The effects of Aldrich humic acids (HA) on the removal of Zero-valent iron (ZVI) was investigated in laboratory systems. In batch, the removal rate of Zn and Ni (5 mg l(-1)) was, respectively, 2.8 and 2.4 times lower in the presence of HA (20 mg l(-1)) than in the absence of HA, presumably due to the formation of HA-heavy metal complexes which prevented the removal reactions at the ZVI surface. Chromate removal was not affected. In a column test, two parallel systems were supplemented with a continuous input of simulated groundwater containing a mixture of the heavy metals Zn, Ni and Cr(VI) (5 mg l(-1) each), with or without HA (at 20 mg l(-1)). Initially, the two column systems efficiently (>90%) removed the heavy metals from the simulated groundwater. When the input heavy metal concentration was increased to 8-10 mg l(-1), a significant breakthrough of Ni and Zn, up to 80%, occurred in the column system fed with HA. Chromate and HA did not significantly break through. After 60 weeks, the effect of HA on leaching of the accumulated metals (approx. 2 mg g(-1)) was investigated. No significant leaching was observed. The results of this study suggest that the impact of dissolved organic matter on the efficiency and lifetime of a ZVI barrier for in situ removal of heavy metals should be considered in the design of the barrier.