Groundwater occurs in a variety of rock types and can be classified according to the rock types or aquifers in which it occurs:
Surficial aquifers occur in alluvial (water-formed) sediments in river valleys, deltas, and basins, in lake or lacustrine sediments, and in aeolian (wind-formed) deposits. They are essentially unconsolidated clay, silt, sand, gravel, and limestone formations, mainly
of Quaternary age. These deposits are easily exploited and are the major sources of potable groundwater.
Figure 1. Groundwater sources in the MDB (Source: National Land
and Water Resources Audit, 1988, Hydrogeology of Australia,
Geoscience Australia 1987).
Sedimentary aquifers occur in consolidated
sediments such as porous sandstones and conglomerates, in which water is stored in the intergranular pores, and limestone, in which water
s stored in solution cavities and joints. These aquifers are generally located in large, continuous sedimentary basins that may be tens or hundreds of metres thick. In terms of quantity, they contain the largest groundwater resources, although much of the water is of marginal quality. Nonetheless, these aquifers are of critical importance over much of inland Australia.
These occur in igneous and metamorphosed hard rocks which have been subjected to disturbance, deformation, or weathering, and which allow water to move through joints, bedding plains, faults, solution cavities, and zones of weathering. Although fractured rock aquifers are found over a wide area, they contain much less available groundwater than surficial and sedimentary aquifers. Due to the difficulty of obtaining high yields, the quantities of water taken from them are relatively low.
