Geochemical cycling of major and minor elements in the Taieri River and Waipori River catchments

The concentrations of the major ions (Na + , K + , Ca 2+ , Mg 2+ , Cl, SO 4 2 , total alkalinity A T and dissolved silica SiO 2 ) and the dissolved trace metals Cu and Zn have been determined in the combined catchments of the Waipori and Taieri Riveis, in Otago, New Zealand In general, the concentrations of the major ions were low, and were similar to levels observed in other uncontaminated rivers in the South Island However elevated concentrations of Na + , Cl, Ca 2+ , HCO, with mole ratios characteristic of seawater, were found in the lower catchment, indicating intrusion of sea salts by tidal mixing Significant inputs of nutrients, Zn, and, to a lesser extent, Cu, resulting from sewage effluent of the Mosgiel treatment plant, were observed at some periods during a time series of samples collected over a 20‐month period Estimates, based on the data, of the annual flux of nutrients and trace metals to the lower Taieri River catchment from this source indicate that the treatment plant contributes approximately 45% of the nutrients NO 3 , PO 4 3 , and dissolved Zn This implies that the recent closure of this treatment plant should result in a significant improvement in the water quality of the lower Taieri River