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 sal t distribution in four catchments in the wheatbelt of Western Australi a. The good correlation (r2 = 0.7-0.9) obtained by relating the appare nt conductivity profiles measured by the downhole EM with salt profile s from soil cores enabled estimation of the salt storage in the uncore d wells. The salt distribution was compared with ground conductivity s urveys (using EM31 and EM34 instruments) in three catchments and with an INPUT aerial electromagnetic survey (AEM) in a fourth catchment. Th e study revealed the presence of five types of apparent electrical con ductivity profiles (ECa) which are correlated with the basin geomorpho logical units. The conductivity profiles are: (1) low ECa (recharge) p rofile; (2) subsurface peak (discharge) profile; (3) two bulge profile s - a single bulge (accumulation-weathering) profile and a split (aqui fer development) profile; (4) reduced bulge (leaching) profile; (5) hi gh 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 d istribution was found to be controlled by the geomorphologic propertie s of the basins, which control the hydrological and hydrogeological pr ocesses, and by the recharge discharge mechanisms which influence salt mobilization and accumulation.