flashcards

Groundwater Flashcards

1
Q

Groundwater recharge

A

precipitation (comes from high inland reaches)

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2
Q

Losing stream

A

Sits above the water table. May exit into groundwater system

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3
Q

Gaining stream

A

Sits below water table, receives groundwater

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4
Q

Groundwater flow

A
  • downhill through aquifers (regions of high porosity)
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5
Q

Aquifers

A

Regions of high porosity

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6
Q

Aquitards

A

Inhibit the flow of water

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7
Q

Confined aquifer

A

aquitard above it, can cause water to flow uphill

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8
Q

Unconfined aquifer

A

connected to the water table, no aquitard above it

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9
Q

Ocean discharge

A

can discharge into ocean or a seawater-freshwater interface can form

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10
Q

Perched aquifer

A

water gets stranded and cant flow out

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11
Q

Conceptual models

A
  • topography
  • aquifers
  • aquitards
  • water table
  • flow direction
  • piezometric surfaces
  • position of wells
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12
Q

Aquifer material

A

gravels, limestone

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13
Q

Aquitard material

A

clay, silt, bedrock, glacial till, shale

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14
Q

Seepage face

A

When water in an aquifer breaks the surface

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15
Q

Protecting a spring for water supply

A

Horizontal, vertical

  • slanted covers, rock entrance, overflow, outflow
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16
Q

Artificial springs (groundwater well)

A
  • recharge
  • discharge
  • ground surface
  • piezometric surface
  • piezometers
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17
Q

How to get ground water

A
  1. drill a hole
  2. keep bad water out (casing, grout)
  3. let good water in (screen)
  4. get good water out (punmp)
  5. improve (sanitary seal, gravel pack, sand trap)
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18
Q

Groundwater depletion

A

pump lots of water out of wells, piezometric surface depresses, springs dry up, well levels and yields affected

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19
Q

Groundwater head

A

elevation that water will rise to or settle at in a semi-infinite straw

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20
Q

Contours

A

fitted through measurements of equal head

water flows perpendicualr to contours

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21
Q

K (Darcys law)

A

conductivity [m/s]

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22
Q

dh/dx

A

hydraulic gradient

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23
Q

A (Darcys law)

A

cross-sectional area

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24
Q

Water table in an unconfined aquifer

A

piezometric surface

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25
Q

Leakage

A

underlying aquifer is imperfectly confined. Direction depends on difference in head between layers

26
Q

Storage

A

The amount of water in a groundwater system that we can extract

27
Q

ΔV (storativity)

A

volume of water removed

28
Q

Δh (storativity)

A

drawdown (m)

29
Q

A (storativity)

A

area over which drawdown occurs

30
Q

Extracting water from unconfined aquifer

A

easy (drinking while letting water in)

31
Q

Extracting water from confined aquifer

A

hard, need large suction forces

32
Q

Storativity

A

the volume of water that is extracted for each unit decline, for each unit area that decline occurs over

33
Q

Storativity in an unconfined aquifer

A

= porosity = specific yield

34
Q

Storativity in a confined aquifer

A

= thickness x specific storage or elastic yield

35
Q

Magnitude of storativity

A

Confined aquifer Ss small, need large Δh to produce small q

Unconfined Sy large, need small Δh to produce large q

36
Q

Water balance (without pumping)

A

Recharge
- precipitation
- upgradient groundwater

Discharge
- spring/stream
- downgradient groundwater

37
Q

Water balance changes with pumping

A
  • steeper hydraulic gradient = larger upgradient recharge
  • shallower hydraulic gradient = smaller downgradient recharge
  • reversal to spring (discharge to recharge)
38
Q

Why use welltests

A
  • can’t look at groundwater directly
  • need to estimate aquifer properties
  • understand aquifer geometry
39
Q

Transmissivity eqn

A

T=Kb

40
Q

Pump test

A
  • temporary drawdown is intentionally induced to measure aquifer properties
  • drawdown caused by pumping at fixed rate or series of fixed steps
  • analyse using theis solution
41
Q

Pump test solution

A
  1. plot drawdown on semi-log graph paper
  2. draw line through linear portion (start is full theis soln, end suggests flow boundary)
  3. use gradient and y-intercept (=log(1))
42
Q

When is Theis solution not appropriate

A
  1. confined aquifer with leakage
    method: Hantush-Jacob solution
    parameters: c = hydraulic resistance of aquitard
  2. Unconfined aquifer
    method: Neuman-Moench semi analytical soln
    parameter: Sy= specific yield
43
Q

Physical meaning of super position

A

drawdown evolution around a well is linear so it can be added for wells at different locations- including imaginary wells

44
Q

Flow barriers and recharge

A

lateral constraints on drawdown- solve by superposition of image wells

45
Q

Well performance

A
  1. Sustainable yield
  2. well efficiency
46
Q

Rorabaugh method

A

y = mx + c

  1. choose value of β that gives linear relationship between x and y
  2. use β to find T and Sy by guessing S and checking T (using t[test])
  3. Then estimate Qsus that limits drawdown to the maximum value (using t[pump])
47
Q

Well components

A
  • sanitary seal
  • borehole
  • grout
  • pump
  • pump intake
  • screen
  • casing
  • sandtrap
  • gravel pack
48
Q

Drilling

A

old: percussion driven steel casing into ground

new: rotary cutting drill driven by mud circulation. Mud returns cuttings to surface for analysis

49
Q

Screen

A

Open section of well, prevents particle entry, minimise entrance velocity

50
Q

Screen slot size

A

Small enough to exclude particle sizes without causing entrance velocity to exceed requirements

51
Q

Gravel pack

A
  • removes fines around the well
52
Q

Screen length

A

as long as possible BUT need to avoid drawdown, deep enough to avoid surface contamination and below top of gravel pack

53
Q

Pump selection

A

Need adequate flow rate and head

54
Q

Other well improvements

A

grout
- cement between casing and borehole (prevents contamination and anchors)

sandtrap
- empty section below screen for sand and silt to settle

sanitary seal
- prevent contaminants entering the well, provides access to well

55
Q

Wellfield design issue

A

Well interface (overlapping cones of depression)

56
Q

Wellfield design goal

A

Use as few pumps as possible with minimum drawdown to meet total pumping requirements

57
Q

Source protection

A

prevent contaminated water reaching well by measuring time of travel

58
Q

Source protection zones

A

Immediate zone (well head), middle zone (microbial viral attenuation), outer zone (persistent pollutants)

59
Q

Time of travel contour lines

A

asymmetric (more uphill than downhill)

60
Q

Adquate time of travel

A

50-100 days

61
Q

c meaning

A

celerity- translation of wave while holding shape constant

62
Q

D1 meaning

A

diffusion constant- rate at which wave spreads out

practice (3 decks)

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