Modeling reactive transport of strontium-90 in heterogeneous variably-saturated subsurface

Li Wang

An accidental release of sodium-bearing waste (SBW) containing high concentration of Sr-90 at the Idaho Nuclear Technology and Engineering Center (INTEC), Idaho National Laboratory (INL), Idaho USA in 1972 has raised public concerns. The vadose zone at the INTEC, composed of surficial alluvium, basaltic rocks and interbedding sediments, ranges 60-270 m in thickness. In order to investigate the transport and fate of Sr-90 through this heterogeneous, variably-saturated subsurface, a 2-dimensional model was conducted using TOUGHREACT. Four different scenarios were selected to represent different mechanisms for perched-water formation, including scenario 1 (base run), with the geometric mean of field-measured interbed permeability used for interbeds; scenario 2, the smallest field-measured interbed permeability used for interbedded sediments; scenario 3, one tenth of the smallest field-measured interbed permeability used for the top layer of interbeds at depths of 20-85 m; and scenario 4, with the smallest field-measured interbed permeability used for the top layer of basaltic rocks underlying interbeds at this range. The results showed that different mechanisms led to different steady-state flow patterns in terms of water saturation, horizontal and vertical pore-water velocities, water residence time, and water travel time from SBW leakage to ground-water table. For all scenarios, though, water flow was vertically dominant. Scenario 2 led to larger areas of saturated zones and longest water travel time from SBW leakage to ground-water table, while scenario 3 resulted longest water residence time for some grid blocks. After ~15 yr, two areas of high Sr2+ concentration could be found at different depths beneath the SBW leakage. A small fraction of Sr plume reached ground-water aquifer in ~45 yr of simulation. After the simulated 200 yr, both Sr2+ concentration in solution and on exchange site still remain the highest in alluvium. Among 1.2 mol of the total Sr input, only a tiny fraction had reached groundwater aquifer, ~99.7% was on the exchange sites, ~0.3% in solution, and ~96.1% still remain in alluvium after 200 yr. The results also indicated that distribution coefficient and retardation factor for Sr2+ changed more than one order of magnitude for the same material because of changing concentrations of Sr2+ and other competing ions, both in solution and on exchange sites.

Modeling reactive transport of strontium-90 in heterogeneous variably-saturated subsurface

Files and links (1)

Abstract

Metrics

Details