Model Simulation of Groundwater Contamination by Cs-137 at Al-Tuwaitha Site

Abstract

This study simulates the groundwater movement and the transfer of Cs-137 contaminants of the aquifer system for Nuclear Research Center at Al-Tuwaitha site, using processing MODFLOW software package version 5.3. The site is located in south of the province of Baghdad near the Tigris river and contaminated with radiation as a result of exposure for bombing and acts of vandalism and looting in 2003. MODFLOW model was used to determine the groundwater movement and the impact of change of water surface elevation of the Tigris river on layers of aquifer system as well as to evaluate the ability of the proposed pumping well on the changing movement of water and contaminant in steady-state. In addition, MT3D model was used to study the contaminant distribution in aquifer and its change with time. The model calibration gave a relatively good agreement between the calculated and observed heads for six observed wells which were chosen for this study. Through the calibration process, an increase of 5% of the hydraulic head for the layers of the aquifer was found whenever the horizontal hydraulic conductivity of the third layer increases. The parameters used as input in model after the final calibration were 2x10-6 , 7x10-5, and 3x10-4 m/s as values of the horizontal hydraulic conductivity, 0.1, 0.26, and 0.32 as values of the effective porosity, and 1x10-9, 1x10-8 and 5x10-8 m2 /s as values of the diffusion coefficient for the first, second and third layers, respectively. The results indicated that any increase in the water elevation of the Tigris river leads to increase in levels of existing wells in the region by taking in consideration that other input variables in the model are constant. In addition, the use of pumping well has reduced the groundwater level and intercepted groundwater movement. The results of transport model showed that the spread of Cs-137 contaminate increased with time and its direction with the ground water movement. The migration of contaminants in horizontal direction is low which the seepage velocity for the first, second and third layer were 0.0357, 0.51, and 1.76 m/year, respectively, with approximately oval shape based on the ratio of longitudinal dispersion and diffusion coefficients. A distance of 690 m that the plume moved during 20 years of the simulation towards the pumping well existing in the direction of the water of the Tigris river. It was a relationship between the surface water (Tigris river), groundwater and the nature of the soil, a change in one of those elements may affect the others.