Ground displacements in the Lower Namoi region
Below the land surface, groundwater occupies tiny spaces known as ‘pores’ between grains of sand and gravel, and in clay. In aquifer systems the groundwater helps support the overlying sediments. If the groundwater level drops by too much for too long, the drop in pore pressure can result in the collapse of the pore spaces and cause compaction of the sediments. Compaction over a large area reduces the amount of groundwater that can be held in storage and is irreversible. In some cases this can lead to land subsidence.
The Lower Namoi aquifer was identified as “at risk to the structural integrity of the groundwater system” during the development of the Water Sharing Plan for the Upper Namoi and Lower Namoi Regulated River Water Sources 2016. Land subsidence and its relation to groundwater extraction in the Lower Namoi aquifer was assessed and quantified in the report ‘Ground displacements in the Lower Namoi region’ (PDF, 36MB).
The study investigated and compared several datasets: surveyed benchmarks, historical groundwater levels, and remoting sensing radar imagery (Interferometric Synthetic Aperture Radar (InSAR).
The report 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 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 - expected in this area.
This work is a benchmark for future studies into land subsidence in the Lower Namoi. The knowledge gained from this work will benefit other areas of the State where land subsidence is a risk.