BASIN GEOMORPHOLOGICAL CONTROLS ON THE MECHANISM OF RECHARGE AND DISCHARGE AND ITS EFFECT ON SALT STORAGE AND MOBILIZATION - COMPARATIVE-STUDY USING GEOPHYSICAL SURVEYS

Downhole electromagnetic (EM) profiles have been used to study the salt distribution in four catchments in the wheatbelt of Western Australia. The good correlation (r2 = 0.7-0.9) obtained by relating the apparent conductivity profiles measured by the downhole EM with salt profiles from soil cores enabled estimation of the salt storage in the uncored wells. The salt distribution was compared with ground conductivity surveys (using EM31 and EM34 instruments) in three catchments and with an INPUT aerial electromagnetic survey (AEM) in a fourth catchment. The study revealed the presence of five types of apparent electrical conductivity profiles (ECa) which are correlated with the basin geomorphological units. The conductivity profiles are: (1) low ECa (recharge) profile; (2) subsurface peak (discharge) profile; (3) two bulge profiles - a single bulge (accumulation-weathering) profile and a split (aquifer development) profile; (4) reduced bulge (leaching) profile; (5) high ECa irregular (palaeochannel) profile. The salt distribution showed a systematic increase in salinity from the watersheds to the valleys with salinity increasing upstream of geological structures. The salt distribution was found to be controlled by the geomorphologic properties of the basins, which control the hydrological and hydrogeological processes, and by the recharge discharge mechanisms which influence salt mobilization and accumulation.