Geophysical investigation of River Murray salinity: a case study using fast-sampling TEM

Abstract

Australian Water Environments (AWE) has been engaged by the Bookpurnong - Lock 4 Environmental Association (BL4EA) to investigate the interception of saline groundwater along the River Murray downstream of Berri, South Australia (Figure 1). Groundwater discharge contributes approximately 70 tonnes per day of salt to this reach of the river. Salt loads are predicted to rise to more than 200 tonnes per day over the next 30 years. The proposed Environmental Enhancement Scheme (EES) comprises a series of shallow pumping wells constructed primarily on the floodplain to intercept saline groundwater before it reaches the river. Intercepted groundwater will be pumped to the Noora Disposal Basin located some 25 km inland. A detailed understanding of the hydrogeological environment is essential to the design and effective operation of the EES and similar groundwater interception schemes. In March 2001, Zonge Engineering conducted a fast-sampling TEM (time-domain EM) traverse to investigate changes in sub-surface resistivity across the highland area adjacent the River Murray and the floodplain within the riverine trench. The highland is planted with citrus and has been irrigated since the 1960?s. The shallow depth to saline groundwater within the Monoman Formation aquifer has seriously degraded the health of native vegetation health across the floodplain. In May 2002, twelve shallow wells were constructed along a narrow stretch of floodplain located approximately one kilometre downstream of the first traverse. Accurate formation samples were collected and logged. Zonge Engineering conducted a second fast-sampling TEM survey in June 2002 along a line parallel to the trial borefield to investigate the correlation between TEM data and variations in hydrogeology identified during drilling. This paper reports the results of the two land-based geophysical surveys and comments on the utility of fast-sampling TEM as a tool for mapping subsurface hydrogeology across the highland and floodplain at Bookpurnong.