In situ treatment of arsenic contaminated groundwater

Groundwater in a sand and gravel aquifer was contaminated by arsenic compounds. The extent and the As concentration of the polluted groundwater plume decreased from 1971 to 1975, whereas the content of free dissolved oxygen increased. High As concentrations (> 1 mg/1) occured in groundwater with typical characteristics of a “reduced” water with negative Eh values and high concentrations of dissolved iron (up to 140 mg/1 in 1971). When plotted into an As stability field diagram, the higher values (> 1 mg As/1) coincided with the fields of trivalent As species, whereas the lower values (< 0.1 mg As/1) fitted to the fields of the pentavalent arsenic species. Therefore it was concluded that an improvement of the oxygen supply should accelerate the natural precipitation processes. By injection of 29,000 kg KMnO 4 into 17 wells and piezometers the soluble As (III) species were oxidized to As (V) species, which were precipitated as FeAsO 4 or Mn 3(AsO 4) 2 or co-precipitated with Mn- and Fe-hydroxides.