||Zero-valent iron (Fe0) based permeable reactive barriers (PRBs) have shown great potential for removing inorganic and organic pollutants from contaminated groundwater as an in situ remediation technology. The mechanisms responsible for contaminants removal are also well understood. However, uncertainties remain concerning the effectiveness of Fe0 PRBs in complicated subsurface environments. Among many influencing factors, the ubiquitous presence of humic substances in subsurface environments is an important issue that might influence the performance of Fe0 PRBs. The present study investigated the influences of humic acid on Cr(VI) and As(V) removal by Fe0 in laboratory batch settings and continuous flow column systems with the presence of various geochemical constituents, such as bicarbonate and Ca2+. The results obtained in this study show that humic acid exerted different influences on Cr(VI) and As(V) removal. It might originate from the distinct removal mechanisms of Cr(VI) and As(V) by Fe0. Cr(VI) removal is a reduction-dominated process, whereas As(V) removal principally involves adsorption onto Fe (hydr)oxides that are generated during iron corrosion. For Cr(VI) removal, the influences of humic acid varied significanly dependent on the presence of Ca2+ in solutions. In the absence of Ca2+, humic acid showed little inhibition to Cr(VI) removal. On the contrary, in the presence of Ca2+, humic acid would greatly co-aggregate with Fe (hydr)oxides colloids and progressively deposit on the Fe0 surfaces, and hence inhibit electrons transfer from the surface of Fe0 to Cr(VI) and largely reduce the effective porosity of the Fe0 matrix. As a result, the Cr(VI) removal capacity of Fe0 was significantly decreased. However, As(V) removal was observed to proceed differently facing the influences induced by humic acid. Humic acid significantly changed As(V) removal kinetics, which could originally be described well using a pseudo first order model, in the ways of inhibiting Fe2+/Fe3+ from forming hydroxides by binding with them and stabilizing the fine Fe hydroxides colloids (<0.45 μm) in solutions. These Fe hydroxides are the major adsorbents responsible for As(V) removal. As a result, the process of As(V) removal was retarded. The findings of this study arouse a consideration in the performance prediction and design of Fe0 PRBs in subsurface environments that are in rich of humic acid. The continuous input of humic acid might adversely affect the removal of contaminants that are based on chemical reduction or adsorption mechanisms. For successful applications of long-term in situ contaminated groundwater remediation using Fe0 PRBs, the amount of humic acid should be monitored and appropriate measures reducing its influences should be taken.