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2010 Case Study: Salinity and intermediate rainfall zone mining
The salinity of water resources is an important issue for us to address in our efforts to protect the quality of water supply catchments. The Darling Range has been divided into three rainfall zones: high, intermediate and low. In high rainfall zones historical monitoring of the streams draining our mining operations has shown that our operations do not affect the salinity of water resources. However the eastern part of our bauxite mining lease extends into the intermediate rainfall zone (IRZ) and low rainfall zone (LRZ), which are more susceptible to changes in salinity. The IRZ and LRZ receive less than 1,100mm of rainfall a year and contain approximately one third of Alcoa's known Darling Range bauxite resources.
While the HRZ has only low levels of salt stored in the soil, the IRZ has sufficient salt stored in the soil so that if groundwater levels rise following the removal of forest vegetation, high salinity groundwater could discharge to streams. Therefore if mining and rehabilitation operations in the IRZ are not planned and managed effectively, there is the possibility for them to cause temporary increases in stream salinity.
Since 1979, Alcoa has been researching the impact bauxite mining will have in the IRZ and examining ways to minimise the impact on the water resources of the region. In 2004 Alcoa began a Trial Mining Project (TMP) within the IRZ Cameron group of experimental catchments.
Final mining in the Cameron’s experimental catchments as part of the project took place in February 2010 and rehabilitation of the last of the mined areas in these catchments commenced later in 2010.
Analysis of trial results has so far indicated a low risk of increased salinity in streams as a result of mining. This is because declining groundwater tables as a result of a drying climate far exceed any observed transient rises in groundwater levels due to clearing for mining.
Results indicate that there have been variable rises in groundwater levels under or close to mine pits, as expected. However the magnitude of the rises is small in relation to the depth at which the groundwater presently sits (up to 30m or more). This means that there is very little likelihood of saline groundwater reaching a level high enough for salts to enter streams.
So far there are no observed effects of mining on either stream flows or stream salinity. Groundwater levels in areas where rises have been observed are expected to stabilise and then decline as the rehabilitation develops, even if we experience a series of wetter than average rainfall years.
A full independent review report of the hydrological response in the area to mining and rehabilitation to date was completed in early 2011 and presented to the Government.
In addition to this in 2006 Alcoa sought and obtained approval to change our reliance on the trial mining project as the sole determinant of approval for general IRZ access. We now have a ‘staged approval process’ whereby discrete and smaller areas within the IRZ are considered and assessed for salinity risk and subsequent mining entry (such as the present O’Neil mining region) – in addition to the ongoing TMP.