Oral Presentation Australasian Groundwater Conference 2017

The role of surface-groundwater interactions in salt delivery from a small upland salt scald in Central west NSW (#41)

David C Mitchell 1
  1. NSW DPI, Orange, NSW, Australia

The objectives of this research was to better understand the processes that mobilise and transport salt from the land to the stream and the role groundwater has, if any, in the process.

Dryland salinity is attributed to the removal of deep rooted vegetation, salt is mobilised from deep in the soil and transported to the soil surface and/or into waterways by rising groundwater.  recent research has challenged that paradigm and indicates that changes are mainly due to climate influences; as well stream EC follows an oscillating path of rising during wet periods and falling during dry periods.

Method 

Two plots (one revegetated) of 1 ha in size were established on a saline scald in central NSW, climate, groundwater and surface runoff from September 2003 to present were measured.

Results

A change point analysis was performed on the event mean concentration (EMC) of the runoff water. Change points were detected in 2006, 2012, 2013 and 2014.  There was a high EMC of around 4 kg/mm from 2003 to 2006, then a low EMC of 0.4 kg/mm from 2006-2012, then a reversion to a high EMC 4 kg/mm, and then a return to an EMC of 0.4 kg/ha. These periods of high EMC coincided with periods of groundwater that within 1.5 m of the ground surface while the periods of low EMC runoff water coincided with ground water being below 1.5m. 

Conclusion

The watertable elevation is the primary driver of salt export coupled with surface runoff events. Only when the watertable is within a critical distance from the soil surface are significant amounts of salt exported. If scaled up, this phenomenon is capable of providing a causal mechanism for the larger-scale observations, particularly the oscillation in stream EC reported particularly in the 2009 NSW Salinity Audit.

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