FIELD STUDIES ON THE MIGRATION OF ARSENIC AND CADMIUM IN A CARBONATE GRAVEL AQUIFER NEAR MUNICH (GERMANY)

The migration of arsenic and cadmium species in ground water was investigated with the aid of tracer experiments at a test site in a fluvioglacial aquifer. Experimental breakthrough curves, obtained under natural flow conditions, characterize the migration of arsenic and cadmium species in the ground water in relation to a conservative tracer. Transport velocities of arsenic species corresponded to those of the groundwater flow and resulted in a rapid breakthrough in the observation wells. The recovery of the arsenic species, calculated with respect to the conservative tracer, averaged about 50%. Intensity of sorption increased with decreasing concentrations of arsenic species. The migration of cadmium species, however, was controlled by chemical precipitation of otavite and a slow redissolution of this compound. The breakthrough of cadmium in the observation wells, thus, was much slower with respect to the water movement. A recovery of about 40% was observed after more than 1 year of observation. Increased concentrations of chloride or/and sulphate resulted in remobilization of cadmium. Breakthrough curves are described by a commonly used dispersion model. It appears that a retardation factor is not sufficient to describe the asymmetrical breakthrough curves; it must be supplemented by recovery rates and a specific dispersivity, defined by the time variance of concentration at a given point. Longer flow distances or lower initial concentrations caused reduced recoveries, higher specific dispersivities and retardations both for arsenic and for cadmium species.