Data collected from two monitoring programmes conducted by Environment Waikato, New Zealand, have been used to estimate the mass flows of Li, B, As, SO4, and Cl from geothermal fluids entering the Waikato River up stream of Lake Whakamaru. Chloride is the major anion in geothermal waters and is closely related to Li, B, and As. The total river water mass flow of Cl is 4600 g s−1at the outlet of Lake Whakamaru and of this, 78% originates from geothermal fluids; 13% flows into Lake Taupo, and 65% enters the river down stream of Lake Taupo. The Wairakei field contributes the largest fraction of 45% and geothermal fluid flows which were not directly measured in the monitoring programmes contribute 23%. The remaining 10% derives from measured flows from 14 of the 16 known geothermal fields. The origins of Li, As, and B closely follow those of Cl. Geothermal SO4is produced from the oxidation of H2S carried by geothermal steam and the estimates made of the mass flows originating from geothermal fluids are less certain than those for Cl.