Chloride and Arsenic Fluxes from the Kawerau Geothermal Field into the Tarawera River, New Zealand

Abstract

The Tarawera River bisects the Kawerau geothermal field. Natural geothermal drainage as well as geothermal production fluid effluent (190 kg s-1) discharge to the river. The concentrations and fluxes of arsenic and chloride were measured upstream and downstream of the field to quantify the proportion of natural inflows of geothermal fluid compared to the discharge of effluent.

Upstream of the geothermal effluent outfalls, the river arsenic and chloride concentrations are about 21 m g L-1 and 39 mg L-1, respectively. The discharge of effluent increases the river concentrations to 29 m g L-1 arsenic and 48 mg L-1 chloride, respectively. Calculated concentrations, given the known discharge of effluent, are 38 m g L-1 arsenic and 50 mg L-1 chloride, respectively. The differences between the measured and calculated concentrations are within the gauging and analytical errors. At minimum and maximum mean river flows (1984-1992), the river concentrations would increase and decrease by 23% and 46%, respectively.

Arsenic appears to be soluble and not associated with suspended solids. However, increased transport of arsenic by suspended solids may be a factor at higher river flows. The input of natural geothermal fluid upstream of the effluent outfalls (estimated <170 kg s-1) could not be detected within the errors.