Water chemistry of lakes in the Taupo Volcanic Zone, New Zealand

Abstract

Dissolved salts in lake waters of the Taupo Volcanic Zone consist predominantly of HCO₃ ^‐ salts from ‘normal’ weathering of rocks and soils by carbonic acid, Cl^‐ salts from precipitation and geothermal water, and SO₄ ^2‐ salts from injection into the lakes or catchments of sulphuric acid from geothermal steam. Precipitation and cold spring waters are considered to be the input and final product, respectively, of normal weathering. The concentrations of major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO₃ ^‐, Cl^‐, SO₄ ^2‐) in these waters and in geothermal water and steam were used to calculate the origins of the dissolved salts in each lake. Rhyolitic strata dominate most lake catchments, but in some the presence of andesite, basalt, or hydrothermal ejecta influences the major ion ratios in the fraction of the dissolved salts derived from weathering. Each of the 32 lakes studied was classified according to the origins of its lakewater dissolved salts and the effect on these salts of its catchment geology.

Keywords:

• Taupo Volcanic Zone
• lake water
• dissolved salts
• major ions
• water
• chemistry
• precipitation
• geothermal water