THE APPLICATION OF AIR, GROUND AND ‘IN RIVER’ ELECTROMAGNETICS IN THE DEFINITION OF SPATIAL PATTERNS OF GROUNDWATER INDUCED SALT ACCUMULATION IN A SALINISING FLOODPLAIN, LOWER RIVER MURRAY, SOUTH AUSTRALIA

Floodplains play an important role in catchment hydrology, representing a zone in which groundwater is shallow, and groundwater - atmosphere interactions through evapotranspiration (ET) are more pronounced. Spatial patterns in evapotranspiration develop due to the variable distribution and type of floodplain sediments, patterns of vegetation type, floodplain elevation and geometry. Where groundwater is saline, as in the lower River Murray in South Australia, evapotranspiration concentrates salt and can lead to floodplain salinisation, vegetation dieback or health decline. Modelling these systems can assist our understanding of which areas of vegetation are at high risk from salinisation in order to target them for management. However, models of this scale require detailed data for parameter population and validation. This paper discusses the potential application of airborne, ground and “inriver” electromagnetic techniques to indicate spatial patterns of groundwater evapotranspiration and baseflow from a salinising floodplain on the lower River Murray. High resolution RESOLVE frequency domain helicopter EM data are examined along with ‘in river’ NanoTEM and ground EM31 data. Their study demonstrated the scale of variations in baseflow was smaller than expected, showing alternation between losing and gaining (from a salt load perspective) groundwater in a river broadly understood to be a gaining system. The HEM and ground data sets are strongly correlated with groundwater salinity and are now being trialed as a basis for validating groundwater models.