Down-slope change in soil hydrogeochemistry due to seasonal water table rise: Implications for groundwater weathering

Landscape-scale chemical weathering has gone through a fundamental re-think due to: 1) the acknowledgement that a majority of solutes in most soils originate from wet fall and dry fall; and 2) that the zone of active weathering in many landscapes is much deeper than previously thought. With the goal of better understanding where weathering is occurring, a toposequence of five soil/regolith profiles from ridge crest to valley bottom in the Mt Lofty Range of South Australia was investigated using whole soil geochemistry, soil exchange pool and soil pore water hydrochemistry; stream water and groundwater hydrochemistry were analysed as well. This investigation aimed to: 1) trace water through soils, into the groundwater, and to the stream; and 2) investigate soil and regolith weathering hydrochemistry. The toposequence consists of podzolic Alfisols or texture-contrast Red-Brown Earths/Brown Chromosol soils which grade into saprolite which in turn grades into Precambrian meta-sedimentary bedrock. The groundwater table is shallow in the valley bottom, deepens upslope and has at least one metre seasonal rise and fall due to winter wet season recharge and summer dry season draw-down. Strontium isotope analysis of the groundwater, stream water, soil pore water, and soil exchange pool has established that the stream water is a mix of soil water and groundwater. This mix varies seasonally due to large changes in soil water input. Furthermore, in the down-slope soil sites, strontium isotope ratios of the soil exchange pool demonstrate a contribution to this pool from the bedrock groundwater. Up-slope soil sites that are well removed from the water table have exchange pool strontium isotope ratios that are much the same as the soil pore water. Thus, the dissolved solids in up-slope soils are dominated by wet fall-dry fall input whereas the dissolved solids in lower slope soils are dominated by weathering of high ratio Precambrian rock and saprolite transported by the groundwater system. This weathering is thought to be concentrated in the vadose zone at the transition between bedrock and saprolite.