The chemical evolution of groundwater in a first-order catchment and the process of salt accumulation in the soil profile

Abstract

The chemical characteristics of surface water, base flow and groundwater in a first-order catchment in the wheatbelt of Western Australia were used to study the weathering process and its relationship to the development of groundwater and stream salinity. Meteoric water infiltrates through the unsaturated zone to the water table aquifer and through the aquifer outcrop in the case of a confined aquifer. The groundwater composition changes in space and time, becoming more saline with depth and distance away from the recharge zone. The concentration of salt in the system can be explained by four main mechanisms: withdrawal of water through uptake by plant roots for transpiration; loss of water during the weathering process and the formation of new minerals; leakage between aquifers; evaporation upstream of geological structures and near discharge zones. The groundwater is mainly of Na-Cl type, and is at saturation with respect to most of the carbonate minerals, chalcedony, talc and tremolite. The water changes in its chemical composition as rock-water interaction takes place. The weathering products are gibbsite and kaolinite, with the release of Na +, K +, Mg 2+, Ca 2+, HCO − 3 and H 4SiO 4. The preclearing weathering products are produced in a system open to CO 2 (through the plant roots), with groundwater under this system having excess Na +. After clearing the system becomes depleted in CO 2 and the groundwater becomes depleted in Na + through exchange with Mg 2+ from the rock surface. Geochemical modelling showed that most of the constituents in groundwater can be accounted for by taking into consideration the constituents of rainfall, with minor additions from the weathering process.