Multivariate statistical methods for assessment of groundwater chemistry in the Waiareka and Deborah aquifers, Otago

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Daughney, C.J. 2008 Multivariate statistical methods for assessment of groundwater chemistry in the Waiareka and Deborah aquifers, Otago. Lower Hutt, N.Z.: GNS Science. GNS Science report 2008/25 31 p. + 1 CD

Abstract: This investigation explored the use of multivariate statistical methods to provide insight into the groundwater chemistry in the Waiareka and Deborah aquifers, North Otago. Prior to this investigation, Otago Regional Council (ORC) had defined five preliminary hydrostratigraphic units within this study area. An independent comparison to the ORC conceptual hydrostratigraphy was provide by hierarchical cluster analysis (HCA), which was used to re-categorise the monitoring wells based on major ion concentrations without any consideration of well location, depth, or assumed hydrostratigraphic unit. Four hydrochemical categories were defined by HCA, and these were generally consistent with the ORC conceptual hydrostratigraphy. However, HCA revealed some cases where a well’s hydrochemistry was inconsistent with the expectation for its assumed hydrostratigraphic unit. Thus discriminant analysis (DA) was used to predict the likelihood that each well taps into each ORC hydrostratigraphic unit, on the basis of major ion chemistry and well depth. The results of DA were also generally consistent with the ORC conceptual hydrostratigraphy, with the DA prediction matching the assumed hydrostratigraphic unit for 78% of the monitoring wells (n=58). Most of the wells for which the DA prediction did not agree with the ORC unit assignment were within a ca. 3 km by 3.5 km area diagonally bisected by SH1, just east of the Waiareka Creek crossing. Overall, this investigation has shown that multivariate statistical methods can be valuable for the development and validation of a conceptual hydrogeological model. (auth)