WATER AND RADIOACTIVE-TRACER FLOW IN A HETEROGENEOUS FIELD-SCALE SYSTEM

A coupled field-scale aquifer pumping and water infiltration test was conducted at the Idaho National Engineering and Environmental Laborato ry in order to evaluate subsurface water and contaminant transport pro cesses in a heterogeneous flow system. The test included an aquifer pu mping test to determine the storage properties of the aquifer and the state of confinement of the aquifer (similar to 190 m below land surfa ce), and a vadose zone infiltration test to determine vertical moistur e and radioactive tracer migration rates, Pump test results indicated that the Snake River Plain Aquifer was locally unconfined with a trans missivity ranging from 5.57 X 10(5) to 9.29 X 10(4) m(2)/day. 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 aver age rate of 5 m/day (based on vertical distance traveled and first arr ival of water at the monitoring points), Analysis of breakthrough curv es for a conservative tracer allowed estimation of the arrival of the peak concentration and yielded an average velocity of 1 m/day, The mig ration velocities from the neutron probe and tracer tests are in good agreement given the scale of the test and difference in analysis techn iques. None of the data sets showed a correlation between migration ve locity (arrival time) and distance from the point source, but they str ongly indicate preferential flow through discrete fractures. Upon reac hing the first continuous sedimentary interbed layer in the basalt for mation, water flow was diverted laterally along the interbed surface w here it spread outward in primarily three areas corresponding to topog raphic lows on the interbed surface, and slowly infiltrated into the i nterbed, The nonpredictable movement of water and tracer through speci fic fractures underlying the site suggests that a priori prediction of transmissive fractures in this media is not possible. Results do sugg est that the continuous sedimentary interbed layers, in general, imped e vertical water flow and contaminant migration.