Monitoring Changes to Freshwaters Lens Systems in a “Sea” of Saline Groundwater across the Murray River Floodplain in SE Australia using Multi-date Airborne EM

Abstract:

Saline aquifers in the Murray River of SE Australia are traversed by freshwater rivers, with adjoining riparian zones and floodplains containing freshwater lenses with widely varying geometries. Bore data and more recent AEM surveys have determined that these lenses are spatially extensive. The maintenance of these lenses, part of the Critical Zone, is important as they support environmentally significant riparian vegetation such as Red Gum (Eucalytpus camaldulensis) and Black Box (Eucalytpus largiflorens). A more complete understanding of their hydrogeology is required to ascertain how they develop and degrade. Limited ground investigations including 14C geochemistry have determined that the lens systems contain recent water (decades), indicating that they are dynamic systems, with their development defined by the relative rates of recharge from the river and mixing with the regional groundwater. Changes in groundwater gradients and floodplain extent, topography or depth to groundwater are believed to control their initial location. The same controls also govern their stability.

We examine the role of airborne EM systems in defining the geometry of these lens systems in 3D. An assessment of their value for monitoring variations associated with these ecosystems, including spatio-temporal processes connected with surface water and groundwater interactions, formed part of the study. The advent of “calibrated” AEM systems and robust inversion tools have given added impetus to their use for monitoring. The focus was over the Katfish Reach of the Murray River in South Australia. Spatio-temporal variations are observed in the near surface (top 20m) from a multi-temporal assessment of Clark’s Floodplain, adjacent to the Bookpurnong Irrigation area in co-incident airborne EM surveys acquired between 2008 and 2015. Spatial changes in ground conductivity, attributed to changing groundwater quality have been observed. The freshwater lens systems appear to have contracted significantly over the last decade. This is attributed, in part, to land use patterns and the development of an irrigation-related groundwater mound on the highlands adjacent to the floodplain, and an increased hydraulic gradient towards the river. The results indicate the geometry of the hyporheic zone may have also changed along the river.