Remediation or treatment of contaminated land, groundwater, or waste materials at their original location without excavation or removal. Techniques include chemical oxidation, bioremediation, stabilisation/solidification, and soil vapour extraction. In-situ treatment requires authorisation under Environmental Permitting Regulations but can be more cost-effective and less disruptive than excavation and off-site treatment. RPS 215 guidance covers small-scale in-situ remediation schemes.