Biomass uptake and fire as controls on groundwater solute evolution on a southeast Australian granite: aboriginal land management hypothesis

The chemical composition of groundwater and surface water is often considered to be dominated by water–rock interactions, particularly weathering; however, it has been increasingly realised that plant uptake can deplete groundwater and surface water of nutrient elements. Here we show, using geochemical mass balance techniques that at our study site in Southwest Victoria, Australia, water–rock interactions do not control the hydrochemistry. Instead the chemical species provided by rainfall are depleted by plant biomass uptake and exported, predominantly through fire. Regular landscape burning by Aboriginal land users is hypothesized to have caused the depletion of chemical species in groundwater for at least the past 20 000 yr by accelerating the export of elements that would otherwise have been stored within the local biomass. These findings are likely to be representative of southeast Australia, as well as similar climatic regions elsewhere in the globe, and contrast with Northern Hemisphere studies of groundwater and surface water chemistry, where water–rock interactions are the dominant hydrochemical control.