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Geog370Hdrlgy > Ground Water > Flashcards

Ground Water Flashcards

1
Q

Hydraulic Head

A

Measure of the mechanical energy of water at a location

- equals pressure head + elevation head

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2
Q

Elevation Head

A

Equal the elevation above some chosen reference level

- Base reference is sea level datum elevation

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3
Q

Pressure Head

A

Equals the length of the column of water above the screen

  • Common practice defines the water pressure at the free interface between air and water as zero
  • Function of density and gravity
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4
Q

Gaining Stream

A

Receives water from local, intermediate, or regional ground water flow.
- Seepage face at water table height

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5
Q

Losing Stream

A

Connected to the water table but loses water to the ground water

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6
Q

Perched Losing Stream

A

Loses water to ground water but is perched above it

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7
Q

Flow Through Stream

A

Connected to the water table where it gains water from one side of cross-section and loses it on the other side

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8
Q

Darcy’s discharge/flow equation

A

Discharge = area cross-section of flow path (length^2) x Hydraulic conductivity (1/T) x Hydraulic Gradient (Change in Hydraulic Head/distance between the 2 points)

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9
Q

Why is hydraulic conductivity relevant for calculating discharge?

A

Relevant for pore (matrix) flow in a saturated, porous medium
- lower in clays than gravels

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10
Q

Hydraulic gradient

A
  • Change in head (elevation) between 2 points at the top of the groundwater table
  • i = change in h/L (distance between 2 points)
  • Function of Hydraulic head
  • Gradient slope of the topic of the groundwater table (indicates direction of movement)
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11
Q

Darcy’s Flux Equation

A
  • Volume of fluid passing through a unit cross-sectional area of A during a unit length of time of L
  • q = Darcy’s flow (Q)/Area = L^3T^-1/L^2 = L/T
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12
Q

Three steps for finding how water moves through the subsurface

A

Step 1: Find discharge (Q)
Step 2: Find Flux, q = (Q/A (length))
Step 3: Find Effective (seepage) velocity (q/effective porosity n)
Step 4: Divide length by effective velocity

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13
Q

5 General rules for groundwater flow

A
  • Water table reflects topography
  • Equipotential contour lines perpendicular to divides, parallel to boundaries
  • Streamlines perpendicular to equipotential contour lines
  • Both horizontal and vertical flow components (down in uplands, up in lowlands)
  • Flow nets complicated by changes in geology and topography
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14
Q

GRACE

A

Satellite that can measure groundwater (gravity as a proxy for GW)

  • Gravity on Earth’s surface varies depending on material at the subsurface
  • Can’t measure soil water b/c soil water is very small value
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15
Q

How does ground water flow vertically?

A
  • Along potentiometric gradient from high to low

- i = change in hydraulic head/ change in elevation head between wells or piezometers

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16
Q

How does ground water flow horizontally?

A

horizontal i = change in h/ change in x

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17
Q

What are equipotential contours?

A

Lines of the same flow energy

- Lines of equal hydraulic head

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18
Q

What defines a flow tube?

A

Flow streamlines that are perpendicular to equipotential contour lines

19
Q

What are some issues with ground water flow nets?

A
  • Geological environment can affect flow greatly
  • Doesn’t work well in fractured rock because water will preferentially flow along fractures and not so much equipotential lines
20
Q

What are the 2 main methods that ground water is recharged naturally?

A

Infiltration and percolation

21
Q

What are the 3 ways that recharge inputs are transferred?

A
  • Flow to adjacent areas via through flow
  • Re-surface as return flow (springs) or base flow (rivers)
  • long term storage in deep aquifers
22
Q

What is an important application of GRACE?

A

It can give a draught indicator from the wetness value of earth’s subsurface over time

23
Q

How is GW in BC derived?

A

Infiltration and snowmelt

24
Q

What are some potential challenges with GW in BC

A
  • shallow and confined aquifers are vulnerable
  • flow routes, rates, and residence times are poorly understood and unmodelled
  • land use impacts changing recharge, storage, discharge
25
Q

What are some impacts on GW in BC relating to quality and quantity?

A
  • Quality: Contamination

- Quantity: Drainage diversion, artificial recharge, and withdraw

26
Q

Point Source

A
  • Easier to identify, has a single known location

- Landfills, gas stations, mines, disposal sites etc.

27
Q

Non-Point Source

A
  • Difficult to identify, protect against, fix, and legislate
  • May not have boundaries, exact source unknown
  • Agriculture, runoff of manure/fertilizer, herbicides, pesticites, urban runoff containing oil/soaps
28
Q

California Aquaduct

A
  • Artificial water feature to mitigate subsidence in california
  • Gravity driven flow from Sierra Nevada Mnts. and power to pump water to some locations
  • Power usage creates problems but getting the water is still more important
29
Q

What are the two principal sources of toxic organic chemicals in water?

A
  • Improper disposal of industrial and household wastes

- Pesticide runoff from farm fields, forests, roads, golf courses, lawns, etc.

30
Q

Walkerton Ontario Incident

A
  • E. coli from cow manure spread into cracked water well due to prolonged heavy rains
  • Shared responsibility of Government and farms/company (privatization, incompetence, unconfined manure, public deception, etc.)
    ~1500 illnesses and fatalities
31
Q

What conditions are needed for effective recharge?

A
  • Water needs to move through the vadose zone
  • Water needs to move in the aquifer, away from infiltration site so as to raise water build up of groundwater mound or ridge
32
Q

What characteristics are desirable for infiltration recharge systems?

A
  • Permeable soil
  • Permeable vadose zone free of clay
  • Confined aquifer that is permeable and thick to avoid groundwater mounds
  • GW table must be deep (at least 10m below surface)
33
Q

What are suitable sites for infiltration recharge?

A
  • Flood plains of rivers, sand dunes, alluvial fans, permeable vadose zones, glacial outwash plains
  • higher concentration of larger soil particles (faster infiltration)
34
Q

What is the solution to building an infiltration basin where there is a clayey surface?

A
  • Surface clay can be removed, infiltration basin can then be built in underlying permeable deposits
35
Q

What is artificial recharge?

A
  • Artificially increasing the amount of water that enters a groundwater reservoir
36
Q

What is artificial recharge used for?

A
  • Used for waste disposal, secondary oil recovery, land subsidence problems, and water resource management
37
Q

How is artificial recharge engineered?

A
  • Injection wells (like a reverse well)
  • Infiltration ponds
  • Drainage ditches
38
Q

Infiltration Basin

A
  • Collects water and gives it opportunity to infiltrate GW in optimal places (above unconfined aquifer)
  • Goal is to build up mound of GW to create pressure difference and stimulate flow to a recovery area
39
Q

What happens if a groundwater mound in an infiltration basin gets too big?

A
  • The water won’t infiltrate further

- Water flow can reverse direction and won’t feed the recovery area

40
Q

What are some purposes of artificial recharge?

A
  • Conserve and dispose of runoff-flood water
  • Supplement natural recharge
  • Reduce or balance salt water intrusion
  • Suppress (but can’t reverse) subsidence
  • Store water in off-seasons to use during growing season
  • Geothermal applications
  • Remove suspended solids by filtration through ground
41
Q

Saltwater Intrusion

A
  • Marine sediments have saltwater in their pores and high hydraulic head that can push saltwater inland
  • Salt water is more dense and can intrude further under fresh GW
  • Pumping will exacerbate this and can draw saltwater in
42
Q

How can saltwater intrusion be combated?

A
  • Artificial recharge of fresh water
  • Take uncontaminated fresh water from wells further away inland and channel to contaminated we and recharge to hold off salt intrusion
43
Q

Iraq and Persian Gulf salt water intrusion problem

A
  • Wetlands reducing due to climate change, drought, land use changes (damming, agriculture)
  • Led to salt water intrusion from gulf
44
Q

Ground subsidence

A
  • land sinking as GW is drawn out faster than it can recharge
  • Venice due to pumping out of GW but is exacerbated by climate change and sea level rise

Geog370Hdrlgy (9 decks)

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