Silica Deposition Along Kiriohinekai Stream, Wairakei, New Zealand

Abstract

This study was undertaken to determine the rate and mechanisms of silica deposition along a 3 km segment of the Kiriohinekai Stream located within Wairakei Geothermal Field. Water samples from six sampling sites were collected for silica (total and monomeric) and colloid particle analysis. Representative silica deposits from the comsponding sampling site wem also collected for XRD and XRF analysis.

The pH of the brine remained almost constant, which is about 8.6 while the temperature decrnased from 77 to 32 "C along the stream. On the other hand, monomeric silica decreased significantly f m 553 ppm to 205 ppm while total silica concentration declined slightly from 565 ppm to 493 ppm. The highest drop in total Si02 was between sampling sites US and TC in the upstream area with a deposition rate of 8.4 Mm-s. Particle size determination showed that the colloidal particle grew fmm 14 nm to 23 nm. The mineralogy of the deposits as given by XRD and XRF analysis are mainly amorphous silica with composition of between 8&97% Si02, respectively.

The mechanism responsible for silica deposition in Kirhhinekai Stream apparently involves a condensation reaction (polymerization) catalyzed by hydmxyl ions. Formation of cdloidal particles invdves decrease in the monomeric silica concentration as a function of temperaturn and degrw of supersaturation. The decrease in monomeric silica downstream coresponds to the amount of colloidal silica hrmed, which is capable of depositing on the streambed. The negligible silica deposition at distal downstream sites is related to particle size. Particles with diameter e20 nm deposit more quickly at temperatures ranging from 77 to 55 "C. Decline in monomeric silica due to polymerization is associated with changes in the morphology of the silica deposits.