2004 Initial Assessments for the T and TX TY Tank Farm Field Investigation Report (FIR): Numerical Simulations

Abstract

In support of CH2M HILL Hanford Group, Inc.’s (CHG) preparation of a Field Investigative Report (FIR) for the Hanford Site Single-Shell Tank Waste Management Area (WMA) T and TX-TY, a suite of numerical simulations of flow and solute transport was executed using the STOMP code to predict the performance of surface barriers for reducing long-term risks from potential groundwater contamination at the T and TX-TY WMA. The scope and parametric data for these simulations were defined by a modeling data package provided by CHG. This report documents the simulation involving 2-D cross sections through the T Tank and the TX-TY Tank Farm. Eight cases were carried out for the cross sections to simulate the effects of interim barrier, water line leak, inventory distribution, and surface recharge on water flow and the transport of long-lived radionuclides (i.e., technecium-99 and uranium) and chemicals (i.e., nitrate and chromium For simulations with barriers, it is assumed that an interim barrier is in place by the year 2010. It was also assumed that, for all simulations, as part of tank farm closure, a closure barrier was in place by the year 2040. The modeling considers the estimated inventories of contaminants within the vadose zone and calculates the associated risk. It assumes that no tanks will leak in the future. Initial conditions for contaminant concentration are provided as part of inventory estimates for uranium, technetium-99, nitrate, and chromium. For moisture flow modeling, Neumann boundary conditions are prescribed at the surface with the flux equal to the recharge rate estimate. For transport modeling, a zero flux boundary is prescribed at the surface for uranium, technetium-99, nitrate, and chromium. The western and eastern boundaries are assigned no-flux boundaries for both flow and transport. The water table boundary is prescribed by water table elevations and the unconfined aquifer hydraulic gradient. No-flux boundaries are used for the lower boundary. Numerical results were obtained for compliance at the WMA boundary, 200 Areas boundary, exclusion boundary beyond the 200 Areas, and the Columbia River (DOE-RL 2000). Streamtube/analytical models were used to route computed contaminant concentrations at the water table to the downstream compliance points. When the interim barrier was applied at 2010, the soil was desaturated gradually. The difference in saturation of the soil with and without the interim barrier was the largest at 2040, the time the closure barrier was applied. After this, the difference in saturation in the two cases became smaller with time. Generally, the solutes broke though faster if there was a water line leak. A relative small five-day leak (Case 4) had little effect on the peak concentration, while a large 20-yr leak (Case 3) increased the peak concentration significantly and reduced the solute travel in the vadose zone. The distribution of the inventory, either uniform or nonuniform, has little effect on peak arrival time; the peak concentrations of the conservative solutes varied by -6.9 to 0.2% for the T tank farm and by 11 to 49.4% for the TX tank farm. The reduction of the meteoric recharge before the barrier was applied led to less soil saturation, as expected, and thus longer solute travel time in the vadose zone and smaller peak fence line concentration. The effect on soil saturation lasted for about another 50 years after the barrier was applied at 2050. However, the reduced recharge rate affected the breakthough curve till the end of the simulation. The fence line concentrations at the year 3000 were always higher for cases with reduced natural recharge than for those of the base case, which indicates that the fundamental impact of the reduced natural recharge is a smoothing of the breakthrough concentrations at the compliance points.