The Ivanhoe Block-its structure, hydrogeology and effect on groundwaters of the Riverine Plain of New South Wales.

The Ivanhoe Block is a faulted and uplifted concealed basement ridge complex underlying the mallee sand dunes adjacent to the New South Wales part of the Western Riverine Plain in the central Murray Basin. The western ridge of the Ivanhoe Block forms the regional divide between the Darling and Lachlan-Murrumbidgee groundwater systems. The parallel basement ridges of the Ivanhoe Block deflect flow south in the deeper aquifers, and thinner beds on top of the ridges result in convergence of flow in the shallower aquifers. The Geera Clay aquitard is enveloped by the Renmark Group on the Ivanhoe Block this produces an additional barrier to lateral groundwater throughflow. Groundwater salinity is strata-bound in the aquifers of the Ivanhoe Block and Western Riverine Plain, and this enhances the value of electric logs in accomplishing a 3-fold subdivision of the Renmark Group- a basal fluvial succession, overlain by a paralic grading to marginal marine sequence which in turn is overlain by a prograding shoreline succession. The Western Riverine Plain is the regional groundwater discharge zone for the eastern Murray Basin in New South Wales, and it has been created by the impeding action of the bounding Ivanhoe Block . On the basis of rates of change of chloride concentration along regional flow lines in the Tertiary aquifers, the Western Riverine Plain is partitioned into the Balranaid- Hatfield discharge zone in its western half and the Moulamein - Mossgiel buffer zone in its eastern half; the former defines the zone of salt production and accumulation in the shallower Tertiary aquifers and the latter defines the maximum historical extent of up-basin propagation of refluxed salts. In recent years the Ivanhoe Block and western Riverine Plain of New South Wales have been the focus of growing community concern about clearing in the mallee lands and the susceptibility of these areas to land salinisation. This paper addresses the second issue. The Balranald Hatfield discharge zone and the lower Willandra Lakes are most at risk from land salinisation if water tables continue to rise. The Moulamein Mossgiel buffer zone is in the second-highest risk category, and the eastern Riverine Plain has a low risk of sa linisation in its non-irrigated lands.