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4 GROUNDWATER Authors: Jim Kellett, Hashim Carey and Ian Mullen

This chapter describes the application of the AEM survey to mapping and understanding the groundwater resources of the lower Macquarie River Valley.

4.1 Introduction Groundwater is the medium for transporting salt in the saturated zone. When water tables rise in

response to increases in recharge, from both natural and anthropogenic causes, dissolved salt is

transported in the groundwater and precipitated in the newly saturated ground and in its capillary zone.

Damage to vegetation occurs if this salt is transported to, and able to accumulate in sufficiently high

concentration in, the root zone. Visible land salinisation, often described in Australia as ‘dryland

salinity’, occurs when the water table rises close to the ground surface. It is therefore important to

know the depth and configuration of the water table and its salinity, and to understand the processes

that may affect its future behaviour. With this in mind, stakeholders from the Central West Catchment

Management Committee and NSW Government representatives on the project Steering Committee

initially developed a number of generic land and water management issues relevant to the lower

Macquarie River Valley that the airborne survey should be designed to address (Kellett et al. 2006).

The principal objective underlying these issues is to understand aqueous salt transport in the lower

Macquarie River Valley, in particular salt loads which may be produced from irrigation activities in

the south of the flight area, and any potential effect on the Macquarie Marshes in the northern (down

gradient) part of the flight area. During the course of the investigation, several meetings were held

with stakeholders and the following subsidiary groundwater-related issues arose upon which it was

hoped the AEM may be able to inform:

1. The lower slopes and plain between Narromine and Dandaloo (on the Bogan River) are

underlain by a buried palaeovalley (the ‘Southern Palaeovalley’) which contains a high-

yielding basal alluvial aquifer. Between 20 000 and 30 000 Megalitres a year of good quality

(EC < 1000 µS/cm) groundwater is extracted from this aquifer for irrigation. Prior to the

survey, the configuration of the Southern Palaeovalley was reasonably well known east of

‘Waterloo’ but was virtually unknown to the west. The question was posed: ‘What is the

western configuration of the Southern Palaeovalley and does it swing north and discharge

ultimately to the Macquarie Marshes?’ The six western fingers (Figure 4.1) were added to the

AEM flight area specifically to answer the second part of this question.

2. With declining flows in the Macquarie River over the past several years, attention from

irrigators has focussed on the possibility of obtaining alternative groundwater supplies. The

question on groundwater resource assessment was: ‘Can the AEM reveal any previously

unknown high quality groundwater resources which may be suitable for irrigation?’

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3. The Macquarie Marshes overlie the Great Artesian Basin (GAB). Bores in the vicinity of the

marshes which are deep enough to intersect the Pilliga Sandstone yield free flowing artesian

groundwater. There has long been speculation that the marshes may derive some of their water

from upward leakage from the Pilliga Sandstone. Hence the question to be addressed was: ‘Is

there any indication from the AEM that there is upward leakage to the Macquarie Marshes

from the Pilliga Sandstone?’

4. Significant flow losses from the Macquarie River in the reach between Narromine and Warren

have been consistently recorded since river discharge measurements began in 1901. A recent

hydrologic study by Keshwan (1995) identified the kink in the river at Buddah (about 20

kilometres north of Narromine) as being the most likely area of flow loss. During high river

stage, water is thought to flow from the river into the alluvium, and ultimately to the Pilliga

Sandstone aquifer. Thus the question was: ‘Can the AEM identify the location(s) where the

Macquarie River loses flow and recharges the aquifers between Narromine and Warren?’

5. The Macquarie River at Warren provides replenishment flows for Marra Creek and the Bogan

River of up to 25 Gigalitres per year. Because of the unreliability of the Bogan River (it only

flows after heavy rain and then dries back into a series of waterholes), water is transferred

from an off-take on Gunningbar Creek along the Albert Priest Channel (an open channel about

62 kilometres long) to the weir pool on the Bogan River at Nyngan. This water provides town

water supplies for Nyngan and Cobar, and three mines, however, according to Cobar and

Bogan Councils, at least 50 per cent of the water is lost in transit along the Albert Priest

Channel. Flow losses are attributed to evaporation, channel seepage and/or unallocated losses.

The question on flow losses was: ‘Can the AEM identify areas along the Albert Priest Channel

where seepage losses occur and what is their order of magnitude?’ The second finger (Figure

4.1) was sited to cover the whole of the Albert Priest Channel to investigate this issue.

4.2 Pre and post-flight drilling The Bureau of Rural Sciences (BRS) undertook a pre-flight drilling (12 holes) and geophysical

logging program (37 drill-holes of which 25 were NSW Department of Water and Energy (DWE)

monitoring bores) in 2006. The post-flight drilling program was undertaken by BRS in 2007 and 26

holes were drilled. As with the pre-flight drilling program, hydrogeological data (standing water level,

airlift rate and EC) were collected wherever possible and the majority of the drill-holes were

completed as monitoring bores. Eight of the drill-holes in the post-flight program were fully cored

either to 60 metres depth, or to relatively unweathered rock. The locations of the pre and post-flight

drill-holes are shown in Figure 4.1.

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4.3 AEM conductivity provinces The AEM partitions the flight area into three general provinces of bulk conductivity in the saturated

zone. Figure 4.2 shows these provinces for the 30 to 40 metre depth slice. Apart from the Southern

Palaeochannel, this depth slice lies within the saturated zone throughout the flight area, and is

therefore a ‘snapshot’ of groundwater salinity.

The southern province is characterised by resistive Cainozoic sediments containing fresh to slightly

brackish groundwater, but also includes some resistive granite and Ordovician rocks in the south-east

corner. It includes all of NSW Groundwater Management Area 008. However, this province also

contains a belt of conductive inliers extending from 5 kilometres south of LMQ03 to 15 kilometres

north of LMQ10. The conductive material is generally saprolite developed on shallowly sub-cropping

Mesozoic rocks (Drildool and Keelindi Beds), but in a few cases it includes saline groundwater in the

overlying shallow Cainozoic aquifers. The northern boundary of this domain is quite sharp and runs

approximately through drill-holes LMQ05 and LMQ09, almost collinear with the Mitchell Highway.

The central province of the flight area is characterised by a fine network of resistive to slightly

conductive Cainozoic sediments with brackish to slightly saline groundwater, interspersed with large

belts of conductive material. This conductive material is saprolite developed on Mesozoic rocks

(Drildool and Keelindi Beds south of Warren, Rolling Downs Group north of Warren). This province

extends northward to Mount Foster and to about 5 kilometres north of LMQ18. Like the southern

(resistive) domain, it includes conductive inliers of saprolite developed on the Mesozoic rocks but on a

larger scale. Figure 4.2 shows the main areas of these conductive inliers are to the west and north of

Warren (e.g. LMQ14), and a prominent tongue of highly conductive material extending from

Bundemar (LMQ12) south-west towards Trangie, including the Gin Gin Hills (PFD01). This tongue of

highly conductive material is saturated saprolite developed on the Drildool Beds.

The northern province is all highly conductive material, and for the 30 to 40 metre depth slice, consists

entirely of saturated saprolite developed on the Rolling Downs Group. Apart from the western fingers,

the water table in this province varies from 5 metres to 10 metres below ground surface, so the 30 to

40 metre depth slice is well within the saturated zone, however the same highly conductive signature

is shown by the AEM 5 to 10 metre depth slice in this domain. The high conductivity in the 5 to 10

metre depth slice is generated by highly saline groundwater in the Quaternary aquifers, some of which

approaches sea water concentrations (50 000 µS/cm EC).

90

Figure 4.1: Location of pre-flight (PFD) and post-flight (LMQ) drill-holes. Monitoring bores are highlighted

91

Figure 4.2: AEM bulk conductivity, 30 to 40 metre depth slice (0 to 0.5 S/m stretch) with groundwater provinces and pre and post flight drill-holes superimposed.

Note: This depth slice can also be viewed as a snapshot of groundwater salinity since it lies at or below the watertable over the majority of the survey area.

92

4.4 Hydrogeology

4.4.1 General hydrogeological setting

Apart from a small area in the south-east corner, the flight area of the lower Macquarie River Valley

and its fingers traversing the Bogan River floodplain, consists of very gently sloping Quaternary

alluvial plains of the eastern Darling Riverine Plain. Palaeozoic basement of the Lachlan Fold Belt

outcrops in a bounding arc, extending from the south-east of the flight area (east of Narromine) to the

western side of the Bogan River. Basement rocks range from deeply weathered Cambro-Ordovician

phyllite and schist (Girilambone Group) with several mafic intrusions in the west, to Siluro-Devonian

sediments, volcanics and granite in the east. The younger basement in the east of the flight area is

dominantly composed of Late Ordovician Cotton Formation metasediments, Middle Devonian Obley

Granite and Late Devonian Hervey Group sandstone. Within the northern half of the flight area,

Middle Devonian monzonite pinnacles occur at Mt Foster, Mt Harris and Little Mount. The Palaeozoic

basement can be considered as hydraulic basement. Aquifers occur in the overlying Mesozoic and

Cainozoic sediments and their hydro-stratigraphy is shown in Figure 4.3.

4.4.2 The ‘Sandstone’ aquifer Late Jurassic and Early Cretaceous sediments of the southern Coonamble Embayment of the Great

Artesian Basin unconformably overlie Palaeozoic basement throughout the entire flight area apart

from a small area of Obley Granite and Cotton Formation outcrop east of Narromine. South of

Warren, sandstone beds within the Drildool and Keelindi Beds comprise a locally important source of

groundwater for stock and domestic bores, known informally as the ‘sandstone aquifer’. Good

supplies of groundwater are obtained where the sandstone beds are silicified and fractured, particularly

in the Keelindi Beds. For example, in the pre-flight drilling a fracture zone from 97 to 103 metres in

the Keelindi Beds yielded 4 Litres a second of good quality water (EC = 1145 µS/cm) in the drill-hole

PFD01. In the post-flight drilling, a fracture zone from 99 to 105 metres in the Keelindi Beds yielded

10 Litres a second of good quality water (EC = 1070 µS/cm) in the drill-hole LMQ11. Properly

constructed production bores would obtain yields much higher than these airlift rates.

Prolonged and intense weathering occurred in the lower Macquarie Valley during the Late Cretaceous

– Late Miocene hiatus in sedimentation. In many places, the weathering was intense enough to remove

all traces of rock fabric in the Mesozoic rocks, and sandstones were completely weathered to loose

sand beds. Many of the driller’s logs of water bores, particularly in the Drildool Beds, describe

unconsolidated ‘sand’ in this rock and many stock and domestic bores obtain supplies from these loose

sand beds. However, their yield is invariably low (of the order of 1 Litre a second or less).

93

Figure 4.3: Generalised hydro-stratigraphy of the Mesozoic and Cainozoic Formations in the AEM flight area.

North of Warren, the Keelindi and Drildool Beds become too fine-grained to yield useful groundwater

supplies and the ‘sandstone’ aquifer in the central and northern provinces is the Pilliga Sandstone.

North of Quambone, bores in the Pilliga Sandstone are free-flowing and produce good quality water

(less than 1000 mg/L TDS) except near the western boundary of the Coonamble Embayment (Bogan

River).

4.4.3 Alluvial aquifers

4.4.3.1 Lower alluvial aquifer This aquifer is restricted to the Southern Palaeovalley in the southern resistive province of the AEM.

The Southern Palaeovalley represents the proto-Macquarie River Valley, which was actively down-

cutting an east to west valley until the Pleistocene when it switched to a northerly course at about

where the town of Narromine now stands. Keshwan (1995) speculated that this switch in the course of

94

the proto-Macquarie River was caused by minor tilting of crustal blocks north of Narromine generated

by movement along a tectonic feature which he called the ‘Buddah Lake Depression fault’ - a lineation

extending in a north-north-west direction from Narromine through Bugaboo Point. The Southern

Palaeovalley extends at least 60 kilometres from south-east of Narromine, initially collinear with the

Backwater Cowal, then westwards towards the Bogan River near Dandaloo. The palaeovalley

shallows and dips beneath the Macquarie River 6 kilometres to the south-east of Narromine and

appears to be hydraulically connected to the river, which seems to be its main source of recharge. Prior

hydrogeological investigations of the palaeovalley have been conducted by Pirard (1974), Keshwan

(1995), Salas (1996), Baird (2001) and Haridharan (2003). These workers established the palaeovalley

contains up to 160 metres of Cainozoic sediments, informally subdivided into three Formations.

Martin (1999) assigned spores and pollen found in the basal aquifer of DNR drill-hole GW096000

(107 to 119 metre depth) at the then known western limit of the palaeochannel near ‘Waterloo’ as

being no older than the Upper P. tuberculatus Zone (Early Miocene). Macphail (2006) assigned spores

and pollens found in the basal aquifer of drill-hole PFD05 (88 to 91 metre depth) at a possible westerly

extension of the palaeochannel near Dandaloo as being M. galeautus Zone (Late Miocene – Early

Pliocene). This places the lower alluvial aquifer as being time equivalent to the Lachlan Formation in

the Lachlan River Palaeovalley and the Calivil Formation in the Murray Basin.

The AEM maps the Southern Palaeovalley as a highly resistive feature (Figure 4.4). The boundary of

the palaeovalley in Figure 4.4 is enhanced by altering the AEM colour stretch to 0 – 0.2 S/m, and by

aggregating the 20 to 100 metre depth slices (the 0 to 20 metre data is truncated to remove the

masking effects of conductive near-surface clay beds). The bores highlighted in Figure 4.4 are those

which intersected the lower alluvial aquifer in the prior streams of the palaeovalley.

The lower alluvial aquifer is composed of several fining-upwards cycles of sub-rounded to rounded

and moderately well sorted quartz grains with minor lithic fragments. Grain size ranges from fine

gravel to coarse sand. Figure 4.5 shows the characteristics of the lower alluvial intersected in drill-hole

PFD03 in the axis of the Southern Palaeovalley (note that clay balls in the chip samples are

contamination from above). At this location the lower alluvial aquifer contains three fining-upwards

sequences (117 to 130 metres, 130 to 135 metres and 135 to 147 metres). The airlift rate from the

slotted section (137 to 143 metres) was too high to measure (well in excess of 10 Lites a second) and

the water quality is good (EC = 1090 µS/cm).

95

Figure 4.4: Extent of the Southern Palaeovalley depicted as a resistive thick belt of sediments by the AEM. Note the 0 to 0.2 S/m colour stretch.

Note: Three curvilinear resistive features are mapped in the west. The AEM also maps a breached subsurface extension of the topographic ridge in the east, and a parallel buried ridge. Highlighted bores intersect the lower alluvial aquifer.

96

Figure 4.5: The lower alluvial aquifer intersected in drill-hole PFD03.

4.4.3.2 Upper alluvial aquifer The upper alluvial aquifer is a composite of meander plain facies of the four Quaternary formations

mapped in the lower Macquarie River Valley by Watkins and Meakins (1996). In ascending order of

age, these units are the Trangie (Pleistocene), Carrabear (Late Pleistocene), Bugwah (Late Pleistocene

to Holocene) and Marra Creek (Holocene) Formations. With the exception of the Marra Creek

Formation, each of these formations are generally composed of fining-upwards sequences, and it is the

basal channel fill sediments which comprise the aquifer material. Thus the ‘upper alluvial aquifer’

should be thought of as a stacked sequence of sand and gravel aquifers with intervening confining

97

beds. The Marra Creek Formation is generally too fine-grained to transmit groundwater in sufficient

amounts to be described as an aquifer, however perched aquifers occur in this formation in some areas

close to modern river channels.

The area of occurrence of the upper alluvial aquifer is throughout the entire flight area, but because of

the cut and fill history of the rivers which deposited it, the aquifer material tends to be laterally

discontinuous. The most permeable, and therefore most hydrogeologically significant, material of the

upper alluvial aquifer are channel fill deposits of the Trangie Formation. Watkins and Meakin (1996)

map a large belt of Trangie Formation meander plain sediments extending from Mungeribar to

Nevertire. Exposures of 10 metres to 15 metres of clay-bound coarse gravel (conglomerate) at the

Myall and Toucan pits testify to the high energy bed load channel systems that laid down these point

bar deposits during the Pleistocene. Northward, the Trangie Formation is buried by the younger

Quaternary formations and its gravel-sized sediments persist at least as far as the Northern Marshes.

For example, red lithic medium-sized gravel of the Trangie Formation was intersected at depth (22.5

metres to 25 metres) in drill-hole LMQ21 in the Northern Marshes.

Mineralogically and texturally, the Trangie Formation sands and gravels are not as mature as the

sediments of the lower alluvial aquifer. The Trangie Formation sediments tend to contain a higher

proportion of lithic fragments and the grains are not as well sorted or rounded as those of the lower

alluvial aquifer.

The thickest known occurrence of the Trangie Formation, and for that matter of the upper alluvial

aquifer, is 72 metres measured in the drill-hole LMQ09 in the Trangie – Nevertire Palaeochannel, a

prominent resistive feature mapped by the AEM and which will be discussed subsequently in

Section 4.5.3. Drill-chip samples for LMQ09 are shown in Figure 4.6. The airlift rate from the slotted

section (41 to 47 metres) was 4 Litres a second and the water quality is good (EC = 812 µS/cm).

Channel deposits within the meander plain facies of the Carrabear and Bugwah Formations also form

part of the upper alluvial aquifer. In general, the aquifer material is dominantly silt and sand, and

therefore permeability tends to be low. Locally important exceptions to this occur within belts closer

than 5 kilometres to the modern channel systems, particularly the Macquarie River. The drill-hole

LMQ13 intersected a fining-upwards sequence of coarse lithic gravel to coarse sand in the interval 14

to 27 metres. This material is interpreted to be channel fill deposits of Bugwah Formation meander

plain facies underlying Marra Creek Formation sediments (0 to 14 metres). The drill chips are shown

in Figure 4.7 – the gravel overlies weathered Drildool Beds, shown here in the 27 to 28 metre interval.

The airlift rate from the slotted section (21 to 27 metres) was greater than 2 Litres a second (it was not

possible to measure an exact rate due to compressor problems) and the water quality is good (EC =

808 µS/cm).

98

Figure 4.6: Trangie Formation (meander plain facies) in the Trangie – Nevertire Palaeochannel (LMQ09).

Sand and gravel of the upper alluvial aquifer extends from 17metres to 72metres. Note the high proportion of lithic fragments (particularly in the top 20metres) and the occurrence of clay bands.

Figure 4.7: Bugwah Formation (meander plain facies), part of the upper alluvial aquifer system (LMQ13)

Note the fining-upwards sequence from coarse gravel to coarse sand in the interval 14 to 27metres, and the dominance of lithic grains over quartz.

99

4.5 Hydrogeological insights provided by the AEM

4.5.1 Western configuration of the Southern Palaeovalley and its possible discharge to the Macquarie Marshes

The AEM maps a thick, prominent belt of resistive material, interpreted to be the Southern

Palaeovalley (Figure 4.4), as extending to the west of ‘Waterloo’ towards the Bogan River at

Dandaloo. Dandaloo lies on the western boundary of the AEM flight area so if the Southern

Palaeovalley extends beyond here it cannot be traced by the AEM. However, since the country to the

east is known to be composed of Girilambone rocks of the Cobar Block, and it is topographically

higher, it is highly likely the Palaeovalley (if indeed it extends beyond the AEM boundary) would dip

beneath the Bogan River and trend in a north-westerly direction. Logs of water bores and mineral

exploration bores provide patchy information on the subsurface structure of the Bogan River

floodplain, and there is no evidence of a continuous buried palaeovalley analogous to the Southern

Palaeovalley beneath the Bogan River floodplain. Whilst there are a few logs which could be

interpreted as possibly intersecting a finer-grained variant of the lower alluvial aquifer at depths of up

to 100 metres, there are many more bores close to the Bogan River which intersect very thin Cainozoic

sediments. The subsurface structure of the Bogan River floodplain seems to typified by a series of

step-wise depressions filled with sand, separated from each other by highs in the Cretaceous or

Palaeozoic substrate.

The most significant contribution the AEM makes in understanding the Southern Palaeovalley is its

detection of three curvilinear resistive features in the west (Figure 4.4). These structures are all narrow

(of the order of 1 kilometre) and consistently exhibit low bulk conductivity values (50 to 100 mS/m).

Drill-hole LMQ06 (Kellett et al., 2008) targeted one of these resistive features and revealed the

resistive feature to be a buried ridge of Hervey Group Sandstone.

Based on the AEM cross-section (Figure 4.8), the buried ridge is a steep-sided structure, similar in

morphology (but buried) to the 600 to 700 metres wide, locally prominent topographic ridge, 15

kilometres south-west of Narromine. This structure is a graben capped with (Devonian) Hervey Group

Sandstone which is manifests as a basement high in the inverted topography of the plain. The

conductive band about 40 metres deep to the north of 6430500 is saprolite developed on the Drildool

Beds, and resistive materials south of here are sediments of the Southern Palaeovalley. The AEM

cannot differentiate the base of the palaeovalley because of the lack of conductivity contrast between

the lower alluvial aquifer and Palaeozoic basement.

To understand the effect on groundwater flow in the palaeovalley, it is necessary to understand

whether this buried ridge is breached further south where it intersects the Southern Palaeovalley.

Figure 4.9 shows flight line 10410, located 2 kilometres west of bore GW096000, the western-most

control bore known to lie on the palaeovalley axis.

100

Figu

re 4

.8:

Flig

ht li

ne 1

0211

(0.0

1 –

1.0

S/m

log

stre

tch)

show

ing

proj

ecte

d lo

catio

n of

LM

Q06

and

stee

p si

ded

burie

d rid

ge o

f Her

vey

Gro

up S

ands

tone

Figu

re 4

.9: F

light

line

104

10 (0

.01

– 1.

0 S/

m lo

g st

retc

h) sh

owin

g po

ssib

le b

reac

h in

the

mid

dle

burie

d rid

ge o

f Her

vey

Gro

up S

ands

tone

w

est o

f bor

e G

W09

6000

101

Since the ridge runs sub-parallel to the flight line, it appears to be wider than it actually is. According

to Figure 4.9, there appears to be a breach through the ridge between 6421000 – 6422000 mN. It

cannot extend any further south than this because of the occurrence of conductive material to depths of

80 metres south of northing 6421000 metres (the AEM signature of the Southern Palaeovalley is a

continuous sequence of resistive sediments below 30 metres). If this breach is real, it is only 1

kilometre wide, and would form a choke to groundwater flow in the palaeovalley.

Figure 4.10 shows the nature of the western-most curvilinear resistive feature through Dandaloo. A

ridge of resistive material, assumed to be Hervey Group Sandstone or at least Devonian red beds, is

indicated in the interval 6424000 – 6428000 mN. As there appears to be no breach through the ridge

or the saprolite, then this structure most likely represents the terminus of the Southern Palaeovalley.

The eastern-most curvilinear resistive feature is intersected by flight line 10670 (Figure 4.11) where it

crosses the Southern Palaeovalley. Along this flight line, the buried ridge lies about 4 metres below

ground surface. The Southern Palaeovalley is interpreted to be the resistive material in the interval

6422200 – 6425000 mN, slightly less than 3 kilometres wide.

In summary, the cumulative effect of the three curvilinear resistive features, interpreted as three buried

ridges, is to impede groundwater through-flow in the Southern Palaeovalley. Chokes are developed

where the palaeovalley narrows significantly. In the eastern-most buried ridge, the AEM indicates the

palaeovalley thins to 3 kilometres, and to only 1kilometre in the central buried ridge. Haridharan

(2003) carried out a direct current resistivity survey (Schlumberger electrode array configuration)

along two traverses about 25 kilometres east of Dandaloo. One of these traverses passed very close to

the control bore GW096000, and Haridharan concluded the palaeovalley here was only of the order of

1 kilometre wide at its base. The AEM data support this interpretation.

The evolution of the buried ridges is unknown. It appears they may have been uplifted in response to

block faulting along one or more of the meridional faults oblique to the Dandaloo area during the

Pliocene. Drainage defeat of the proto-Macquarie River appears to have been accomplished by the

western-most buried ridge which shows no signs of having been breached. In the gentle gradients

which prevail in this area, it would not have taken much differential movement to generate major

changes in drainage patterns.

The second part of the issue related to the Southern Palaeovalley in Section 4.1 was the possibility of

the palaeovalley swinging northward and ultimately discharging to the Macquarie Marshes. Clearly

this is impossible because of the terminus and drainage defeat of the proto-Macquarie River system at

Dandaloo.

102

Figu

re 4

.10:

Flig

ht li

ne 1

0050

(0.0

1 –

1.0

S/m

log

stre

tch)

show

ing

exte

nt o

f bur

ied

ridge

ben

eath

Dan

dalo

o

Figu

re 4

.11:

Flig

ht li

ne 1

0670

(0.0

1 –

1.0

S/m

log

stre

tch)

show

ing

exte

nt o

f eas

tern

-mos

t cur

vilin

ear r

esis

tive

feat

ure

103

4.5.2: Subsurface extension of the Hervey Group Sandstone outcrop at the eastern end of the Southern Palaeovalley

The exposed north-north-west topographic ridge of Hervey Group Sandstone 15 kilometres south-west

of Narromine has long been known to obstruct groundwater flow from the Southern Palaeovalley to

the east (Salas 1996). Bore data indicate the ridge deflects the course of the palaeovalley by at least 4

kilometres to the north-north-west and that the ridge forms a choke to groundwater flow indicated by

the abrupt fall of about 30 metres in water levels in bores from the eastern to the western side of the

ridge.

The AEM maps a continuous subsurface resistive feature extending for 6 kilometres in a north-north-

west direction from the outcrop (Figure 4.4). The drill-hole LMQ08 was sited 2 kilometres along this

buried feature. Drilling revealed 18 kilometres of Quaternary sediments overlying Hervey Group

Sandstone on which 8 metres of saprolite has developed. While the buried ridge lies 20 to 25 metres

below ground surface before plunging beneath the Mesozoic rocks, it must have been breached

somewhere by the proto-Macquarie River which formed the Southern Palaeovalley. The flight line

11430 shows this breach underlies the Backwater Cowal (Figure 4.12). The breach is discernable

because of the small difference in conductivity between the groundwater in the Cainozoic sediments

and the Hervey Group Sandstone basement. Although the groundwater in the Cainozoic sediments is

fresh (EC < 800 µS/cm), the aquifer is still more conductive than the Hervey Group Sandstone. The

breach appears to be about 80 metres deep. Deeper, slightly more conductive material south of

6432000 mN in Figure 4.12 is not a breach – this material lies on the western flank of the ridge and

dips toward the paleovalley.

The AEM also maps a resistive feature at about 25 metres below ground surface running parallel to the

topographic ridge (Figure 4.14). This structure is interpreted as a parallel buried ridge of Hervey

Group Sandstone which is covered by 25 metres of saprolite and shallow Quaternary material, and

terminates abruptly at 6423000 mN.

104

Figu

re 4

.12:

Flig

ht li

ne 1

1430

(0.0

1 –

1.0

S/m

log

stre

tch)

show

ing

brea

ch in

bur

ied

ridge

at 6

4325

00 m

N b

enea

th th

e B

ackw

ater

Cow

al

Figu

re 4

.13:

Flig

ht li

ne 1

1340

(0.0

1 –

1.0

S/m

log

stre

tch)

show

ing

north

ern

extre

mity

of b

urie

d rid

ge (6

4370

00 –

643

9300

mN

). C

ondu

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e m

ater

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uth

of th

e rid

ge is

sapr

olite

dev

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ed o

n th

e D

rildo

ol B

eds

105

Figure 4.14: Flight line 11670 (0.01 – 1.0 S/m log stretch) showing parallel (eastern) buried ridge terminating abruptly at 6423000 mN

4.5.3 Fresh groundwater resources revealed by the AEM The AEM indicates the only fresh groundwater resources in the Cainozoic sediments of the flight area

are restricted to the Southern Resistive Province. While the Central Province is characterised by a fine

network of resistive to slightly conductive saturated Cainozoic sediments, these features are mostly all

analogous to shoestring sands and gravels which would not yield groundwater in sufficient quantities

to be considered a viable groundwater resource.

The AEM shows that within the Southern Province, fresh groundwater occurs in three distinct

domains:

• floodplain sediments and channel sands east of Narromine (note that a small proportion of the

resistive material includes Ordovician rocks and granite basement)

• the Southern Palaeovalley, in which the majority of the fresh groundwater is yielded by the

lower alluvial aquifer

• the Trangie – Nevertire Palaeochannel, in which fresh groundwater occurs in the upper

alluvial aquifer.

Figure 4.15 shows the induction conductivity log of drill-hole LMQ09, which is located in the

Trangie-Nevertire Palaeochannel, approximately halfway between Trangie and Nevertire.

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Figure 4.15: Induction conductivity log of drill-hole LMQ09, showing resistive saturated sediments (30 to 45 metres) underlain by conductive saturated sediments (45 to 72 metres) in the upper alluvial aquifer

4.5.3.1 The Trangie – Nevertire Palaeochannel This palaeochannel is a prominent linear resistive feature which also defines the northern boundary of

the Southern Resistive Province. The AEM maps the Tangie – Nevertire Palaeochannel as a low

conductivity strip about 2 to 3 kilometres wide running approximately underneath the Mitchell

Highway between Mungeribar and Nevertire. It cannot be traced up-gradient from Mungeribar

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because of the lack of vertical conductivity contrast. The palaeochannel first becomes evident at about

20 metres depth and extends down to 60 to 70 metres. As noted previously, the Trangie – Nevertire

Palaeochannel is filled with Trangie Formation sediments and appears to be the Pleistocene channel of

the proto-Macquarie River following drainage defeat in the Southern Palaeovalley by tilting which

induced the river to migrate to the north-west.

The conductivity in the sand and gravel increases from 100 mS/m in the interval 20 to 45 metres (with

no change from unsaturated to saturated conditions), to 400 to 550 mS/m in the basal section 45 to 72

metres. The aquifer is unconfined and groundwater in the upper low conductivity section is fresh

(EC = 812 µS/cm), but the EC of the groundwater in the basal section of the aquifer would be many

times more than this. The conductivity in the upper part of the aquifer gradually increases down-

gradient from 100 mS/m near Trangie to 200 mS/m at Nevertire, as a consequence of infiltration of

progressively more evaporated water down the flow line. Figure 4.16 shows the structure of the

channel along flight line 25690. The palaeochannel is around 2 kilometres wide.

The Trangie – Nevertire Palaeochannel bifurcates near Trangie and a southern arm is depicted by the

AEM as running beneath Quartpot Lagoon on the Backwater Cowal towards ‘Thornton’. The southern

arm appears to either dissipate at the western edge of the AEM flight area, or to swing in a northerly

direction to rejoin the main palaeochannel near ‘Ingelara’ on the second finger. The southern arm was

targeted by the drill-hole LMQ10. Drilling revealed the Cainozoic sediments to be 63 metres thick

(compared with 72 metres at LMQ09) and finer grained than at LMQ09, which is to expected in a

lower energy distributary of the main channel. Groundwater from the basal aquifer (58 to 63 metres) in

LMQ10 is slightly brackish (EC = 1310 µS/cm).

The drill-holes LMQ15 and LMQ15A were put down on ‘Ingelara’ where the AEM indicates the

Trangie – Nevertire Palaeochannel starts to swing to the north-west. The channel-fill sediments here

are only 53 metres thick, and the aquifer becomes semi-unconfined. Like LMQ09, the aquifer here

exhibits salinity stratification, ranging from an EC value of 4930 µS/cm at the top of the aquifer (36 to

42 metres) to an EC value of 7520 µS/cm at the base of the aquifer (50 to 53 metres).

By the time the Trangie – Nevertire Palaeochannel reaches the third finger, its basal aquifer has

become confined. The northern-most drill-hole in the palaeochannel is LMQ17, 24 kilometres north-

west of LMQ15 and LMQ15A. The AEM indicates the palaeochannel is a continuous feature

extending from Mungeribar to Billybingbone on Marra Creek (sixth finger). Drilling at LMQ17

revealed the channel fill sediments are 60 metres thick and the sands are generally finer-grained and

not as well sorted as those further south in the palaeochannel. Groundwater in the basal aquifer at

LMQ17 (55 to 60 metres) has an EC value of 3530 µS/cm, significantly less saline than the

groundwater up-gradient in drill-holes LMQ15 and LMQ15A. This appears to indicate dilution and

freshening of the groundwater, probably by leakage from Gunningbar Creek which overlies the

palaeochannel between the second and third fingers.

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The AEM maps the northern part of the Trangie – Nevertire Palaeochannel as continuing in a north-

north-west direction beneath the fourth and fifth fingers, with conductivity values in the range 300 to

400 mS/m, indicating increasing groundwater salinity down-gradient. It appears to deepen to about

100 metres and widen in the most northern flight line – shown in cross-section in Figure 4.17.

4.5.4 Discussion The largest fresh groundwater resources in the flight area occur in the lower alluvial aquifer of the

Southern Palaeovalley. The structure and extent of the Southern Palaeovalley had been reasonably

well known before the survey, but the chokes formed by the buried ridges of Hervey Group Sandstone

in the west of the palaeovalley were only discovered by using the AEM. Following several years of

investigation drilling and water level monitoring in the Southern Palaeovalley, Salas (1996) calculated

the aquifer storage in the eastern half of the palaeovalley as 4.8 x 106 Megalitres; he also estimated a

sustainable yield of 36 000 megalitres a year for this part of the palaeovalley, the same order of

magnitude as current abstractions.

While it was previously known that fresh groundwater was yielded by the upper alluvial aquifer of the

Trangie – Nevertire Palaeochannel, the full extent of the palaeochannel was unknown. The AEM has

now revealed the Trangie – Nevertire Palaeochannel extends all the way from Mungeribar through to

the north-west tip of the flight area at Billybingbone.

The volume of fresh groundwater potentially suitable for irrigation that is held in storage in the

Trangie – Nevertire Palaeochannel may be calculated using the following parameters: the length of the

resistive part of the palaeochannel is 50 kilometres (Mungeribar to Nevertire), its width is on average

2 kilometres, its saturated thickness is 14 metres (based on the difference between the water table and

the inflexion at 44 metres in the induction conductivity log of LMQ09), and an estimated

(conservative) porosity of 0.2. Multiplication of these parameters gives a volume of 280 000

megalitres, an order of magnitude lower than the storage calculated by Salas (1996) for the eastern half

of the Southern Palaeovalley.

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Because the Trangie-Nevertire Palaeochannel is unconfined, it receives diffuse recharge from rainfall

infiltration, but it may, in addition, receive leakage from the Macquarie River immediately

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downstream from Narromine, or leakage from the Backwater Cowal during flooding. The diffuse

recharge component may be estimated using deep drainage figures for Trangie meander plain soils

(Hulme 2003). Deep drainage beyond the root zone (i.e. groundwater recharge) is given by Hulme

(2003) as ranging from 2 millimetres a year to 8 millimetres a year under 460 millimetre a year

average annual rainfall. These values equate to a recharge from rainfall of 200 megalitres a year to

800 megalitres a year for the 50 kilometre long palaeochannel between Mungeribar and Nevertire.

However, when the soil is loaded with 8 megalitres per hectare of irrigation water, Hulme (2003)

estimates deep drainage rates of 20 millimetres a year to 105 millimetres a year. These equate to deep

drainage accessions (groundwater recharge rates) of 2000 megalitres a year to 10 500 megalitres a

year.

The AEM indicates the groundwater in the Trangie – Nevertire Palaeochannel becomes too saline for

irrigation down gradient from Nevertire. Nevertheless, the groundwater in the palaeochannel is

suitable for stock, except perhaps for the section between the fourth and fifth fingers, where the AEM

indicates increasing groundwater salinity which may render it marginal for stock.

4.6 Flow losses in the Macquarie River between Narromine and Warren

4.6.1 Background Griffin (1966) documented mean annual flows in the Macquarie River at Narromine of 830 000 acre-

feet per annum and at Warren of 590 000 acre-feet per annum. These figures were based on 62 years

of concurrent records to 1965, three years before Burrendong Dam was built. Thus, in the unregulated

Macquarie River, there was an average flow loss of 240 000 acre-feet per annum (809 megalitres a

day) between Narromine and Warren over 62 years of record – the majority of these years up until

1950 were low-flow years. Griffin (1966) attributed the flow loss to river leakage into the Pilliga

Sandstone aquifer via the alluvium.

Keshwan (1995) reported unaccounted flow losses between Baroona (13 kilometres upstream of

Narromine) and Wambool Bridge (9 kilometres upstream of Gin Gin Weir) of 480 megalitres a day

when the Macquarie River was in high flow in 1993 (4670 megalitres a day at Dubbo). The same set

of hydrographic measurements carried out two days later produced an unaccounted for flow loss

between these two stations of 440 megalitres a day. Keshwan (1995) estimated that when flow

exceeded 2500 megalitres a day and river levels were high enough to saturate the exposed permeable

alluvium on the river banks, that leakage to groundwater from the river could be as high as

450 megalitres a day. Since Keshwan’s hydrographic measurements gave unaccounted for flow losses

of 340 megalitres a day from a station upstream of Buddah to a station downstream of Buddah, he

concluded that the majority of the river loss between Baroona and Gin Gin occurred in the 3 kilometre

long stretch of the river at Buddah.

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An independent hydrochemistry study by Radke et al. (2000) supports the hypothesis of recharge to

the Pilliga Sandstone by leakage from the Macquarie River where it crosses the intake beds.

4.6.2 Indications of river leakage and aquifer recharge from the AEM Aquifer recharge by leakage from the Macquarie River should be manifested in the AEM as a resistive

plume emanating from the river and being propagated down-gradient through the alluvium. Figure

4.18 shows the 0 to 10 metre depth slice of the AEM at an extremely low colour stretch (0 to 0.1 S/m),

for the Macquarie River between Baroona and Warren.

Figure 4.18 shows a resistive (бb ≤ 20 mS/m) band 2 to 3 kilometres wide enveloping the Macquarie

River between Baroona and Narromine, which is interpreted as the wetted perimeter of the river,

where alluvium adjacent to the channel is recharged by fresh water during high river flows and floods.

Note the AEM was flown after several years of drought, and the wetted perimeter would be expected

to expand during wet years of high river flow.

The resistive band continues from Narromine to Buddah and is about 2 kilometres wide, except at

Timbrebongie where it reaches 4 kilometres width; however, some of this resistive material is granite.

The resistive band continues along the Macquarie River from Buddah to a point 5 kilometres upstream

of Gin Gin (2 kilometres downstream of Gin Gin Weir), where it is generally less than 1 kilometres

wide and patchy. The AEM indicates (Figure 4.18) that the next 8 kilometres of the river is

conductive, interpreted as indicating there is no wetted perimeter and no river leakage in the Gin Gin

area.

The resistive band continues along the river from 3 kilometres downstream of Gin Gin to 3 kilometres

upstream of Warren (and 2 kilometres downstream of Warren Weir). The band is 1 to 2 kilometres

wide and is not quite as resistive (20 ≤ бb ≤ 30 mS/m) as the wetted perimeter upstream of Buddah,

which probably reflects slowly increasing river salinity downstream. Again, the conductive reach of

the river 3 kilometre upstream of Warren (and beyond) is interpreted as a zone of negligible hydraulic

connection between the river and the alluvium.

The AEM identifies two resistive plumes extending from the river into the alluvium. The most

prominent plume extends 15 kilometres from a point 4 kilometres north of Rocky Point to Bundemar.

The plume lies in sandy Bugwah Formation meander plain sediments in a belt 1 to 2 kilometres wide

enveloping Crooked Creek. However, the elevation of the top of the plume at Bundemar is about 20

metres higher than the confluence of Crooked Creek and the Macquarie River, therefore this plume

represents a zone of fresh groundwater seepage towards the river.

The second resistive plume is much smaller and runs from Buddah for about 8 kilometres along the

Trangie Cowal. The plume tapers from 2 kilometres width at Buddah to less than 1 kkilomtre width

after the first 4 kilometres. It is also slightly more conductive (20 ≤ бb ≤ 40 mS/m) than the wetted

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perimeter of the river at Buddah, but unlike the Bundemar plume, the Trangie Cowal plume appears to

represent leakage from the river to the alluvium.

Figure 4.18: AEM bulk conductivity, 0 to 10 metre depth slice (0 to 0.1 S/m linear stretch) for the Macquarie River, Baroona to Warren

4.7 Seepage losses from the Albert Priest Channel The Albert Priest Channel was constructed in 1942, and when full, has a capacity 64 megalitres a day.

The channel is an open canal 62 kilometres long and runs from Gunningbar Creek near Warren to the

weir pool at Nyngan on the Bogan River. The Albert Priest Channel provides a permanent water

supply to the towns of Nyngan and Cobar, and to three mines (Peak Gold Mine, CSA Mine and

Endeavour Mine). The channel also supplies stock and domestic and irrigation water to landholders

along its length.