This research work investigates the iron chromatographic displacement from the granular activated carbon (GAC) columns that occurred in the Vars, Ontario groundwater treatment plant. Natural organic matter (NOM) (XAD-8) fractionation showed that 30% of NOM was hydrophobic with the capability of forming complexes with iron. CHELEX-100 isolation indicated that 37–40% of the iron was organically-bound. The quantification of complexed iron performed on GAC microcolumn effluent at iron exhaustion point showed that this iron was neither adsorbed nor split from NOM. This indicates that NOM–iron complexes were not the cause of the observed chromatographic displacement of iron and suggests that competitive adsorption between iron and NOM forced the previously adsorbed iron to be desorbed. Adding greensand treatment prior to the GAC columns resulted in full iron removal, which suggests that NOM–iron complexes must have been either broken by potassium permanganate oxidation and/or adsorbed by the greensand filter.