An investigation into the water quality of the municipal supply for Alexandra was undertaken to characterise the water composition and its origin, and explore the interaction between surface and groundwater resources. Water samples were collected fortnightly from the Clutha River, the municipal water bore field and five private bores over an eight month period. The water samples were analysed for Ca2+, Mg2+, K+, Na+, Cl-, Fl-, Br-, SO42-, NO3-, PO43- and total alkalinity (HCO3-). These ions as well as a range of field parameters such as pH, electrical conductivity, total dissolved solids and temperature were used to assess the quality of the water. Analysis of the groundwater composition shows water dominated by CaCO3, with modest amounts of base cations, which are consistent for schist-derived gravel aquifers in Central Otago. Stable isotopes 18O and 2H were also analysed from each water sample. Isotopic analysis of 18O and 2H shows that there is a spatially variable connectivity to the Clutha River, and that the bore field that supplies the municipal drinking water supply is isotopically indistinguishable from the Clutha River. However, analysis of the chemical composition of the Clutha River reveals alkalinity levels up to seven times lower than that of the groundwater, despite the town supply bores being located roughly 15 metres from the river bank. This suggests that water is primarily derived from the Clutha River, but that over a very short distance the groundwater is chemically altered by subsurface chemical exchanges. Thus, this study suggests that by drawing the river water through the aquifer results in a rapid deterioration of the water quality, and substantial hardening of the domestic water supply.
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Assessing groundwater / surface water connectivity and the effect on groundwater quality in Alexandra, Central Otago
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