A two-column reactor was designed to remove dissolved As and Cd from comtaminated water. The reactor functions by equilibrating the targered water with CO2 and directing it via saturated flow through acolumn of crushed siderite. This results in siderite dissolution and an increase in dissolved Fe(Ⅱ). The feedwater is them directed into the top of a second, aerated cloumn of crushed limestone, where it passes by unsaturated flow. The Fe2+ ion oxidizes quickly to Fe3+ and precipitates as Fe(Ⅲ) oxyhydroxide, which is an effective sorbent of AsO34-. The aeration that occurs in the second column also removes dissolved CO2 from the feedwater. This causes precipitation of Ca and Cd carbonates. Together, the processes reduce As and Cd concentrations from 1 and 3mg/1, respectively, to below detection (respectively<0.005and <0.01mg/1). A time-limited reduction in Cr conentration also occurred. Much of the As was removed in the first column of the reactor, because Fe(Ⅲ) oxyhydroxides also formed there. This was due to oxidation of Fe(Ⅱ) by Cr(Ⅵ) and other oxidants present on the input wastewater. Although As is removed in the reactor columns by a sorption mechanism, the sorbent responsible, Fe(Ⅲ) oxyhydroxide, is continuously produced during the operation of the reactor. Thus, unlike attenuation in a system with a fixed amount of sorbent, breakthrough of the As contaminant should never occur.