The fate of antimony in a major lowland river system, the Waikato River, New Zealand

Antimony is an element that is becoming of increasing concern as an environmental contaminant. Geothermal systems are a source of Sb into some fresh waters of New Zealand’s North Island. The purpose of this research was to determine the factors controlling the behaviour of geothermally-derived Sb in the large lowland Waikato River system. The Waikato River is New Zealand’s longest and most utilised river. Antimony in the system exhibited mainly conservative behaviour, and seasonally variable dilution was found to be the most important control on Sb concentrations. The most significant potential removal process was identified as adsorption of Sb onto suspended particulate material (SPM). The adsorption of Sb onto the SPM is enhanced at low (<5) pH conditions, and in the anoxic base of stratified lakes. There was evidence that the adsorption of Sb is mainly onto Fe oxides in SPM, and changes with changing Fe concentrations. Therefore, Sb adsorption was higher in winter (when Fe concentrations in SPM were higher) than in summer. In Lake Ohakuri, which was stratified during the late summer/early autumn of 2007, there was also potential for removal of Sb as Sb2S3 in the presence of sulfide formed in the anoxic layer. The behaviour of Sb was conservative through the estuary at the mouth of the river.Antimony was compared to As, a metalloid often assumed to exhibit behaviour similar to Sb in aquatic environments. It was found that while the removal processes affecting Sb will also affect As, the inverse did not necessarily apply. Arsenic will adsorb more readily to SPM than Sb and, while there was evidence for bioaccumulation of As by freshwater macrophytes, there was no such evidence for Sb.