Water and Radioactive Tracer Flow in a Heterogeneous Field‐Scale System

Abstract

AbstractA coupled field‐scale aquifer pumping and water infiltration test was conducted at the Idaho National Engineering and Environmental Laboratory in order to evaluate subsurface water and contaminant transport processes in a heterogeneous flow system. The test included an aquifer pumping test to determine the storage properties of the aquifer and the state of confinement of the aquifer (∼190 m below land surface), and a vadose zone infiltration test to determine vertical moisture and radioactive tracer migration rates. Pump test results indicated that the Snake River Plain Aquifer was locally unconfined with a transmissivity ranging from 5.57 × 105 to 9.29 × 104 m2day. Moisture monitoring with neutron probes indicated that infiltrating water was initially transported vertically through the upper basalt layer of the vadose zone, primarily through fractures and rubble zones, at an average rate of 5 m/day (based on vertical distance traveled and first arrival of water at the monitoring points). Analysis of breakthrough curves for a conservative tracer allowed estimation of the arrival of the peak concentration and yielded an average velocity of 1 m/day. The migration velocities from the neutron probe and tracer tests are in good agreement given the scale of the test and difference in analysis techniques. None of the data sets showed a correlation between migration velocity (arrival time) and distance from the point source, but they strongly indicate preferential flow through discrete fractures. Upon reaching the first continuous sedimentary interbed layer in the basalt formation, water flow was diverted laterally along the interbed surface where it spread outward in primarily three areas corresponding to topographic lows on the interbed surface, and slowly infiltrated into the interbed. The nonpredictable movement of water and tracer through specific fractures underlying the site suggests that a priori prediction of trans‐missive fractures in this media is not possible. Results do suggest that the continuous sedimentary interbed layers, in general, impede vertical water flow and contaminant migration.