Effects of reinjecting diluted mineral pool water into the Rotorua Geothermal System

Abstract

This work presents the results of a geochemical modelling study into the potential effects of reinjection of dilute geothermal brine (i.e. dilution by bathing waters) into the geothermal aquifer of the Rotorua Geothermal System (RGS). It also summarizes the potential environmental effects from reinjection or discharge to surface waters, from both the release of pathogenic microorganisms, and the potential for biochemical changes to the environment by microbes.
The results of this study show that reinjection will have a small effect on the water composition of the shallow geothermal aquifer fluid proportional to the reinjection rate. At reinjection rates of 50 and 100 t day-1 (both at fluid temperatures of 120°C and 150°C) changes to the aquifer chemistry are minimal. Changes in the concentration of conservative elements will be insignificant as these are modelled to be within the ± 2% uncertainty of the aquifer brine composition. Less mineral precipitates are expected when injecting diluted brine compared to undiluted.
A measurable change in pH is found only at high reinjection rate (800 tonne day-1) up to 800 m from the well. There will be a measurable pH change and substantial decrease in the SiO2 concentration, which could impact surrounding wells and surface geothermal features.
Reinjection of water from mineral pools, or discharge to surface waters, poses a risk of release of human pathogens. Pathogens may infiltrate groundwater or surface water and present a hazard through inhalation, skin contact, or ingestion of contaminated food. Microorganisms identified in nearby surface features indicate that there may also be environmental risks from the natural geothermal microbial populations. These range from increased biofilm formation leading to reduced permeability, to the production of toxic
and corrosive H2S. The gradient of risk depends on the types of microorganism, volumes and dilutions of water reinjected, and the receiving environment, particularly temperature and pH. Filtration is recommended to remove the natural microbial population and pathogens and reduce the risk of adverse effects from their metabolic processes.