Abstract by Dr Kevin Pucillo

The Quaternary alluvial and aeolian sediments underlying the eastern portion of the Riverine Plain have been examined to assess their impact on groundwater access and movement in the Coleambally district. Over 9800 borehole logs from the Coleambally Irrigation Area (CIA) and surrounding districts were digitized using GIS and database software and supplemented with 632 borehole logs from the Department of Land and Water Conservation (Leeton) to form the platform for stratigraphic and groundwater investigations conducted in this study.

The borehole data were summarised into two sediment classification schemes, the first to delineate the distribtuion of palaeochannel sediments and the second to assess spatial distribtuion of acquitards and aquifers. A series of detailed cross-sections differentiated between at least four distinct palaeochannel sequences identified within the Upper Shepparton Formation overlying the clay-dominated Lower Shepparton Formation. The two deepest sequences, the new Gumblebogie and Ugobit Members, comprise thick (3-10 m), laterally extensive (up to 25 km wide) sheets of coarse sandy alluvium that occur to the north of the CIA at depths between 12 to 35 metres below the surface. These deposits are evidence of highly active alluvial phases on the plain, more vigorous than any since. Slightly higher in the sequence (typically 10-20 m depth) is a thick (2-15 m), laterally extensive (up to 10 km wide) mixed-load sequence (the new Duderbang Member), which is statigraphically disconnected from the deeper sand-dominated units. Near-surface palaeochannel deposits, which consist of less extensive (up to 3 km wide) coarse sandy alluvium at depth and a combination of mixed-and bed-load sequences closer to the surface, make up the youngest palaeochannel deposits in the area. The size and extent of preserved palaeochannel sequences beneath the study area have decreated markedly since what is interpreted as the mid Quaternary and is probably symptomatic of declining fluvial activity on the Riverine Plain through to the present.

The development of source-bordering dunes associated with belts of palaeochannel material in the area was examined using shallow geophysics (GPR), topograhic surveys, laser particle size analysis and thermoluminescence dating. Dune building in the area occurred in conjunction with channel activity during the Kerarbury (55-35 ka) and Coleambally (105-80 ka) palaeochannel phases (Page et al, 1996) when sediment supply conditions were favourable, probably due to strongly seasonal discharges draining the southeastern highlands. The presence of stabilising vegetation on the channel margins is believed to have played a key role in the development of dunes in the area, which show poorly developed internal structure. The increased precipitation that enhanced channel discharges must also have sustained dune vegetation. Increased aridity and reduced vegetation approaching and during the LGM appears to have caused the widespread remobilization of dune crests.

The complex alluvial and aeolian stratigraphy of the Shepparton Formation strongly influences groundwater movement in the Coleambally district. Piezometric levels show that deeper, highly permeable units (e.g. Ugobit Member) in the northern part of the study area act as conduits for the discharge of groundwater, helping to maintain much lower watertables in the north as well as induce groundwater flow from the CIA. 'Sediment vs depth' analysis demonstrates that there is limited vertical connection through the thick, clay-dominated sequence of the Lower Shepparton Formation to the highly transmissive sands and gravels of the underlying Calivil Formation. The tendency of the Lower Shepparton Formation to restrict downward leakage is likely to both contribute to the problem of shallow irrigation-induced watertables in the CIA as well as impede efforts to lower watertables by pumping from deeper aquifers. The upper 3 metres of the Shepparton Formation are clay-dominated with a low water storage capacity, causing near-surface watertables to be highly responsive to recharge by surface water. The available data indicated that when piezometric levels fall below this clay-rich zone they are able to absorbe short-term increases in recharge due to the higher proportion permeable alluvium in the unsaturated zone.