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

ABSTRACT

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.

KEYWORDS:

• Weathering
• Sr isotopes
• dissolved cations
• dissolved Si
• seasonality
• hydroelectric dams
• Southern Alps

Acknowledgments

We thank Claudine Stirling, Malcolm Reid and Dave Barr for their generous help with the analyses undertaken in the Centre for Trace Element Analysis. Christine McLachlan, Briar Taylor-Silva and Rebecca Parker guided VP through measuring silica by spectrophotometry. Katrin Wellnitz helped with the analytical work and PHREEQC. Luke Easterbrook-Clarke was a great help with making figures. We thank Michael Bau, Sylvia Sander, Nathalie van Havre and Birthe Kortner for their support during the first water collecting field trip. Thanks to hydrologists Kathy Walter from NIWA and Eddie Stead from Meridian Energy Ltd. for providing unpublished river and canal flow data. Nick Mortimer and two anonymous reviewers provided suggestions that significantly improved this manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.