Water being discharged to the Waikato River has physically eroded the cement, exposing the larger stones. This process has been accelerated by acids that are formed when Hydrogen sulfide gas dissolves in the condenser water – Image: Heurisko Ltd.
Excess hot water is disposed of by Discharge or Re-injection. The proportions of each have changed over time.
Discharge is when water is drained into the Waikato River. Originally this was the only method used at Wairakei.
Currently about 60 000 tonnes per day is discharged into the Waikato River.
The pipeline in the foreground carries hot water that will be used to recharge underground water at the edges of the geothermal field. The ponds behind are part of a prawn industry made possible by the use of waste heat energy – Image: Heurisko Ltd.
Re-injection is when hot water is pumped deep underground either within the geothermal system itself or outside the system into cold groundwater. Re-injection investigations began in 1978, twenty years after generation began in an attempt to avoid the contamination of the Waikato River by elements such as arsenic in geothermal water.
Currently about 46, 000 tonnes per day is re-injected.