Download Water Occurrence in Arid Areas

Water Occurrence in Arid Areas
Robert Read
AIG Conference 2015

After English
et al. 2012
Not much moves without underground water
 Towns
Tourism
 Pastoral
Mining
Characteristics of the Arid Zone

Erratic rainfall

For stations with long
records wettest 5
consecutive years 9
to 10 times as much
rainfall as driest 5
consecutive years.

Mostly low relief

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Relatively well
vegetated.
Plants that are
very effective at
scavenging
water.
Aquifers

Widespread, but not always where needed.

Variable water quality.
Recharge

Average low to very low .

Erratic, often in La Ninya years

Characteristic “saw-tooth” hydrographs
RN 5586
548
500
400
300
546
200
545
100
544
0
1/3/65
12/17/75
11/29/86
11/11/97
10/24/08
Monthly r ainfall mm
WL in m AHD
547
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Average water levels deeper than in humid
areas.
Overall dissolved solids are higher, ranging up
to hypersaline in salt lake areas.
Ti-Tree Basin and a catchment in N Oman
Wadi Zarub
Ti-Tree Basin and a catchment in N Oman
Ti-Tree Basin and a catchment in N Oman
Recharge and Vegetation
Plants may be able to take water from about 20 m
depth.
Mulga, Acacia aneura



Pre-dawn leaf
potentials as low as 7 Mpa.
Taking water from
silty soils at shallow
depth.
Water use declines in
dry periods. (95%
decline in Ti-Tree Basin,
April to Nov 2007.)

Evapotranspiration 0.5 to
0.2 mm/d
Bloodwood, Corymbia opaca





Like mulga perseveres
through drought.
Water used does not
decline much in dry
periods.
Pre-dawn leaf potential 2 Mpa.
Appears to be taking
water from about 6 m,
deep in the unsaturated
zone.
Water use remains near
constant in dry periods
Eucalyptus camaldulensis
Grows along creek lines
Pre-dawn leaf potential as low as -1.2 Mpa
Evapotranspiration 2.4 mm/day
How does any recharge water get
down?
How does any recharge water get
down?


Recharge is probably
largely due to macropermeability.
Burrows (e.g.
termites) in the top
few m, fractures and
fissures below that.
Termites, lizards,
scorpions, small
mammals all burrow.
Kay Kessing “Spinifex
Times”


Oral account of a well-sinker finding a “native
rat” burrow at 6 m in weathered crystalline rock.
Some small caves in calcrete reach the water
table.
Effect of geology

Soil over
silty sand,
water may
get down
through
macropores,
but will
move very
slowly
below.

Soil over
fissured
calcrete,
more
infiltrating
water will
get down
to the
water
table
Ti-Tree Basin

Lower salinity water
not under creeks, but
under areas of
shallow calcrete.
• This catchment
discharges from a
permanent spring,
easy to estimate
recharge.
Rapid direct recharge
of about of 25 mm/y,
about 10% of rainfall.
Aquifer types
Broadly the aquifers in the arid zone can be
classified into four types, though the
classifications overlap:

Major basins, with mostly primary porosity.

Palaeo-channels and epeirogenic basins


Fractured and weathered sedimentary rocks,
usually older than the major basins.
Fractured and weathered crystalline rocks.
Palaeovalleys
Palaeovalleys range in age from Quaternary (Alice
Springs Town Basin) to possibly Permian.
Aquifers in them include coarse fluvial sands
and fissured silcrete and calcrete. They may be
evident in outcrop and imagery or completely
obscured by dune fields.

The map above is the major palaeovalleys,
there are many more minor ones down to
channels 50 m wide with bores that provide
small stock supplies.
Pipalyatjara

Palaeovalley, far NW
of SA, clearly marked
by calcrete.

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Eastern end of the
Mann Range, 125 km
from previous.
Palaeovalley found by
blind chance.
Older Sedimentary Basins
Generally the rocks in older (say mid Palaeozoic)
sedimentary basins will have low primary
permeability. They are good aquifers when
permeability and porosity have been enhanced
by weathering. Aquifers are localised where
topography and structural features have allowed
the greatest weathering. Yields may range from
low to 100 L/s for the Alice Springs town supply.
Weathering history is
very important for the
formation of aquifers.


Downhole optical scan
of bore at Roe Creek,
showing large fissure.
Limestones, dolomites
vary hugely along
strike depending on
solution fissures.
Example

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Siltstone with magnesite pellets.
In some locations magnesite had been
dissolved out leaving a very permeable rock.
Fractured crystalline rocks
These are the most frustrating. The deep water
tables result in shallow fractures that might yield
water in more humid areas being dry. In
general, yields are only enough for pastoral and
small communities. On rare occasions aquifers
in highly weathered rocks yield up to 90 L/s in
the short term.
Evaluating fractured rock aquifers

This is a nightmare.

Pump test evaluation is difficult.


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Some tests show Theis or Jacob straight line
responses.
Some one dimensional flow responses.
Drawdown plotted against arithmetic square
root of time is close to a straight line.
Fractional dimensions (Barker, 1988) often
appear to fit.

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Extrapolating to the long term is problematic.
For single hole tests square root of time plots (1
dimensional flow) will generally give reasonable
estimates for about 6 months to 2 years.
Only experience will show long term yields.
Engawala


Community, population of 140
supplied by production bores in a
fault zone in gneiss with generally
low permeability.
Despite the apparently limited
storage and the presence of saline
water nearby the supply has been
maintained for 40 years.
Engawala
Engawala
Yuendumu

Community of about 800 people, 300 km NW of
Alice Springs.

Designated growth centre.

Average age 25.
Yuendumu
Yuendumu


Historically developed on small fracture aquifers
in crystalline rock , long since outgrown.
Current water supply is from fractured Devonian
and Carboniferous sandstones in the Ngalia
Basin.

Recharge is very small.
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Aquifers have progressively dewatered.

Will require development of further aquifers in
the future.
Yuendumu
Yuendumu
Roe Creek Borefield, Alice Springs

After various attempts
at digital modelling
best results by
plotting water level
against cumulative
extraction.
Water Quality
Even in areas where salinity is low, water may not
be suitable for human consumption because of
high nitrates and or fluorides.
Nitrates
Nitrates are largely associated with direct
recharge, and appear to originate from termites
which are very common in the arid zone.


Surfer plot of all nitrate
values in the NT.
Nitrates stop at the
edge of the Barkly
blacksoil where no
termites live.
Fluorides
Fluorides are common in hot areas throughout the
world, especially in crystalline rocks with fluoride
containing minerals.

The arid areas offer enough challenges to keep
many more generations of hydrogeologists
entertained.