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Salinity and IRZ Mining

Salinity is a major issue Alcoa must deal with to protect the quality of our water supply catchments. The Darling Range has been divided into three rainfall zones; high, intermediate and low. Alcoa currently mines within the High Rainfall Zone (HRZ) where historical monitoring of the streams draining our mining operations has clearly shown that we do not cause an impact to 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). The IRZ and LRZ receive less than 1100 mm 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 that if groundwater levels were to rise following the removal of forest vegetation, high salinity groundwater could discharge to streams. This means if mining and rehabilitation operations in the IRZ are not planned and managed effectively temporary increases in stream salinity might occur.
 
Alcoa has made a commitment that it will not mine in the lower rainfall zones of its lease until research has shown that mining can be accomplished without significantly adversely impacting the salinity of water resources.   IRZ mining has now commenced following BHC approval in the O'Neil region.
 
Since 1979 long term research has been in progress to demonstrate that Alcoa can mine sustainably in areas where stream salinity may be an issue. We have collaborated with representatives from government agencies, research organisations and universities to determine what impact bauxite mining will have in the IRZ and what management practices we can use to minimise its impact on the water resources of the region.
 
In 2004 Alcoa began a trial and demonstration mining project within the IRZ Cameron group of experimental catchments.  Complete rehabilitation of the area is due for completion by 2011.
 
In 2007 a formal approval process for Alcoa to follow to gain access to ore in the IRZ was developed and agreed with the MMPLG and the Bauxite Hydrology Committee (BHC, a technical subcommittee of the MMPLG).  This process allows the BHC and the MMPLG to consider areas in the IRZ on a case by case basis, and requires a significant salinity risk assessment to be completed and, where necessary, a risk management plan to be developed prior to mining.
 


Map of rainfall zones

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Alcoa's mining lease incorporates high, intermediate and low rainfall zones.

Streamzone

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A streamzone in the Intermediate Rainfall Zone.