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An iron-rich water treatment residue (WTR) consisting mainly of ferrihydrite was used for immobilization of arsenic and chromium in a soil contaminated by wood preservatives. A leaching batch experiment was conducted using two soils, a highly contaminated soil (1033mgkg⁻¹ As and 371mgkg⁻¹ Cr) and slightly contaminated soil (225mgkg⁻¹ As and 27mgkg⁻¹ Cr). Compared to an untreated reference soil, amendment with 5% WTR reduced leaching in the highly contaminated soil by 91% for Cr and 98% for As. No aging effect was observed after 103d. In a small field experiment, soil was mixed with 2.5% WTR in situ. Pore water was extracted during 3years from the amended soil and a control site. Pore water arsenic concentrations in the amended soil were more than two orders of magnitude lower than in the control for the upper samplers. An increased release of arsenic was observed during winter in both …