marine

THE SALINISATION OF MAJOR RIVER SYSTEMS IN AUSTRALIA, AND IN OTHER COUNTRIES, IS PRIMARILY DETERMINED BY THE UPWARD MOVEMENT OF SALINE WATER FROM REGIONAL AQUIFERS INTO THE RIVER. THE MIGRATION HAS BEEN ACCELERATED DUE TO IRRIGATION SCHEMES AND FARMING PRACTICES THAT HAVE CHANGED REGIONAL HYDRAULIC GRADIENTS DRIVING SALINE-WATER IN AQUIFERS TOWARDS THE MAJOR DRAINAGE POINTS IN THE LANDSCAPE. IN THIS PAPER, WE DESCRIBE RESULTS FROM A TRANSIENT ELECTROMAGNETIC (TEM) SYSTEM THAT HAS BEEN DEPLOYED TO MONITOR THE INFLUX OF SALINE WATER THROUGH SUB-RIVERBED SEDIMENTS. THE DEPLOYMENT WAS A FLOATING ARRANGEMENT OF A COMMERCIAL FAST SAMPLING (HIGH RESOLUTION) TEM SYSTEM THAT IS SENSITIVE TO SHALLOW ( THE TECHNIQUE HAS BEEN EXTENSIVELY TRIALED AROUND THE RIVER MURRAY TOWN OF WAIKERIE IN SOUTH AUSTRALIA. AN INITIAL SERIES OF SURVEYS ALONG A 40 KM SECTION OF RIVER SHOWED A RANGE OF SUB-RIVERBED RESISTIVITIES BETWEEN 1 AND 20 OHM-M, WITH A TOP LAYER OF ABOUT 10-15 OHM-M CLOSELY FOLLOWING THE WATER DEPTH. REGIONS OF HIGH-RESISTIVITY IN THE RIVERBED SEDIMENTS CORRELATED WELL WITH SALINE-AQUIFER BOREHOLE PUMPING LOCATIONS, INDICATING A LOCALIZED DRAWDOWN OF FRESHER RIVER WATER. LOW-RESISTIVITY ANOMALIES HAVE BEEN INTERPRETED AS REGIONS OF SALINE WATER INFLUX INTO THE RIVER. THE TECHNIQUE IS NOW USED FOR ROUTINE MAPPING IN AUSTRALIA, WITH OVER 800 KM OF THE MURRAY SURVEYED, AND HAS POTENTIAL APPLICATION TO OTHER MAJOR WORLD RIVER SYSTEMS.

INCREASING SALINITY IN THE RIVER MURRAY IS WELL DOCUMENTED AND IS OF CONCERN ENVIRONMENTALLY, ECONOMICALLY AND SOCIALLY. THE MURRAY DARLING BASIN COMMISSION AND THE MALLEE CATCHMENT MANAGEMENT AUTHORITY ENGAGED THE AUTHORS TO COLLECT BASE-LINE IN-STREAM NANOTEM DATA WITHIN THE RIVER MURRAY FROM LOCK 1 TO MALLEE CLIFFS (675 KM). THIS IS A NEW APPLICATION OF A HIGH RESOLUTION FAST SAMPLING TRANSIENT ELECTRO-MAGNETIC (TEM) SYSTEM, TOWED BEHIND A BOAT, TAKING SOUNDINGS EVERY SEVEN TO TEN METRES ALONG THE RIVER. THE OBSERVED NANOTEM RESPONSE WAS INTERPRETED AGAINST THE CURRENT UNDERSTANDING OF THE REGIONAL HYDROGEOLOGY AND GROUNDWATER PROCESSES IN AND AROUND THE RIVER. THIS PAPER SUMMARISES SOME OF THE RESULTS FROM THIS INVESTIGATION. THE OBSERVED RESPONSE CORRELATES STRONGLY WITH PREVIOUSLY MAPPED MAJOR CHANGES IN UNDERLYING LITHOSTRATIGRAPHY ALONG THE MURRAY RIVER, AND WITH GAINING AND LOSING REACHES OF THE RIVER. THE EXTENSIVE LENGTH OF THE SURVEY PROVIDES AN INSIGHT INTO POTENTIAL INTERACTIONS BETWEEN THE RIVER, FLOODPLAIN AND GROUNDWATER, BUT DOES NOT REPLACE THE NEED FOR FOCUSSED GROUND-TRUTHING PROGRAMS TO EXAMINE SPECIFIC CORRELATIONS. THIS RAPID, PORTABLE TECHNIQUE SHOULD BE APPLICABLE OUTSIDE THE MURRAY-DARLING BASIN AS WELL AS AT ADDITIONAL LOCATIONS WITHIN THE BASIN.