Hexachlorocyclohexanes (HCHs) waste stockpiles are the single largest source of POPs. Sites contaminated with high levels of HCHs pose serious threats to public health and environmental safety, but there remains a lack of proper environmentally sound solution. To provide guidance on this issue, the remediation of contaminated soil with high levels of gamma-HCH (similar to 800 mg/kg) by persulfate with three commonly applied field activation methods, namely heat activation at 40 degrees C, high pH and Fe(III)-EDTA activation, was investigated. The effects of activation method and persulfate dosage on the degradation and dechlorination of gamma-HCH were studied, and degradation pathways were proposed. The results show that heat-activated persulfate is highly effective for the remediation of gamma-HCH, achieving complete degradation and dechlorination of gamma-HCH in 14 days. Under high pH conditions, gamma-HCH rapidly dehydrochlorizes into trichlorobenzenes, which are further degraded by persulfate. On the other hand, persulfate alone at 20 degrees C and Fe(III)-EDTA activated persulfate are not effective for the degradation of gamma-HCH, due to limited generation of reactive radical species resulting from the inefficient activation of persulfate, and the competition of soil organic materials and Fe(III)-EDTA with gamma-HCH for reactive radicals. (C) 2015 Elsevier B.V. All rights reserved.