Geochemical processes affecting the major ion composition of rivers in theSouth Island, New Zealand

The major ion (Na+, K+, Mg2+, Ca2+, Cl-, SO42- and reactive SiO2) compositi ons of seven rivers in the South Island, New Zealand (Manuherikia, Clutha, Takaka-Cobb, Haast, Grey, Aorere and Rakaia rivers) are reported. All the r ivers were pristine in comparison with global average river waters, and the concentrations of most major ions were close to the 1% percentile levels f or global river water compositions. A geochemical model that attributes ion compositions to both rain input and to the weathering of various simple mi neral types in the catchment was applied to these data, and to data already published. This model, although simplistic, accounts for the main features of major ion composition in the rivers studied. The dominant sources of ma jor ions were the weathering of limestones and feldspars. Atmospheric input was relatively small except for Na+ and Cl-.