Surface-temperature silica springs of the eastern Great Artesian Basin – Hydrogeology and hydrochemistry

• The recharge area of the Hutton Sandstone aquifer is less than previously estimated. • The Eastern Eromanga springs are water-table windows. • They are fed by flow in subhorizontal joints within a surface silcrete layer. • The springs precipitate siliceous minerals despite being at surface temperature. • High dissolved Si levels are due to dissolution of silcrete microcrystallites. An unusual north–south line of ∼ 50 springs along the eastern margin of the Eromanga Basin, northeastern Australia, discharge from the outcrop margin of the Hutton Sandstone aquifer, which is folded so the springs are fed by easterly groundwater flow, in contrast to the dominant westwards flow within the Eromanga Basin. This means that the effective recharge area of the Hutton Sandstone in this region is less than previously estimated. The springs occur as pools which represent water-table windows, with groundwater ‘streams’ flowing from one side to the other along subhorizontal joints within a near-surface silcrete layer developed on the Hutton Sandstone. The springs are recharged through fractures in the silcrete, feeding laminar groundwater flow through the Hutton Sandstone until its outcrop terminates. At this point flow transfers into the overlying silcrete as concentrated pathways probably localised along broad, shallow troughs in the silcrete beneath surface drainage lines. The springs are surrounded by white siliceous precipitates with a groundmass of intergrown amorphous silica and kaolinite; this may have been allophane originally. Most silica springs are geothermal, yet the eastern Alice Tableland springs have surface temperatures. The elevated dissolved Si levels in these springs are due to dissolution of relatively soluble silica microcrystallites within the silcrete through which the spring water flows. The lack of calcite precipitation from the springs reflects the low Ca concentrations of the groundwater, probably due to strong Ca uptake by plants.