Modeling in situ iron removal from groundwater with trace elements such as As

The cyclic injection of oxygenated water in an aquifer may induce in situ iron removal from groundwater. During injection of aerated water, sorbed ferrous iron is displaced by cations, oxidized in the pore space, and precipitated as ferric iron oxyhydroxide. During pumping, ferrous iron is sorbed from groundwater on the exchange and sorption sites, and the breakthrough of dissolved iron is retarded. Other trace elements such as arsenic may be eliminated jointly with iron by sorption or co-precipitation. The volume of iron-free groundwater that can be pumped per volume of injected, aerated water defines the efficiency of the process. The efficiency is determined by the ratio of the retardations of oxygen during injection and of iron during pumping. This chapter shows how these retardations can be calculated for given water qualities and aquifer compositions. The first seven cycles of an in situ iron removal project in The Netherlands were simulated with the hydrogeochemical transport model PHREEQC (version 2). The concentration changes of CH4, NH4 , Mn, Fe, PO4 3and As are discussed in detail. Arsenic shows concentration jumps in pumped groundwater which are related to oxidation/reduction and sorption/desorption reactions resulting from the water quality variations.