Sources and downstream variation of surface water chemistry in the dammed Waitaki catchment, South Island, New Zealand

This article presents the results of a pilot study exploring downstream changes in water chemistry in the Waitaki catchment, which drains the eastern slopes of the Southern Alps. The headwaters of this catchment are glaciated, and both natural glacial lakes and hydroelectric reservoirs occur in the catchment. The dominant lithology is calcite-poor quartzo-feldspathic metasedimentary rock. We sampled surface waters from streams and the inflow and outflow of lakes in the catchment on four occasions over the course of 1year, at c. 3-monthly intervals between summer 2013 and spring 2014. We also sampled ice released from the terminus of the Tasman Glacier. Small but measurable variations were observed in the major element (Ca, Mg, K, Na, Si), trace element (Al, Fe, Rb, Sr) and Sr isotopic composition of the surface waters in the catchment. The composition of the waters is interpreted to reflect the change from calcite-dominated weathering near the top of the catchment toward more silicate weathering down the catchment that has been observed in other studies. The composition of samples collected from the uppermost part of the catchment displays more temporal variability than those samples collected from dammed lakes, suggesting that seasonality is dampened by anthropogenic modification of the catchment.