Fitzgerald RH, Scott BJ, Charlton DH, Magill CR. 2022. Geothermal hazard and impact review for the Department of Conservation Te Papa Atawhai. Lower Hutt (NZ): GNS Science. 71 p. (GNS Science report; 2022/19). doi:10.21420/QHAT-6M84.
Abstract
The Department of Conservation (DOC) has been undertaking a review of geological-based hazards and the management of those hazards on Public Conservation Lands and Waters. Deligne et al. (2020) reported on Part 6 of this work, which considered eruptive volcanic and geothermal hazards and defined two categories of volcanic or geothermal eruptions. However, the scope of this previous work did not allow the more commonly experienced hazards in geothermal areas to be addressed, and geothermal areas not associated with active cone volcanoes were not explicitly considered. This report, commissioned by DOC, fills these gaps by providing a comprehensive review of geothermal hazards and their potential impacts.
Geothermal areas can be defined and mapped in multiple ways, including geophysical surveys to define heat source or heat flow at depth or by expressions at the surface. Here, we define geothermal systems by the expression of features at the surface from which hazards are produced. Geothermal surface features include primary and mixed flowing springs, non-flowing pools, mud pools, mud pots, geysers, heated or steaming ground and fumaroles.
Multiple hazards can originate from geothermal surface features – both eruptive and non-eruptive. These include: geothermal gases; weak and unstable ground; heated ground; hot water, mud and steam within existing surface features; ground collapse; mild explosive activity such as jetting, splashing and bubbling; shallow earthquakes; infrastructure failures, including deterioration compromising boardwalks and viewing platforms and failures of bores, drill rigs and wells that cause artificial eruptions; hydrothermal eruptions, further divided into small, moderate and large eruptions; and phreatic eruptions.
Hydrothermal and phreatic eruptions can produce multiple hazards including gas, slugs/geysers of water or mud, ballistic projectiles, pyroclastic density currents and wet ash (mud rain).
Each hazard can impact different-sized areas, ranging from hot water, mud and steam being contained within surface features, and therefore dependent on the size of the feature, to large hydrothermal eruptions that eject material up to 4 km from the surface feature.
Geothermal processes have various properties that make them hazardous, and these properties can vary in their ability to cause negative impacts. Temperature is an important hazard metric for heated or weak/unstable ground, ground collapse, hot water, mud and steam within features, mild explosive activity and pyroclastic density currents. Temperatures of surface features (and therefore any material ejected from them) can range between 20 and 100°C, while pyroclastic density currents can measure up to 250°C. These temperatures can cause burns and fatal injuries. Ballistic projectiles and pyroclastic density currents can travel with enough energy to injure or kill those exposed. Depending on the concentration of geothermal gases, particularly hydrogen sulphide and carbon dioxide, exposure can range from a nuisance to deadly.
While many of the hazards produced in geothermal areas can cause casualties and fatalities, the frequency at which these are produced is important. Since 1800, around the time that records in New Zealand began, on average there has been one hydrothermal eruption every 1.1 years in New Zealand. In this same time period, there have been no recorded phreatic eruptions from geothermal areas in New Zealand, with the last documented occurrence around 700 years ago. A recorded ground collapse event has occurred on average every 3.8 years, while an artificial eruption or creation of a surface feature from infrastructure failure has occurred on average every 2.5 years.
The risk from these hazards can be managed in multiple ways, including hazard and risk assessment, land-use planning, engineering solutions, communication and education products and processes, access restrictions and monitoring/surveillance. Monitoring is an important risk-mitigation strategy for eruptions from volcanoes and can provide some warning of impending volcanic eruptions. Monitoring of geothermal systems may include regular surveillance of surface features tracking any observed changes. This is usually based around staff walking all tracks on a regular basis to check for safety issues, including infrastructure health, ground collapse, landslides and enhanced feature activity. Deligne et al. (2020) included geothermal eruptions in Category A of their two-category classification (eruptions without useful precursory activity that occur due to shallow magma). However, this current review identified that the Category A definition does not adequately cover geothermal hazards. Geothermal eruptions can occur either with or without measurable precursory activity.
In geothermal systems not associated with active cone volcanoes, eruptions are not driven by shallow magma but rather by deep magmatic sources, and non-eruptive hazards are also not included within the existing Category A. Additionally, although rare, magmatic eruptions can occur at geothermal systems associated with calderas. As such, we recommend that the Deligne et al. (2020) Category B (eruptions with precursory activity) be kept in relation to magmatic eruptions from geothermal areas not associated with active cone volcanoes, that Category A be used only for geothermal systems associated with active cone volcanoes and for those volcanoes themselves and that two new categories be created: Category C: Steam-driven eruptive hazards from geothermal systems not associated with active cone volcanoes – this includes all sizes of hydrothermal eruptions, phreatic eruptions and mild explosive activity from surface features; and Category D: Non-eruptive hazards from geothermal systems not associated with active cone volcanoes – this includes gas, ground collapse, heated or steaming ground, weak/unstable ground, failure of infrastructure associated with visitor access, and earthquakes. (The authors)
