GROUND WATER FLOW, AQUIFER GEOMETRY, AND GEOTHERMAL INTERACTIONS INFERRED FROM TEMPERATURE DISTRIBUTION: SNAKE RIVER PLAIN AQUIFER, SOUTH-EASTERN IDAHO

High crustal heat flow from the eastern Snake River Plain (ESRP) (~110 mWm-2) is largely masked by the rapidly flowing cool water of the Snake River Plain aquifer, but makes itself known in warm springs at the margins of the ESRP, in deep wells that penetrate the aquifer bottom, and by occurrence of warm zones in the aquifer. Temperature logs of over 150 wells provide a 3D picture of temperature distribution in this fractured basalt aquifer. Because of the interaction of geothermal heat (conduction) and fluids (convection) with the aquifer waters, the temperature distribution reveals groundwater flow characteristics and aquifer geometry. Temperatures at the water table range from <8oC in areas strongly recharged by underflow from nearby mountains to >18oC in local areas. Temperatures as high as 22oC occur at the base of the aquifer. Temperature profiles of a few deep wells show that the actively flowing part of the aquifer varies in thickness from <100 to >350 m. Temperature profiles of six deep wells provide a data set for recognition of the aquifer bottom. Discontinuous warm zones occur along the margins of the ESRP where the aquifer is very thin, and where aquifer chemistry and isotope ratios suggest significant input of geothermal waters. Other warm zones occur along the axis of the ESRP, where geothermal input may be vigorous, and in areas of abundant fine-grained sedimentary interbeds where reduced hydraulic conductivity may slow the flow of aquifer water. In the deepest part of the aquifer low groundwater temperatures persist to the base of the aquifer, defining a deep channel of high permeability with abrupt transition to conductive temperature gradients below. Areas where deep, narrow plumes of cold recharge water from adjacent drainages penetrate into anomalously warm zones show that preferential pathways exist on a large scale. Prominent ridges in the water table are caused by mounding of cold underflow recharge water where two major drainages enter the ESRP.