The Environmental Fate of Geothermal Arsenic in a Major River System, New Zealand

Arsenic is released from various geothermal fields in the central North Island of New Zealand, into the Waikato River. Arsenic concentrations in the river water typically range from 0.02 to 0.06 mg/L. A review of As behaviour in this large catchment system is presented, based on over 10 years of water quality monitoring data and the results of more detailed, specific studies of As speciation and toxicity, and uptake or release from plants and biota, suspended particulate material and lake bed sediments. The primarily conservative behaviour of dissolved As in this river and its estuary, has significantly reduced the likelihood of long term adverse effects from As discharge. However, a portion of the As is retained in the river system, principally due to two processes; adsorption onto suspended particulate matter in winter/early spring, followed by settlement of the particulates into the lakes dammed for hydroelectricity, and uptake by diatoms and aquatic plants. The environmental and health implications of As retention in the river system are examined, and placed in the context of changing local government policy on the development and discharge from geothermal fields in the Waikato region.