Dissolved trace metals and their relationship to major elements in the Manuherikia River, a pristine subalpine catchment in central Otago, New Zealand

Abstract

The spatial distributions of the dissolved trace metals Cu, Ni, Cd, Zn and Pb and of reactive Hg have been measured in parallel with the major elements Na, K, Mg and Ca and electrical conductivity in three sampling events in the Manuherikia River, New Zealand. The geographical span of the sampling sites ranged from a pristine subalpine upper catchment area through lower altitude sites that are increasingly affected by both natural weathering processes and moderate agricultural and domestic runoff. Major-element concentrations in the upper catchment were at or below the 1% frequency minimum of the spectrum of global river compositions, indicating a highly pristine system. Concen- trations increased steadily downstream to levels similar to world average river water. All trace metals exhibited the same spatial trends as the major ions. Typical upper-catchment concentrations were very low by global standards: Cu (150 ng L^-1), Ni (100-150 ng L^-1), Cd (8 ng L-I), Zn (150-200 ng L^-1), Pb (20-30 ng L^-1) and Hg (0.3 ng L^-1). Oceanic residence times calculated from the data for Zn (80 000 years) and Cd (230 000 years) for which few reliable estimates have been made, are consistent with observed correlations between the oceanographic distributions of these elements and dissolved silicon or phosphate, respectively.