Gas Transfer Through Ruapehu Crater Lake: Insights gained from a Recent Water-borne Survey

Between ca. 100 to 2500 T/D of CO2 and associated volcanic gases are emitted through the crater lake system of Mt Ruapehu on a daily basis, usually with no evidence of surface bubbling or ebullition. Two vents normally supply the bulk of the gas discharge, and these often have suspension-laden upwellings associated with them during periods of high emission rate. On 5 March 2010, a combined water-borne sonar mapping, CO2 diffusive flux, and discreet water sampling survey of Ruapehu Crater Lake was conducted during a period of low lake temperatures and subdued gas emissions. The goals were to identify the locations of the vent discharges on the lake floor, and to characterise the variability of solute gas compositions at the lake surface with a view to evaluating the degassing processes operating in the lake. Discreet upwellings from the two main vents were not observed at the lake surface on the day of sampling, but several subsurface bubble/suspension columns were detected by sonar. Only rarely, however, were bubbles observed breaching the surface, and even then these were ephemeral in nature. This suggests that gas discharging into the lake was completely dissolving during ascent as bubbles through the water column, and that gas transfer to the atmosphere was occurring largely by diffusion across the water-atmosphere interface. In this regard, modelling of the absorption process shows that rate of gas supply and bubble size are the key factors controlling breakthrough of a separate gas phase to the surface. Total gas contents in the water varied by a factor of ~2 across the lake, with CO2 comprising the major species in every case (maximum of 2.16x10-3 molal). There was reasonable spatial correspondence between total gas concentrations and known vent locations, but there was poor spatial agreement between areas of highest water CO2 concentration and CO2 flux, which we feel is in part attributable to surface wind effects on the diffusion process. Removing values which appear to have been affected by wind-induced turbulence, the integrated diffusive flux of CO2 from the lake was 96 T/d, which compares favourably with the airborne emission measurement of 190 T/d.