The geochemical character of upland waters of northeast New South Wales

Abstract

The water chemistry of 18 small reservoirs in the New England Tablelands of northeast New South Wales was strongly influenced by catchment geology. Cluster analysis distinguished three types of waters. One group, strongly dominated by Mg2+ and HCO3− and also having high Ca2+, pH, and total conductivity, was characteristic of catchments underlain by alkali olivine basalt. A second group dominated by Na+ and HCO3−, and having a relatively low conductivity and pH, was characteristic of waters draining granitic rocks of adamellite composition. A third less well‐defined group, similar to group two waters but having high conductivity, drained catchments underlain by acid igneous rocks with a slightly more basic geochemistry and also mixed igneous and sedimentary catchments. Ratios of major cations in waters were highly determined by ratios in igncous catchment rocks (Mg2 +, R2 = 0.983; Na+, R2 = 0.917; Ca2+, R2 = 0.801; K+, R2 = 0.390). The importance of marine salt varied from <7% of ions for waters draining basalts to as high as 65% for dilute water draining a granitic catchment. The leaching of accumulated cyclic salt from the soil was inferred to be the principal source of these ions for all but the waters draining the younger basalts. The presence of relict sea salts in the soils of the acid igneous and sedimentary rock is believed to be due to elevated accession during arid periods of the Cretaceous and early Tertiary. Present‐day cyclic salt accession, connate salts, and anthropogenic sources were unimportant ion sources. The waters deviated substantially from the world average freshwater and also from the world water chemistry model of Gibbs.