RELATING PORE WATER COMPOSITION TO SEDIMENT PROPERTIES IN ARSENIC- AND IRON-RICH SEDIMENTS

Arsenic from the Los Angeles drinking water supply is removed near Olancha (CA) by addition of ferric chloride solution to the surface waters. The As- and Fe-enriched flocs are then deposited downstream in the Haiwee Reservoir sediments, thus providing a relatively simple physico-chemical setting to investigate the factors controlling arsenic mobilization in reducing ground waters conditions. Pore water composition, obtained by equilibration with polyacrylamide gel slabs, are analyzed for total dissolved arsenic, iron, manganese, phosphorous, and silica. A strong positive correlation between dissolved Fe and As suggests that arsenic mobilization results from reductive dissolution of iron oxyhydroxides. In addition, positive correlation between arsenic and phosphate also indicate that competitive sorption might play an additional role in these environments. Sectioned sediment cores are subjected to sequential extraction to determine the nature of association of arsenic to the different mineral phases. Preliminary results indicate that more than 50% of the arsenic is adsorbed to the mineral phases, and the rest is extracted with 1% HCl solution, targeted mostly to dissolve the amorphous iron oxides. Such association of arsenic does not change with depth in the sediments. Sorption experiments are currently conducted to determine the sorption constants for arsenic and phosphate and the binding capacity of the sediments. A surface complexation model, constrained by the sediment properties and the pore water composition, will be presented.