MODELING HEAT TRANSPORT AS A GROUNDWATER TRACER AT THE HANFORD IFRC SITE

There is growing interest in using groundwater temperature data as a tracer to supplement groundwater head and solute tracer data to characterize the aquifer heterogeneity because of recent advances in temperature sensing technologies and availability of relatively inexpensive temperature data loggers. The purpose of this study is to evaluate the utility of temperature data in comparison with Br tracer data to quantify the three-dimensional hydraulic conductivity distribution at the Integrated Subsurface Field Research Challenge (IFRC) Site in the 300 Area of the U.S. Department of Energy Hanford Site along the Columbia River in southeast Washington State. A field experiment was conducted in March 2009, in which a solution with a temperature of 9^oC and Br concentration of about 100 mg/L was injected into the aquifer through an injection well for 9 hours with a flow rate of 16.3 m³/hr. The background groundwater temperature is about 16.8^oC. After fluid injection, the groundwater temperature was measured at different depths in 28 wells and the groundwater was also sampled from 35 wells for Br measurement. For Br sampling, only three cluster wells were screened at different depths while all other wells were screened over the entire saturated depth. Thus, most of the Br concentrations were depth-mixed. Groundwater flow simulation was carried out with the MODFLOW code, while heat and Br tracer transport simulation was conducted with the MT3DMS code under the assumption of constant fluid density and viscosity. The inverse code PEST was employed to help calibrate the flow and transport model. The simulation results show that the groundwater head data alone are insufficient to constrain the hydraulic conductivity distribution, especially because the Hanford aquifer is highly permeable with the hydraulic conductivity in the range of 7000 m/day. While the Br tracer data are very important to improving the estimation of hydraulic conductivity, continuous Br data in 3D are expensive to acquire and not available at the Hanford IFRC site. The temperate data are shown to be a cost-effective proxy for conservative solute tracers such as the Br tracer.