Groundwater investigations on the site of a former pulp mill with high content of arsenic showed that several heavy and semi-metals were mobilized over decades out of iron-slag deposits in the topsoil (residues from pyrite burning). With the exception of arsenic, all elements were fixed directly in the underlying water-saturated region due to sulfate reducing aquifer conditions. Arsenic forms under those conditions unique soluble arsenic-sulfur species (up to 2 mg As/L in the hot spot), which have been identified as thioarsenates by IC-ICP/MS. However, within a flow distance of about 60 m, the thioarsenates are immobilized in the aquifer. Laboratory tests and thermodynamic calculations suggest that in the prevailing sulfate-reducing, iron-containing environment arsenic is immobilized as arsenopyrite. These findings and a mass balance for the involved reacting agents enabled the prognosis of long term arsenic immobilization. This was a pre-condition of the authorities to establish a Monitored Natural Attenuation (MNA) concept in 2002. The results from subsequent 8 years of hydraulic and analytical monitoring confirmed the adequacy of this approach.