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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
20 20 40 60 80
20
40
60
80
20
40
60
80
20
40
60
80
20
40
60
80
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
20
40
60
80 80
60
40
20
20
40
60
80
20
40
60
80
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.