Groundwater resource investigations for Sydney's water supply at Leonay, Western Sydney, NSW

Posted 21 April 2009

Centred on the city of Sydney, the Sydney Basin extends along the east coast of New South Wales from Bateman's Bay to Newcastle, and west to the Blue Mountains.

G.E. Hawkes Parsons Brinckerhoff and J.B. Ross currently at Sydney Catchment Authority

Following encouraging results from a preliminary groundwater investigation, a second more extensive, groundwater resource investigation commenced in November 2006 to quantify the groundwater resource potential of the Hawkesbury Sandstone aquifers in western Sydney.

The program comprised of drilling eight production bores and 10 observation bores, at seven locations in the Leonay-Emu Plains area. These boreholes fully and partially penetrate the Hawkesbury Sandstone, to allow assessment of aquifer parameters, resource potential, possible interaction between surface water (Nepean River) and groundwater, and recharge/discharge characteristics. A number of multidisciplinary approaches have been undertaken to characterise the groundwater resource and to quantify potential extraction rates. These approaches include a 58 day pumping trial, multiple pumping tests using a variety of test pumping analytical techniques to interpret the test pumping data, and collection of groundwater for isotope analysis to assess recharge mechanisms and groundwater residence times.

The area at Leonay under investigation is approximately 20 km2 in size. Leonay is located approximately 60 km west of Sydney, at the foothills of the Blue Mountains, next to the Nepean River. The investigation area is within an urban environment consisting largely of residential dwellings and parkland.

Structurally the study area is located within the central portion of the Sydney Basin. The target aquifer system is the Triassic Hawkesbury Sandstone, the most widespread regional aquifer and one of the main hydrogeological units within the Sydney Basin. The Hawkesbury Sandstone aquifers can be generally described as semi-confined, but they are confined in some areas where the unit is overlain by shales of the Wianamatta Group.

The main geological feature in the Leonay area is the north-south oriented Lapstone Monocline which is aligned north-south along the foothills of the Blue Mountains. The Lapstone Monocline is a downwarped structural feature onto the Cumberland Plain with occasional high angle reverse faults. The cross-section A-A' shown in the figure shows test bores fully penetrating the Hawkesbury sandstone and the groundwater flow direction towards the Nepean River.

Groundwater within the Hawkesbury Sandstone is generally of good quality within the primary matrix and secondary structural features such as fractures, joints, shears and bedding planes. The Hawkesbury Sandstone is essentially a fractured rock aquifer. Borehole construction, lithological descriptions and geophysical logs for test bore L2B are shown in the section.

Groundwater quality within the upper Leonay aquifer is variable ranging from fresh to brackish. It is likely that leakage of saline groundwater from the overlying Ashfield Shale at some sites contributes to the brackish groundwater conditions in the upper Leonay aquifer. Major ions in groundwater of the lower Leonay aquifer indicate that groundwater there is dominated by bicarbonate (rather than chloride), which chemically evolves along the flowpath and increasing in alkalinity. The increase in alkalinity is attributable to the dissolution of carbonate minerals including calcite (calcium carbonate CaCO3) and siderite (iron carbonate, FeCO3). Throughout test pumping, the chemical composition of the groundwater changed little.

Several test pumping programs were initiated throughout this investigation to assess the performance of the Leonay aquifer and to calculate aquifer hydraulic parameters. Individual test pumping programs were conducted on each of the test production bores to assess aquifer parameters and to estimate a suitable yield for each production bore during an extended pumping trial.

An extended pumping trial was conducted for 58 days by pumping four test production bores simultaneously - the deep (L4A) and intermediate (L4B) production bores at the River Road Reserve site, and bores at Leonay Oval (L1A) and Koloona Reserve (L2A). Over this period the Leonay aquifer was pumped at a rate around 80 L/sec, discharging a total of 398 megalitres (ML) to local stormwater drains and the Nepean River. Groundwater recovery was measured sixteen weeks after the cessation of pumping and groundwater levels, in all pumping bores and the majority of observation bores had recovered by at least 80%. Throughout the test pumping program noise reduction was required in this urban environment.

The field component of the groundwater investigations has been completed at Leonay, in western Sydney to assess the groundwater resource potential within the targeted Hawkesbury Sandstone aquifers. The investigations are being carried out by the Sydney Catchment Authority as part of the diversification of potable supplies and to identify new drought water sources under the Metropolitan Water Plan initiative.

The conclusions to date from the groundwater investigations are:

  • Eight test production holes and ten observation bores were constructed to assess the Leonay aquifer and its characteristics on a broad scale.
  • The highest yields obtained in these investigations occurred in the fractured portions of the Hawkesbury Sandstone.
  • With the exception of iron and manganese, groundwater quality is good and generally suitable as a raw water source for potable supply.
  • Radiocarbon and tritium dating indicates that the age of water in the sandstone strata increases from west to east. This is consistent with up gradient recharge areas in the lower Blue Mountains and down gradient discharge areas.
  • A pumping trial conducted using four test production bores over 58 days pumping at a rate of around 80 L/sec. Groundwater drawdown levels were within the predicted range and quality varied little throughout the test.
  • Monitoring of shallow observation bores throughout the pumping trial suggested there was no hydraulic connection with the Cranebrook Formation. Therefore, borefield development would be unlikely to affect shallow aquifers associated with the Nepean River at Leonay.

The investigations suggest that a prospective aquifer exists in the Hawkesbury Sandstone along the Lapstone Monocline structural feature. Further data analysis and modeling is under way to confirm the resource potential. This groundwater resource investigation was conducted in a scientifically rigorous manner to quantify the resource potential and environmental consequences of developing the Leonay aquifer. The investigation has been invaluable in adding to our hydrogeological knowledge of the Sydney Basin which until recently, was poorly understood.

Source:

IAH News, December 2008.

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