Engineers Australia

Hydrogeology of the Botany Basin

R. K. Hatley

Abstract

This paper seeks to review the current knowledge of the geology and hydrogeology of the Botany Basin, and focuses on how the latter impacts on its geomechanical behaviour. It will consider, briefly, the basin’s encapsulating bedrock foundation rocks, their intersection with the basin fill sediments, the latter’s structure, stratigraphy, lithology, distribution and how these impact on its hydrogeological behaviour. The discussion will then consider the basin’s economic and beneficial value, development within the basin and how this has historically impacted on the basin’s hydrogeology, hydrogeochemistry, water quality and economic value and how development projects need to consider their impact on the basin’s condition and the existing development it supports. Two case studies are presented to illustrate the latter.

At the outset, it is appropriate to define what the term ‘Botany Basin’ constitutes. Rickwood (1998) notes that there “…are those geologists who regard it as a tectonically formed bedrock depression that is the result of post Triassic uplift and warping, and is the smaller part of the larger Sydney Basin” (referencing Roy, 1983), which contrasts with the general view held by hydrogeologists that tend to apply ‘… the name Botany Basin to the topographic depression that is covered by the unconsolidated sediments that form the Botany Sands aquifer’ (referencing Griffin, 1963). Rickwood (1998) further develops the interpretation of the Botany Basin as being “… an easily verifiable bedrock basin … centred on Botany Bay and approximates to the catchment area, but excludes(ing) the extensive drainages of the major rivers entering the basin.” Rickwood then considers the relevance of the Pleistocene basin area and that of the modern basin, settling on the latter as the general basis for outlining the extent of the Botany Basin boundaries.

This paper broadly adopts the general extent of the modern basin as the definition of the Botany Basin, and focuses on the Quaternary sediments contained within that basin and how this aquifer interacts with older bedrock formations which comprise the Pleistocene bedrock paleochannel/paleobasin. The hydrogeological aspects of the bedrock formations, primarily the Hawkesbury Sandstone and Ashfield Shales, are discussed in detail in papers elsewhere in this volume.