Monitor well responses at the Raft River, Idaho, Geothermal Site

Effects of geothermal fluid production and injection on overlying ground-water aquifers have been studied at the Raft River Geothermal Site in southcentral Idaho. Data collected from 13 monitor wells indicate a complex fractured and porous media controlled ground-water flow system affected by natural recharge and discharge, irrigation withdrawal, and geothermal withdrawal and injection. The monitor wells are completed in aquifers and aquitards overlying the principal geothermal aquifers. Potentiometric heads and water quality are significantly affected by natural upward geothermal leakage via faults and matrix seepage. No significant change in water quality data has been observed, but potentiometric head changes resulted due to geothermal resource testing and utilization. Long-term hydrographs for the wells exhibit three distinct patterns, with superimposed responses due to geothermal pumping and injection. Well hydrographs typical of the Shallow aquifer exhibit effects of natural recharge and irrigation withdrawals. For selected wells, pressure declines due to injection and pressure buildup associated with pumping are observed. The latter effect is presumably due to the elastic deformation of geologic material overlying the stressed aquifers. A second distinct pattern occurs in two wells believed to be hydraulically connected to the underlying Intermediate aquifer via faults. These wells exhibit marked buildup effects duemore » to injection as well as responses typical of the Shallow aquifer. The third pattern is demonstrated by three monitor wells near the principal production wells. This group of wells exhibits no seasonal potentiometric head fluctuations. Fluctuations which do occur are due to injection and pumpage. The three distinct hydrograph patterns are composites of the potentiometric head responses occurring in the various aquifers underlying the Raft River Site.« less