- 1Ludwig-Maximilians-Universität München, Earth and Environmental Sciences, Munich, Germany (roberto.davoli@min.uni-muenchen.de)
- 2Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 1, Via di Vigna Murata 605, 000143, Rome, Italy
- 3Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Viale Carlo Berti Pichat, 6/2, 40127, Bologna, Italy
- 4School of Environment, University of Auckland, Science Centre, Building 302 23 Symonds Street, Auckland Central, New Zealand
- 5Mercury NZ Ltd., 283 Vaughan Rd, Rotorua 3010, New Zealand
- 6Lancaster Environment Centre, Lancaster University, Library Ave, Bailrigg, Lancaster, LA1 4YQ, United Kingdom
The surface flux of geothermal gases is driven by diffusion and advection, influenced by soil temperature, steam-heated ground, seismic activity, as well as seasonal or meteorological variations. Soil permeability, shaped by natural and anthropogenic processes, plays a key role in fluid migration, while hydrothermal fluid-solid interactions can alter permeability, affecting gas migration and surface release dynamics.
The Rotokawa geothermal field in New Zealand exemplifies how anthropogenic activities can modify natural surficial degassing processes. Ash- to pumice-rich Taupo ignimbrite units built the surficial layers in this geothermal field. In undisturbed areas, degassing occurs through the variably altered soil layers. Excavation induced by anthropogenic activities into the near-surface clay-rich horizons within the ash- to pumice-rich Taupo ignimbrite created new fumarolic areas, springs, and mud pools, while producing thick sulphur crusts and silica patina over time. As a result, in many excavated sites the areal permeability was reduced and the degassing concentrated along cracks. In undisturbed areas, degassing occurred through the variably altered soil layers.
This study integrates petrophysical and geochemical analyses to quantify permeability and gas flux across surficial soil layers. During a field campaign in February 2023, six vertical and three horizontal gas profiles were analysed by inserting a metallic rod into exposed soil and subsurface layers to extract and measure accumulated CO2, CH4, and H2O. The CO2 concentrations ranged from 432 to 99,370 ppm, CH4 from 2 to 754 ppm, and H2O from 23,514 to 41,555 ppm. Further we measured the vertical and horizontal permeability for key layers in these profiles. Samples of these key layers were taken for grain size and componentry analysis. Comparing the gas measurements with the petrophysical properties of the different soil layers provided insight into the vertical and lateral fluid movement and the influence of permeability in individual horizons. Our results enhance our understanding of how alteration and soil properties with and without anthropogenic influences affect geothermal gas emissions and will help improve future soil gas flux assessments.
How to cite: Davoli, R., Tamburello, G., Ricci, T., Montanaro, C., Murphy, B., Jones, T., Brooks, I., Cronin, S., and Scheu, B.: Impact of alteration and soil properties on geothermal gas emissions at Rotokawa, New Zealand, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18615, https://doi.org/10.5194/egusphere-egu25-18615, 2025.
