Diurnal variations in the chemistry of geothermal fluids after discharge, Champagne Pool, Waiotapu, New Zealand

Abstract

Champagne Pool, Waiotapu, New Zealand discharges reduced geothermal fluids rich in CO 2, H 2S trace elements, and over-saturated with respect to several sulfide minerals including orpiment, stibnite and carlinite. After discharge from Champagne Pool, the geothermal fluid cools, degasses and flows over a silica-sintered terrace that hosts a diverse microbial community. Geothermal fluid from Champagne Pool was sampled every 2 h for a 24-h period at five sites between the spring and above the first major downstream dilution. Systematic diurnal variations in composition occur at the three most downstream sample sites whereas the composition of the two upstream sites remains comparatively constant. At the three downstream sites, daytime increase in the concentration of dissolved As, Sb and Tl coincides with peak photosynthetic dissolved oxygen (DO) concentrations. Field observations, analyses and modelling with PHREEQC indicate that sulphur, and amorphous equivalents of orpiment, stibnite and carlinite precipitate at night within Champagne Pool and on some of the sinter terrace area. However, on the sinter terrace, daytime increases in DO concentrations from photosynthetic bacteria are sufficient to destabilise the sulfide minerals and sulfur that precipitate at night, enabling oxidation of the amorphous precipitates during the day. Oxidation of As, Sb and Tl sulfides and sulfur is probably biologically catalysed and causes a pronounced daytime decrease, >2 pH, at the two most downstream sites. Diurnal increases in the concentrations of Be, Ba and Sr at the two furthest downstream sites are likely to be related to the pH decrease caused by upstream oxidation. The geochemical mechanism responsible for daytime concentration increases in Be, Ba and Sr is unclear. Two processes that could be disrupted by daytime pH decrease are precipitation of silicate, sulfate or carbonate minerals on the sinter terrace or adsorption of trace elements onto clay minerals, viable/non-viable organic material or amorphous sulfide precipitates. These findings have implications for sampling programmes within geothermal areas and other environments such as estuaries and wetlands, where reduced mineral species could interact with photosynthetic biological processes.