Conceptual Hydrogeologic Models for the Eastern Snake River Plain, Idaho

We develop two alternative conceptual hydrogeologic models of the Eastern Snake River Plain (ESRP) aquifer to explain the spatial distribution of strontium and uranium isotope data. The ESRP aquifer flows southwesterly within a northeastern trending structural basin 200 miles long and 50 to 70 miles wide. The basin is composed of 3000 to 10000 feet of Late Cenozoic basalt, eolian sand and loess, and alluvial and lacustrine sediments. Up to 85 percent of the deposit is basalt from hundreds of flows where individual flow areas range from 2 to 900 square miles. The bulk of the recharge for the basin is derived from the Yellowstone Plateau, but other recharge waters with high (>.71100) 87Sr/86Sr ratios enter the basin from a series of alluvial valleys north of the ESRP. The first conceptual model includes regional-scale preferential flowpaths to explain the 87Sr/86Sr distribution, whereas groundwater mixing is the primary process in the second. Common factors in the conceptual model such as faulting, discontinuity of basalt flows, rift zones and sedimentary interbeds in the basin strongly affect groundwater flow. Additionally, a high geothermal gradient in the basin with associated upwelling waters may also influence the distribution of isotope ratios. Other factors also include local sediment sources for high 87Sr/86Sr ratios in the basin itself in addition to that entering form the valleys north of the plain. Estimates of critical time and length scales for each hydrologic process are compared.