TRANSPORT MODELING OF TRIBUTYLTIN COUPLED TO VARIABLE SALINITY

Leakage of tributyltin (TBT) to coastal environment is documented globally and represents environmental hazards because of long half lives in anaerobic environment and accumulation in biota. Concentrations below 1 ng/l influence marine organisms. Hence, pollution control authorities request abatements to mitigate toxic concentration levels. Dredging of contaminated sediments is one abatement strategy, but this approach requires safe landfill repositories. An alternative strategy is to cover contaminated sea bottom areas by cap layers of favourable composition and thereby reduce TBT exposure to the environment. Both abatement strategies require understanding of chemical and physical processes involved at the pore scale and at the field scale. In the present study pore scale processes are investigated by laboratory experiments on dredged sediments from contaminated sea bottom and integrated to field scale by numerical simulations. The distribution of TBT between solid and water phase is a function of several parameters viz texture; composition of clay minerals; content of organic matter; pH; and salinity in the pore water. The influences of these parameters on the mobility of TBT in sediments are studied with emphasize on variable salinity flux through the porous media. In our data the mobility of TBT increases as a function of decreasing salinity. Long term leakage of TBT is simulated in a typified near shore landfill with initial marine salt water concentration in the pore water. Initial TBT concentration in pore water is estimated to 30 ng/l, which is corresponding to a chemical equilibrium of 1 mg/kg of TBT in the sediments. Because sorption of TBT varies as a function of salinity, the transport of TBT has to be coupled to concentration of NaCl in the pore water. The pore water flow depends not only on the relation between infiltration of meteoric water and permeability of the sediments, but also on the concentration of NaCl, thus Darcy law has to be coupled to transport of NaCl. In this way transport of TBT require a coupling of three types of physics. First is Darcy pore water flow coupled to concentration of NaCl, and then concentration of NaCl is coupled to desorption of TBT. Based on experimental results we estimated partitioning coefficients which indicate retardation factors in the order of 1·10 9 to 1·10 11 . These factors imply very low liquid transport of TBT. Calculations indicate that in a time span of 10,000 years less than 1‰ of the initial TBT mass in sediments will be mobilized in the liquid phase and transported to the environment.