Geothermal Springs at Wairakei; Lessons for Future Developers Sophie Frances Milloy1, Juliet Newson2 and Fabian Sepulveda3 1,2, 3 Contact Energy, Wairakei Power Station, Private Bag 2001, Taupo 3352, NZ 1Sophie.Milloy@contactenergy.co.nz 2Juliet.Newson@contactenergy.co.nz 3Fabian.Sepulveda@contactenergy.co.nz Geothermal surface features such as hot springs and geysers represent the heat and mass flow through the top boundary of the geothermal system. As such, these flows can be affected by changes in the subsurface system. At Wairakei, the surface activity has changed significantly since the beginning of well discharge for electricity generation in 1953. Changes in spring flow rate, temperature, and chemistry occurred within five years of the start of generation. The changes have been analysed in space and over time using data from over 20 springs, collected between 1951 to 1970, which have been compiled into a complete set of surface feature data from Geyser Valley at Wairakei. This time frame was chosen because this is the period of maximum rate of drawdown in reservoir pressure, from the natural state starting point. The data collected included temperature, and flow rate, and a range of chemical characteristics such as pH, major chemical constituents including (in some cases) gas content, and if applicable, geyser periodicity. The largest dataset is the chloride concentration of spring water. It is possible to identify trends, and characteristics of the surface feature changes. There is a clear trend, originally identified by Dick Glover of the Department of Scientific and Industrial Research, NZ, of a decline in chloride concentration, and cessation of flow over the time period. This is well correlated with the volume of fluid extracted from the reservoir. However, we have also found that the rate of chloride decline is weakly correlated to elevation. A possible physical explanation is that the springs are originally a mixture of geothermal (high chloride concentration) fluid and groundwater. As the reservoir pressure changes the proportions of these fluids change in a way that is related to elevation. Conversely, the temperature data trends with time are less obvious. There is a small decline in average temperature (

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