USING ENVIRONMENTAL ISOTOPES, GEOCHEMISTRY, AND AQUIFER TEMPERATURE TO ADDRESS FLOW REGIMES WITHIN THE EASTERN SNAKE RIVER PLAIN AQUIFER AT THE IDAHO NATIONAL ENGINEERING AND ENVIRONMENTAL LABORATORY

Beginning in 1997 a series of studies utilizing uranium and strontium isotopes were undertaken to characterize the Eastern Snake River Plain (ESRP) aquifer at the Idaho National Engineering and Environmental Laboratory (INEEL). These studies identified fast flow and slow flow zones within the ESRP aquifer at the INEEL. The work presented here is the result of continued study to characterize the physical properties of the aquifer. Especially the implications and origins of large-scale (10's of kilometers) slow flow zones located beneath the 2300 km2 site. Coupling strontium and uranium isotope data from water samples with regional temperature, geophysical, and geologic data has proven to be a robust way to investigate large-scale flow characteristics in the aquifer. Depth-temperature profiles show that effective aquifer thickness varies dramatically across the INEEL from less than 100-m to over 350-m. Isotopic ratios support the conclusion that the thinner portions of the aquifer are "slow flow" zones in which the residence time of groundwater in the rock matrix is sufficient to allow significant water rock interactions to take place. The "slow flow" zones may be zones of increased diagenesis and pore filling mineralization caused by large volumes of upwelling geothermal waters penetrating into the upper, conductive portion of the aquifer. Our current compilation of the large database of isotope, chemical, temperature, hydrologic, and geophysical data in a GIS format enables us to use 50 years of data collected from observation wells and regional surveys to increase our understanding of a complex fractured-rock aquifer.