Hydrochemistry of the Hawkesbury

Sandstone Aquifers in Western Sydney

and the Upper Nepean catchment

W. McLean1 & J. Ross2

1Parsons Brinckerhoff, Sydney NSW AUSTRALIA2Sydney Catchment Authority, Penrith, NSW, AUSTRALIA

Project Background

NSW Government

Metropolitan

Water Plan

Desalination

Recycling

Deep water in dams

Groundwater

Groundwater Investigations

Sydney

Leonay

Wallacia

Upper

Nepean

Sydney Water Supply Investigations –

Scope of works

�Drilling and construction of test production and monitoring bores

�Monitoring of yield and water quality

�Geophysical logging

�Water level monitoring

�Test monitoring

�Safe yield estimates

�Numerical modelling

�Hydrogeochemical and isotope studies

Sydney Water Supply Investigations

Chemical, stable and radiogenic

isotopes were assessed to:

� Determine chemical

characteristics of aquifers

� Delineate recharge/discharge

zones

� Age date groundwater

� Assess groundwater surface-

water interactions

� Assess feasibility of borefield

development

Hydrogeological setting: Hawkesbury

Sandstone aquifer

� Hawkesbury Sandstone is the main productive aquifer

� Thickness of up to 250 m

� Semi-confined to confined aquifer

� Groundwater flow is highly variable

� Primary and secondary porosity (fractures)

� Yields: 5 – 40 L/s

Primary porosity

Fractures

Background: Leonay

� Leonay is located 60 km west of

Sydney

� Located at foothills of the Blue

Mountains

� Centred on the Lapstone Monocline, a major structural

feature of the Sydney Basin

� Investigation involved construction of 18 boreholes,

including 8 deep production

bores in excess of 300 metres

Background: Wallacia

�Wallacia is located ~20 km south

of Leonay

�Located east of the Lapstone Monocline and Nepean Fault

�Investigation involved

construction of 14 boreholes at 5

sites, including 7 deep bores up to 340 m deep.

Geology: Leonay & Wallacia

� Geological target is the Hawkesbury Sandstone (HS) (Triassic) (up to 260 m thick)

� HS is overlain by Ashfield Shale, and Tertiary and Quaternary Alluvium/Colluvium

� At Leonay Cranebrook Formation (Quaternary) is 8-14 m thick and Rickabys Creek Gravels (Tertiary) is 2-12 m thick

� At Wallacia Cranebrook Formation equivalent (Pleistocene) 15-20 m thick, Talus and colluvium (Tertiary) up to 9 m thick

Geological structure: Leonay - Wallacia

� Lapstone Monocline is a major structural feature

� Separates Blue Mountains Plateau in the west from Cumberland Plain to the east

� North-south orientation, 160 km length

� Increased fracturing is associated with the feature & this varies between a series of en-echlon faults and monoclines

Wallacia

investigation area

Leonay

investigation area

Geological structure: Leonay - Wallacia

Hydrogeological setting: Leonay-

Wallacia

� Groundwater recharge occurs on the

Blue Mountains Plateau

� Groundwater flow is eastwards

towards Nepean River

� Steep groundwater gradient across the Lapstone Structural Complex

� At Wallacia groundwater is very

shallow or artesian

� To the west of the Nepean Fault groundwater levels are very deep (up

to 100 mbgl)

Hydrochemistry: Leonay

� Differences in chemical composition of groundwater in upper HS

(<100 m depth) and lower HS (100-300 m depth)

� Higher EC in upper HS (up to 1,700 µS/cm)

� Higher EC due to higher clay content and leakage from overlying

Ashfield Shale

� Lower EC in lower HS (250 - 400 µS/cm)

� Lower EC due to cleaner, coarser grained sandstone at depth

Hydrochemistry: Leonay

�Upper HS dominated by Na, Mg and Cl

�Cl and increased salinity in the upper aquifer is attributed to leakage from Ashfield Shale and higher clay content

�Lower HS evolved from mixed cation (Cl-HCO3) to mixed cation (HCO3-Cl)

�Increasing alkalinity due to dissolution of carbonate minerals (siderite and calcite)

80

60

40

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80

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60

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Ca Na+K HCO3 Cl

Mg SO4

<=H

CO

3Na+

K=>

<=Ca

+ MgC

l + S

O4=

>

Piper Plot

E

E

E

C

C

C

D

D

D

E

E

E

D

D

D

E

E

E

B

B

B

B

B

B

B

B

B

Le gendLege nd

E L1C

C L1D

D L2B

E L2C

D L3B

E L3C

B L5A

B L6A

B L7A

Hydrochemistry: Leonay

� Elevated concentrations of iron and manganese are typical of Hawkesbury Sandstone

� Sources of iron: siderite, and iron oxyhydroxides and hydroxides

� Large spatial variability in iron and manganese concentrations

� Iron concentrations – 49.1 mg/L (max)

� Manganese concentrations – 3.17 mg/L (max)

Isotope hydrology: Leonay

� Radiocarbon ages ranged from 5,300 years BP to 25,700 years BP (uncorrected) and from 3,550 years BP to 21,600 years BP (corrected).

� There is generally an increase in groundwater residence time in the Hawkesbury Sandstone aquifer along the flow path (west to east).

� Groundwater age also increases with depth.

� Tritium values were low in all bores penetrating the Hawkesbury Sandstone with the highest tritium value measuring 0.9 TU.

� Tritium values indicate that the proportion of modern water present in the aquifer ranges from 3 to 30%.

Hydrochemistry: Wallacia

� Differences in chemical composition of groundwater in upper HS (<100 m depth) and lower HS (100-300 m depth)

� Upper HS – Maximum EC ~4,300 µS/cm

� Lower HS – EC ranged from 400-1,600 µS/cm

� Salinity increases along flow path (west to east)

� Higher salinity due to leakage from overlying shales and upward migration of brackish water along faults

Hydrochemistry: Wallacia

Hydrochemistry: Wallacia

� High iron concentrations – 1.57 mg/L to 60.4 mg/L

� High manganese concentrations –0.22 to 2.26 mg/L

� High trace elements, CO2 and CH4in some bores – upward leakage along faults, ingassing from underlying Banks Wall Sandstone or Permian Coal Measures

� Variation in chemistry in area of Norton’s Diatreme

Groundwater age: Wallacia

14C (uncorrected) 8,500 to 42,000 yrs BP

14C (corrected) 5,000 to 30,500 yrs BP

Background: Upper Nepean

� Located near Robertson, in the Southern Highlands of NSW

� Target aquifer: Hawkesbury Sandstone

� Identified as a potential drought resource after an initial drilling program in April-May 2005

� Drilling and pilot testing program commenced in August 2005

� Site identified for borefield development

Geology: Upper Nepean

� Robertson Basalt (Tertiary)� caps elevated areas in Kangaloon-Robertson area

� mainly olivine basalt

� Ashfield Shale � outcrops in the Robertson area (south of investigation area)

� predominantly dark grey to black sideritic siltstone

� Mittagong Formation� thin unit (average 2 m thick)� fine-grained quartzose sandstone

� Hawkesbury Sandstone � up to 180 m thick

� overlies Triassic Narrabeen Group and Permian Illawarra Coal Measures

Geological structure: Upper Nepean

� General dip to the northeast of approximately 1.3°°°°

� Structural deformation since deposition produced gentle warps and folds

� Mittagong area dominated by west-northwest to east-southeast trending horst and graben structures

� New west-northwest trending faults and dome structure (Mt Butler intrusive) identified from recent drilling and aeromagnetic survey

Site 2

Site 11

Site 1

Site 4

Site 7

Site 10

Site 8

Site 5

Site 3

Site 9

Geological structure: Upper Nepean

Geological cross: section

Hydrogeology: Upper Nepean

� Groundwater flow is generally to the north

� Groundwater-surface water linkage for Nepean River and tributaries is connected-gaining

� Water levels typically <30 mbgl

� Deepest levels occur in the north (due to incised creek/river

� Recharge occurs in elevated areas of exposed sandstone

Primary recharge zone

Groundwater chemistry: Upper Nepean

� Lower salinity than Wallacia and Leonay

� EC varied from 50 to 1,660 µS/cm, typically <400 µS/cm in upper HS and <200 µS/cm in lower HS

� Higher salinity occurred in bores located in shale outcrop areas

� Lowest salinity in recharge zone (uplifted horst block)

� Salinity decreased with depth

� Recharge zone had rapid rainfall recharge, characterised by low salinity (<100 µS/cm) and acidic pH (pH<5)

Groundwater chemistry: Upper Nepean

Groundwater chemistry: Upper Nepean

� Chemical composition: Na-Cl and Na-Mg-Cl (recharge zones or higher salinity waters) and Na-(Mg)-HCO3-Cl downgradient of recharge zones (to the north and west)

� Iron: <0.05 to 47 mg/L

� Manganese: 0.01 to 3.44 mg/L80 60 40 20 20 40 60 80

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Ca Na+K HCO3 Cl

Mg SO4

Site 1 (s)

Site 1 (PB)

Site 2 (a)

Site 2 (p)

Site 2 (s)

Site 2 (d)

Site 2 (PB)

Site 3 (PB)

Site 4 (s)

Site 4 (PB)

Site 5 (s)

Site 5 (PB)

Site 6 (PB)

Site 7 (PB)

Site 8 (PB)

Site 9 (p)

Site 9 (s)

Sie 9 (d )

Site 9 (PB)

Site 1 0 (PB)

Site 1 1 (PB)

Site 1 2 (PB)

Spring

Surfa ce Water

Primary recharge zone

Groundwater age – Upper Nepean

Radiocarbon and tritium data

The radiocarbon and tritium data indicated that at least three flow systems are present in the Hawkesbury Sandstone aquifer in the Upper Nepean catchment:

1. A shallow flow system: young water (<50 yrs (corrected)), recharged by rainfall.

2. An intermediate zone: a mixing zone between the shallow and deep groundwater (<3,500 yrs old (corrected)).

3. A deep groundwater system: oldest water (~3,500 to 8,000 yrs BP (corrected))

Conclusions

The hydrochemistry and groundwater age data allowed a better

understanding of the groundwater systems and suitability of the

Hawkesbury Sandstone aquifer as a drinking water source.