Influence of buoyancy forces on movement of liquid radioactive waste from deep injection disposal site in the Tomsk region, Russian Federation: analytical estimate and numerical modeling

Deep injection disposal of liquid hazardous waste into aquifers confined from top and bottom by low permeable rock layers is examined. The injected waste represents aqueous solutions which are more dense than the reservoir water. Since the aquifer is not strictly horizontal, heterogeneity of the groundwater density causes a resultant buoyancy force which is directed down the dip of the aquifer. We considered a mathematical model of the contaminant plume movement taking into account both topography-driven (regional) and buoyancy induced components of the groundwater flow. Boussinesq’s approximation is used. A simple analytical solution, which is obtained by general functions theory, permits to determine conditions when the buoyancy forces suppress the regional flow what can substantially increase safety of the injection disposal. The obtained result is applied to analysis of contaminant plume movement at injection disposal site of liquid radioactive waste from Siberian Chemical Plant in Tomsk region. A numerical modeling of plume movement at the injection site is carried out. Heterogeneity of water transmissivity of the reservoir bed is taken into account as well as heterogeneous relief of its upper and lower boundaries. We showed on the basis of the analytical solution that buoyancy forces caused by elevated salinity of the injected waste and slope of the reservoir bed at the injection site exert a substantial influence on direction of the contaminant plume movement. Numerical modeling of contaminant plume movement is in good agreement with the analytical estimation of the buoyancy forces influence. Velocity of the regional groundwater flow is directed to the southwest of the injection site. However only dilute solutions are carried southwest by the regional flow. Dense solutions move westward because the buoyancy forces hamper southern component of the regional flow. This leads to an increase in travel time of a substantial part of the radionuclides to the river what increases safety of the injection site.