Characterising aquifer geometry in the Northern Botany Basin, Sydney, Australia

Rebecca Higgins
Supervisor: Associate Professor Ian Acworth and Dr. Wendy Timms

Introduction

This project aims to determine the geographical extent of groundwater within the northern part of the Botany Basin, and to use field and numerical methods to ascertain the depth of the groundwater within this region.

Research Methods

A literature review was initially undertaken. Mapping of the bedrock extent was conducted through site observations. Water levels were observed in ponds and monitoring bores over a 48 hour period for use in the analysis. The numerical modelling package Surfer 8 was used to construct contour maps and surfaces.

Data Utilised

Geological Data
A total of 96 bores records were used to construct a bedrock contour map. These were sourced from numerous historical records, and all the bore records used extended to bedrock or ended in silt or clay. Lithological records indicate several highly pervious water bearing layers, comprised of aeolian and marine sands with thin, discontinuous peat lenses.

Water Level Data
Field work was undertaken at seven monitoring bores and at 16 surface water bodies to gain an appreciation of the extent of the aquifer. A dip meter was used to measure depth to water table, and each water body was measured as height relative to a surveyed point, or if none existed, relative to a height estimated from the Botany Bay 1:25000 Topographic Map. Surface waters were important, as these are easily accessed, and provide a ‘window’ to the aquifer levels.

Figure 1 - Bedrock contour map of the Botany Basin

Results

Geographical Extent
Figure 1 shows the extremities of the northern part of the aquifer as outlined through observations and with reference to Griffin (1963). This outline is a significant change to what was previously believed to be the aquifer boundary, resulting in a smaller aquifer than believed. The aquifer is bounded by Hawkesbury sandstone in the north, and by thick clay deposits in the west. Within the eastern part of the aquifer numerous bedrock outcrops were observed. The artificial southern boundary was chosen for flow analysis to be undertaken at a later time.

Figure 2 - Thickness of Botany Aquifer

Hidden Geological Boundaries
The bedrock topography is also shown on Figure 1, with the boundary showing where bedrock reaches the surface. The presence of a paleochannel is easily seen as a marked decrease in height of the bedrock from the northern area around Centennial Park down to the west of UNSW. This signifies a large storage capacity within the relatively low relief sand that has accumulated since the channel was carved.

Aquifer Thickness
Figure 2 shows the thickness of the Botany aquifer as determined using Surfer 8. The paleochannel is where the thickest part of the aquifer occurs, and it is from this area that many industries withdraw water.

Recommendations for further Research

The installation of additional monitoring bores within the eastern sector of the study area is imperative to building a realistic model of the aquifer. Withdrawal from the aquifer should be quantified to develop an effective understanding of the local water cycle. A comprehensive recharge and quality study is required to assess resource sustainability, at current and future pumping rates.

There is scope for chemical analysis of the aquifer on a larger scale than has previously been attempted. Implementation of independent data loggers in all monitoring bores could determine short term groundwater levels and variability in groundwater salinity.

Figure 3 - Centennial Park Ponds (Photo courtesy of Randwick City Council)

Conclusions

The Botany Basin is an invaluable resource to Sydney. With the natural beauty of Centennial Park and Eastlakes, and the numerous industries dependent on it for their existence, a greater understanding of the aquifer is imperative for the sustainability of our environment, and our quality of life.

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