REGEMP 2022: Waikato regional geothermal monitoring programme

Abstract

Waikato Regional Council collects and records monitoring data on geothermal features in the Waikato Region. Among other information, the database contains more than 2400 chemical analyses of geothermal features with variable analytical suites included, some dating back more than 100 years. Every few years more samples throughout the region are collected and analysed, and along with data from other sources, interpreted to determine regional temporal and spatial trends.
This paper presents an interpretation of the REGEMP dataset using classical geothermal classification based on major anions to indicate regional trends in geothermal geochemistry. The major anion plot distinguishes the types of geothermal processes that contribute to spring chemistry. In the REGEMP data the TVZ samples plot throughout the possible composition range on a major anion plot, whereas the North Waikato and Tongariro regions have a more restricted range of compositions. The North Waikato samples are concentrated along the Cl- HCO3 axis, possibly indicating that this area is depleted in S compared to Tongariro and TVZ area. This could indicate a systematic change in geothermal fluid composition in areas more distal from volcanism. Different geothermal fields also plot in relatively restricted, but overlapping, compositional fields on the major anion plot.
Summary recommendations include that major cation and anion ratios (as displayed in ternary diagrams) be used as the key tools to interpretation of the REGEMP dataset, and that alternative monitoring datasets such as satellite data or drone based thermal imaging should be considered.