Ground displacements in New South Wales
Below the land surface, groundwater is found in tiny spaces known as ‘pores’ between grains of sand and gravel, and in clay. Sand, gravel and clay are also called sediments. Groundwater can help to support the structure of sediments by filling these tiny pore spaces. If the groundwater level drops by too much for too long, this can cause pore spaces to collapse. This can then cause compaction of the sediments. Compaction over a large area permanently reduces the size and amount of pore spaces between the sediments. This reduces the amount of groundwater that can be stored in that area. In some cases, compaction can lead to land subsidence. This is where the land surface sinks as the sediments underground have moved. Land subsidence is also known as ground displacements.
On the behalf of the department, CSIRO studied the impact of groundwater extraction on ground displacements in the Lower Namoi groundwater source and groundwater sources in southern inland NSW including the Murray, Murrumbidgee and Lachlan. To investigate ground displacements, CSIRO compared several datasets: surveyed benchmarks, historical groundwater levels, and remote sensing radar imagery (Interferometric Synthetic Aperture Radar (InSAR).
For the Lower Namoi, the study found that some land subsidence has occurred during the 45 years of field benchmark data. However, there is no identifiable land subsidence that can be attributed to groundwater extraction during the three-year period from 2015 to 2018 during which the InSAR analysis was done. The land displacement during this three-year period appears to be due to periodic surficial shrinking / swelling of clays which is expected in this area. Read more about Ground displacements in the Lower Namoi region.
For the southern inland areas, the study found land subsidence of up to 30 mm/yr in some isolated areas near Hilston and Coleambally due to groundwater extraction. However, some of this subsidence could have occurred prior to the study, such as during the Millennium drought when groundwater levels were dropping due to groundwater extraction and the significant dry climate. Read more about Ground displacements over alluvial aquifers in southern inland NSW.
This work is a benchmark for future studies into land subsidence in NSW. The knowledge gained from this work will benefit other areas of the State where land subsidence is a risk.