Transcript GWD2_PP_Characteristics of groundwater systems

WASH Cluster – Emergency Training
GWD
Groundwater Development
and Drilling
Session 2
Characteristics of
Groundwater Systems
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Debunking Groundwater Myths
• You can’t drill anywhere and get groundwater
• Groundwater is not static – it flows through
pores in rocks
• Aquifers are not underground rivers – only really
in caves
• Not all groundwater obtained will be drinkable
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Interconnection and Flow
Interconnection of the pores in rock or soil allows
water to:
•enter the groundwater,
•be stored in an aquifer below ground,
•flow through the subsurface,
•discharge naturally to a river, swamp, spring
or the sea,
•be recovered in a well or bore
This all relates to the geology of the area and the
different types of soil and rock.
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Groundwater Recharge
Recharge is the process by
which water gets into an aquifer.
Recharge quiz
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Rainfall to Groundwater
RAINFALL
Some rainfall runs off
across ground surface,
Water entering the soil is
used by plants
(Evapotranspiration)
Some evaporates at the
surface
Remainder enters soil
Storage of water in the
soil (unsaturated
zone)
Becoming wetter
closer to water table
Water that isn’t used or
stored in the soil seeps
down below water table
– becomes Groundwater
- known as recharge
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Aquifer
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Groundwater Flow
• Groundwater flows through an aquifer
under an hydraulic gradient
• effectively the difference in potential
energy of the water
– (hydrogeologists and groundwater engineers refer to the
potential as the “head”).
• Groundwater flows from areas of higher
to lower head
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EXERCISE 1
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EXERCISE 1 Cross section
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Rate of groundwater flow
Rate of groundwater flow is a function of
the hydraulic gradient
Flow through the soil and within aquifers
occurs by different pathways through
different types of rocks
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Permeability
Rock permeability governs the ease at which groundwater flows
K (cm/s)
10²
10
Permeability
0
10 =1
10
−1
10
−2
Pervious
Aquifer potential
Unconsolidated
sediments / Soils
1
10
−4
10
−5
10
−6
Clean Sand or Sand &
Gravel
10
−7
10
−8
10
−9
10
−10
Impervious
Poor
None
Very Fine Sand, Silt, Loess,
Loam
Peat
Consolidated Rocks*
10
Semi-Pervious
Good
Clean
Gravel
−3
Stratified clay
Oil Reservoir Rocks
Sandstone
Unweathered Clay
Limestone,
Dolomite
Granite
after Bear and Verruijt, 1987
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Impact on well supply
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After: Don Armstrong, Australian Groundwater School,
AMF course notes 1987 (unpublished)
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Aquifers and Aquitards
Aquifer: A layer of soil or rock that has
relatively higher porosity and
permeability than the surrounding
layers, enabling usable quantities
of water to be extracted.
Aquitard:
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A layer of soil or rock that has
relatively lower porosity and/or
permeability than the surrounding
layers, limiting the movement of
groundwater through it and the
capacity
to
extract
useable
quantities of water.
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Activity
•
•
•
•
•
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Fill the kettle
Plug spout with cork
Keep filling
Use different things to plug spout
Time how long it takes for water to come
out
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Confined and Unconfined aquifers
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Groundwater Discharge
• Water flows preferentially through higher
permeability zones
• Water will move to areas that have least
restriction to flow
• Groundwater will discharge at a point
where constraints are released
DISCHARGE QUIZ
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Discharge sites
after Bear and Verruijt, 1987
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EXERCISE 2 cross section
Seepage - springs or soaks
Seepage - springs or soaks
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Climatic influence on
Groundwater occurrence
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Springs
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Interaction with Rivers
• Water table above
river
• Groundwater
discharge to river
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Interaction with Rivers
High flow in the
river can lead to
seepage into the
groundwater
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• River above the
water table
• Downward seepage
into groundwater
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Coastal Zones
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Variability of Water Quality
Groundwater salinity can vary within the same aquifer
From Elrail et al 2009
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