Contamination-free sample collection and handling protocols for trace elements in natural fresh waters

Abstract

Techniques that minimize contamination artefacts in the collection, handling and analysis of natural freshwater samples for dissolved trace metals are described. Methods were evolved during a 5-year study of a pristine high-altitude catchment in New Zealand, during which the dominating influence of contamination artefacts arising during the sampling operation was realized. Even with clean-room analysis facilities, order-of-magnitude contamination at the microgram per litre level was easily introduced at this earlier stage. The study calls into question much of the published literature that reports freshwater concentrations of trace elements in the microgram per litre range and below, particularly for Zn, Cd, Hg and Pb in systems whose pollutant sources are not definitively identified. We suggest that a base criterion for the reliability of trace-element measurements in natural fresh waters should be a similarity of spatial and temporal trends with those of conventionally measured properties such as major ions, alkalinity and electrical conductivity.