TEMPERATURE DISTRIBUTION IN THE SNAKE RIVER PLAIN AQUIFER BENEATH THE IDAHO NATIONAL ENGINEERING AND ENVIRONMENTAL LABORATORY (INEEL): IMPLICATIONS FOR AQUIFER FLOW AND GEOTHERMAL INPUT

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 the presence of warm zones in the aquifer. Temperature logs of ~100 wells near the 2300 km2 INEEL provide a 3D picture of aquifer temperature distribution. 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 <8°C in areas strongly recharged by underflow from nearby mountainous terrain to >18°C near the axis of the ESRP. Temperatures as high as 22°C 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 wells provide an areally extensive data set for recognition of the aquifer bottom. A discontinuous warm zone occurs along the NW edge 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 an area 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 where rapidly flowing, cool waters mask any geothermal input from depth. In the northern part of the INEEL, a deep, narrow plume of cold recharge water from a drainage north of the ESRP penetrates southward into an anomalously warm zone.