Application of a New TEM Technique to Assist in Evaluation of Changes in Soil Condition to Support Management of Environmental Water to Floodplains in a Heavily Regulated River System

The Murray-Darling Basin, Australia, supports a vast ecosystem (1.06 million km2) and $7 billion of irrigated horticulture. River regulation and extraction has resulted in longer return intervals between unregulated floods, reduced duration of floods, and marked declines in ecosystem function including widespread die-off of floodplain trees. The sources of soil water that support tree growth include relatively low salinity groundwater with a moderate depth below ground level, rainfall, lateral-bank recharge during periods of elevated flows, and vertical recharge during floods. Monitoring and maintaining an adequate supply of biologically available water in the unsaturated soil zone is a key challenge to sustaining floodplain vegetation, particularly where rainfall is low (<250 mm/yr) and shallow depth to highly saline groundwater (often >1,000 mS/m) is a stressor. Current soil and groundwater monitoring approaches supply information at discrete locations from monitoring groundwater wells (<10 m) and shallow (5.5 m) soil cores. Near-surface geophysical techniques offer a pathway to provide high resolution information on soil conductivity that can fill in gaps between sites. In May 2021, baseline geophysical data sets (~100 km) were collected at predetermined locations over two large floodplains (6,700 and 9,000 ha) in South Australia where infrastructure has been constructed to enable managed delivery of environmental water during flows at which the floodplains would otherwise remain in a dry phase. Data was collected using LoupeEM, a new time domain electromagnetics (TEM) system designed to collect conductivity information from the surface to ~30 m depth. LoupeEM is a backpack-portable, two-person system that allows data to be collected continuously, over terrain that may be difficult to access. The baseline results show excellent correlation with previously mapped areas of low salinity groundwater, as well as high salinity zones. Planned repeat surveys are expected to provide a line of evidence to support prioritisation of managed delivery of environmental water. These types of surveys will allow evaluation of vertical and lateral movement of groundwater, redistribution of stored salt, and recharge of the soil profile with low salinity surface water.