Hydrogeology around Jamestown, South Australia, revealed through airborne geophysics

Abstract

The farming communities around Jamestown in the Northern Agricultural District of South Australia have experienced periodic crop losses due to salinity and waterlogging. Detailed soil maps and localised studies had not provided a comprehensive understanding of the causes of salinity, and a larger-scale approach was required. The area was flown with airborne electromagnetics (AEM - TEMPEST), magnetics and radiometrics in mid-2002 over three parallel N-S valleys with different expressions of salinity: Caltowie Valley suffers from surface scalding, infrastructure damage and very shallow, saline watertables. Jamestown/Bundaleer Valley has regions of crop failure across the valley floor, seasonal waterlogging and periodic shallow, saline watertables. In Belalie Valley isolated soil degradation and scalding occurs only on north-facing slopes, with minimal salinity issues on the valley floor. Topography shows an increase in valley size from Caltowie to Jamestown to Belalie. Surface drainage is minimal (rainfall 400-500mm/a), but seasonal floods over thick clay soils result in almost yearly waterlogging, particularly at bottlenecks in the topography: in Caltowie and Jamestown Valleys. Airborne magnetics reveal an extensive sub-surface drainage system, defining preferential groundwater pathways beneath broad, flat plains. This has important differences from the surface drainage, helping explain the observed patterns of salinity. The AEM confirms these pathways adding the element of depth; explaining why and where salinity is, and is not, expressed at the surface. Radiometrics highlight surface distribution of materials through and between valleys, particularly the association with salt-bearing waters. Down-hole and groundwater chemistry confirms the relationships shown by the airborne geophysics, defining the scale of groundwater flow systems responsible for the salinity concerns of the region.