The Sydney LPG Cavern is located at Port Botany, New South Wales. The cavern is an unlined rock storage cavern mined out of the Hawkesbury Sandstone and located at a depth of approximately 124 metres below ground for storing Liquefied Petroleum Gas (LPG). A ‘water curtain’ was constructed approximately 15 metres above the top of the storage galleries to maintain saturation and to ensure permanent groundwater flow towards the mined cavern, which is particularly important when considering impacts of climate change on long-term groundwater levels.
The LPG cavern has been operational for 19 years. During the early years of operation, seepage inflows to the cavern were becoming more saline. This trend was mostly reversed with the injection of large volumes of fresh water into the water curtain above the cavern (>2,000 kL/month). However, since early 2018, water curtain injection volumes have been low (<250 kL/month), and the net contribution of higher salinity groundwater to cavern seepage has increased.
Groundwater and process water chemistry and water level data has been collected every three months since 2000. Hydrogeological and hydrogeochemical factors were examined to better understand the salinity trends in the deep groundwater and cavern seepage waters. Groundwater levels remain lower than pre-construction levels, however data from piezometers down-gradient of the cavern suggests seawater intrusion from Botany Bay, the result of a reversal of hydraulic gradients, is no longer occurring. The large difference in hydraulic head between the upper and lower aquifers causes a downward flux of saline groundwater from the Botany Sands aquifer, from the upper to the lower sandstone aquifers.
Chloride/bromide ratios and d18O data showed that the lower Hawkesbury Sandstone remained fresh on the seaward side of the cavern.