Deep weathering profile and groundwater characteristics within a low-lying coastal pine plantation, southern Queensland—relationship to water-logging and salinisation

Abstract

Summary Exotic pine plantations are a major landuse within the coastal lowlands of southern Queensland. Seasonal water-logging and the potential for salinity are of significant concern to forest managers. In the northern Tuan State Forest, a typical deep weathering profile is characterised by soils at the top, ferricrete (discontinuous), mottled saprolite, fine saprolite and coarse saprolite at the base. This zonation exerts a major control on the occurrence and geochemical character of groundwaters within the profile. Three groundwater zones are identified: (1) shallow groundwater (fresh) perched on top of the ferricrete or mottled saprolite; (2) intermediate groundwater (brackish) on top of the fine saprolite, and (3) a deep confined groundwater (fresh) aquifer within the coarse saprolite. Water-logging is caused by the perched shallow groundwater and the risk is largely controlled by the depth of restrictive layer that is related to geology. Discharge of the intermediate groundwater at breaks in slope often causes localised salinity. Over the decades, increased water uptake by mature pines has mitigated overall soil salinity induced by clearing of native vegetation for the Pinus establishment, but future management should avoid large-scale harvesting as rising water tables of the intermediate groundwater due to increased recharge could re-transport the salts upwards towards the ground surface.